java编程思想读书笔记
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java编程思想读书笔记
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多態(tài)
任何域的訪問操作都將有編譯器解析，如果某個方法是靜態(tài)的，它的行為就不具有多態(tài)性
java默認對象的銷毀順序與初始化順序相反
編寫構(gòu)造器時有一條有效的準則：“盡可能用簡單的方法使對象進入正常狀態(tài)，如果可以的話，避免調(diào)用其他方法”，下面的實例說明
//: polymorphism/PolyConstructors.java // Constructors and polymorphism // don't produce what you might expect. import static net.mindview.util.Print.*;class Glyph { void draw() { print("Glyph.draw()"); } Glyph() {print("Glyph() before dr aw()");draw();//invoke RoundGlyph.draw(),this is because that 后期綁定print("Glyph() after draw()"); } } class RoundGlyph extends Glyph { private int radius = 1; RoundGlyph(int r) {radius = r;print("RoundGlyph.RoundGlyph(), radius = " + radius); } void draw() {print("RoundGlyph.draw(), radius = " + radius); } } public class PolyConstructors { public static void main(String[] args) {new RoundGlyph(5); } } /* Output: Glyph() before draw() RoundGlyph.draw(), radius = 0 Glyph() after draw() RoundGlyph.RoundGlyph(), radius = 5 *///:~向上轉(zhuǎn)型（在繼承層次中向上轉(zhuǎn)型）會丟失東西，對應(yīng)于窄化轉(zhuǎn)換，會丟失一些信息，這個主要丟失的是類型信息。向上轉(zhuǎn)型是安全的（這里跟窄化轉(zhuǎn)換相反，可以思考下為什么？）因為基類接口小于等于導(dǎo)出類接口的。
有時候組合模式能夠使我們在運行期間獲得動態(tài)靈活性（狀態(tài)模式），如下面代碼所示
//: polymorphism/Transmogrify.java // Dynamically changing the behavior of an object // via composition (the "State" design pattern). import static net.mindview.util.Print.*;class Actor { public void act() {} }class HappyActor extends Actor { public void act() { print("HappyActor"); } }class SadActor extends Actor { public void act() { print("SadActor"); } }class Stage { private Actor actor = new HappyActor(); public void change() { actor = new SadActor(); }//this show that dynamic flexibility public void performPlay() { actor.act(); } }public class Transmogrify { public static void main(String[] args) {Stage stage = new Stage();stage.performPlay();stage.change();stage.performPlay(); } } /* Output: HappyActor SadActor *///:~is a關(guān)系可以采用繼承的方式，has a關(guān)系采用組合的方式
接口
（理解不了的句子打？，下同）接口和內(nèi)部類為我們提供了中將接口與實現(xiàn)分離的更加結(jié)構(gòu)化的方法
（？）抽象類是普通類與接口之間的中庸之道
創(chuàng)建一個不含任何抽象方法的抽象類的意義在于有個類想要阻止用戶產(chǎn)生該類的實例且方法中沒有用abstract的必要。
（？）interface關(guān)鍵字使抽象類的概念更向前邁進一步，可以將它的作用說成是建立類與類之間的協(xié)議
接口也可以包含域（變量，實例），但是在底層隱式的是static和final
兩者關(guān)系如果感覺是超級抽象（至少通過名字是感受不出來有關(guān)系），但是有微弱的聯(lián)系，就可以用接口
使用接口的核心原因是：單個類能夠向上轉(zhuǎn)型為多個基類型，第二個原因是與抽象基類使用的原因相同，不允許用戶創(chuàng)建該類的對象（抽象基類和接口不允許new）。
如果知道某事物應(yīng)該成為一個基類，那么第一選擇應(yīng)該是使它成為一個接口
不要在不同的接口中使用相同的方法名
（？）策略模式，適配器模式（需要總結(jié)），這兩種模式利用接口實現(xiàn)，概述如下（以Scanner為例）：
public interface Readable {/*** Attempts to read characters into the specified character buffer.* The buffer is used as a repository of characters as-is: the only* changes made are the results of a put operation. No flipping or* rewinding of the buffer is performed.** @param cb the buffer to read characters into* @return The number of {@code char} values added to the buffer,* or -1 if this source of characters is at its end* @throws IOException if an I/O error occurs* @throws NullPointerException if cb is null* @throws java.nio.ReadOnlyBufferException if cb is a read only buffer*/public int read(java.nio.CharBuffer cb) throws IOException;}/**Readable is interface,we can use any class that implement the Readable as the construtor's parameter*/public Scanner(Readable source) {this(Objects.requireNonNull(source, "source"), WHITESPACE_PATTERN);}- 之前可以用interface創(chuàng)建具有static, final類型的常量，現(xiàn)在用enum
- 接口在工廠設(shè)計模式上的應(yīng)用，使用匿名內(nèi)部類實現(xiàn)的工廠模式
- 一個類大多數(shù)情況下相當(dāng)于接口和工廠
內(nèi)部類
它允許你把一些邏輯相關(guān)的類組織在一起，并控制位于內(nèi)部的類的可視性，內(nèi)部類寫出的代碼更加優(yōu)雅而清晰，盡管并不總是這樣
內(nèi)部類能訪問其外圍對象的所有成員，用該性質(zhì)可以實現(xiàn)迭代器設(shè)計模式
package com.innerclass;import java.util.Iterator;interface Selector { boolean end(); Object current(); void next(); } public class Sequence { private Object[] items; private int next = 0; public Sequence(int size) { items = new Object[size]; } public void add(Object x) {if(next < items.length)items[next++] = x; }//innerClass that implements Selector,it can use any objects of outerClass private class SequenceSelector implements Selector {private int i = 0;public boolean end() { return i == items.length; }//access items of outerClasspublic Object current() { return items[i]; }public void next() { if(i < items.length) i++; } } public Selector selector() {return new SequenceSelector(); } public static void main(String[] args) {Sequence sequence = new Sequence(10);for(int i = 0; i < 10; i++)sequence.add(Integer.toString(i));Selector selector = sequence.selector();//while(!selector.end()) {System.out.print(selector.current() + " ");selector.next();} } } /* Output: 0 1 2 3 4 5 6 7 8 9 *///:~并且我們可以看到在java源碼中ArrayList的迭代器的實現(xiàn)也是采用類似的方法
內(nèi)部類new的方法：DotNew dn = new DotNew();DotNew.Inner dni = dn.new Inner()
內(nèi)部類可以更靈活的進行權(quán)限的控制，比如：某個類可以訪問，但其他類不能訪問，就可以用private關(guān)鍵字去實現(xiàn)
10.5略過
匿名內(nèi)部類，解釋代碼如下：
package com.innerclass;//: innerclasses/Parcel7.java and Contents.java // Returning an instance of an anonymous inner class.public class Parcel7 { public interface Contents {int value();} ///:~public Contents contents() { //匿名內(nèi)部類的現(xiàn)象return new Contents() { // auto implements Contentsprivate int i = 11;public int value() { return i; }}; // Semicolon required in this case } public static void main(String[] args) {Parcel7 p = new Parcel7();Contents c = p.contents(); } } ///:~其可以用來優(yōu)化工廠設(shè)計模式，代碼如下：
package com.innerclass; interface Service { void method1(); void method2(); } interface ServiceFactory { Service getService(); } class Implementation1 implements Service { private Implementation1() {} public void method1() {System.out.println("Implementation1 method1");} public void method2() {System.out.println("Implementation1 method2");} public static ServiceFactory factory =new ServiceFactory() {public Service getService() {return new Implementation1();}}; } class Implementation2 implements Service { private Implementation2() {} public void method1() {System.out.println("Implementation2 method1");} public void method2() {System.out.println("Implementation2 method2");} //匿名內(nèi)部類，可以直接引用變量，static是為了保證單一的工廠對象 public static ServiceFactory factory =new ServiceFactory() {public Service getService() {return new Implementation2();}}; } public class Factories { public static void serviceConsumer(ServiceFactory fact) {Service s = fact.getService();s.method1();s.method2(); } public static void main(String[] args) {serviceConsumer(Implementation1.factory);// Implementations are completely interchangeable:serviceConsumer(Implementation2.factory); } } /* Output: Implementation1 method1 Implementation1 method2 Implementation2 method1 Implementation2 method2 *///:~嵌套類就是在內(nèi)部類基礎(chǔ)上前面加個static關(guān)鍵字，通過static關(guān)鍵字能得到嵌套類的幾個性質(zhì)：
1.要創(chuàng)建嵌套類的對象，并不需要其外圍類的對象
? 2.不能從嵌套類的對象中訪問非靜態(tài)的外圍類對象寫測試類代碼的時候，可以用嵌套類如下這種方式，這樣就可以只用寫一個main了，代碼如下：
package com.innerclass;//: innerclasses/TestBed.java // Putting test code in a nested class. // {main: TestBed$Tester}public class TestBed { public void f() { System.out.println("f()"); } public static class Tester {public static void main(String[] args) {Factories f = new Factories();Tester t = new Tester();//此處不用new TestBed.Tester();TestBed t = new TestBed();t.f();} } } /* Output: f() *///:~為什么要使用內(nèi)部類：
//: innerclasses/MultiImplementation.java // With concrete or abstract classes, inner // classes are the only way to produce the effect // of "multiple implementation inheritance." package innerclasses;class D {} abstract class E {}class Z extends D { E makeE() { return new E() {}; } }public class MultiImplementation { static void takesD(D d) {} static void takesE(E e) {} public static void main(String[] args) {Z z = new Z();takesD(z);takesE(z.makeE()); } } ///:~
1.當(dāng)擁有抽象類或者具體類的時候，我們只能用內(nèi)部類實現(xiàn)多重繼承，代碼如下????2.創(chuàng)建內(nèi)部類后，使用變量是比較靈活，方便的，通過迭代器模式就能夠看出來
（？）閉包,lambda表達式與回調(diào)
package com.innerclass;//: innerclasses/Callbacks.java // Using inner classes for callbacksinterface Incrementable { void increment(); }// Very simple to just implement the interface: class Callee1 implements Incrementable { private int i = 0; public void increment() {i++;System.out.println(i); } } class MyIncrement { public void increment() { System.out.println("Other operation"); } static void f(MyIncrement mi) { mi.increment(); } } // If your class must implement increment() in // some other way, you must use an inner class: class Callee2 extends MyIncrement { private int i = 0; public void increment() {super.increment();i++;System.out.println(i); } private class Closure implements Incrementable {public void increment() {// Specify outer-class method, otherwise// you'd get an infinite recursion:Callee2.this.increment();System.out.println("closure");} } Incrementable getCallbackReference() {return new Closure(); } } class Caller { private Incrementable callbackReference; Caller(Incrementable cbh) { callbackReference = cbh; } void go() { callbackReference.increment(); System.out.println("go");}//調(diào)用了匿名內(nèi)部類的函數(shù) }public class Callbacks { public static void main(String[] args) {Callee2 c2 = new Callee2();MyIncrement.f(c2);//以匿名內(nèi)部類實現(xiàn)的回調(diào)new Caller(new Incrementable() {//callback the function@Overridepublic void increment() {// TODO Auto-generated method stubSystem.out.println("callback");} }).go();//以非匿名類內(nèi)部類回調(diào)new Caller(c2.getCallbackReference()).go(); } }
閉包可以理解為匿名內(nèi)部類就可以了，其跟lambda表達式的定義和演算相關(guān)，回調(diào)是基于內(nèi)部類實現(xiàn)的,代碼如下：（？需要體悟）設(shè)計模式總是將變化的事務(wù)與保持不變的事務(wù)分離開
（？需要感悟）內(nèi)部類是很像多重繼承的,多重繼承可以使用其父類的public后者protected變量，而內(nèi)部類也可以這樣。闡述代碼如下：
public class GreenhouseControls extends Controller { private boolean light = false; public class LightOn extends Event {public LightOn(long delayTime) { super(delayTime); }public void action() {// Put hardware control code here to// physically turn on the light.light = true;}public String toString() { return "Light is on"; } } // public class LightOff extends Event {public LightOff(long delayTime) { super(delayTime); }public void action() {// Put hardware control code here to// physically turn off the light.light = false;}public String toString() { return "Light is off"; } } }相當(dāng)于extends Controller, Event盡管java沒有這樣的語法，也就是說當(dāng)你感覺需要繼承多個類的時候，不防試一試內(nèi)部類。
（？有點難和偏）內(nèi)部類的繼承，內(nèi)部類方法可以被覆蓋嗎？
- （？自己的問題）接口和內(nèi)部類跟c++虛函數(shù)和純虛函數(shù)的關(guān)系？
- 這些特性的使用應(yīng)該是在設(shè)計階段就能夠確定的問題。
持有對象
該主題解決的問題是運行時（動態(tài)決定）決定持有對象的數(shù)量甚至類型，提出容器類的概念。
基本類型是存儲在堆棧中，其要求聲明size。
比較詳細的解釋了容器類，更詳細的還在后面
Collection：獨立的元素序列，Map：由鍵值對組成的對象，可以通過鍵來查找值。
（誤區(qū)），代碼闡述如下
public static void main(String[] args){Base b = new Sub();List<String> list = new ArrayList<>();//[1]List<String> list2 = new LinkedList<>();//[2]LinkedList<String> list3 = new LinkedList<>();//[3]}[2]中的list2只能d調(diào)用List中實現(xiàn)的函數(shù)和LinkedList實現(xiàn)函數(shù)的交集而[3]中的list3同理
字符串
每一個修改String的值，在內(nèi)部實現(xiàn)中都是創(chuàng)建過一個或者多個String對象，而最初的String對象則絲毫未動
如果toString方法比較簡單（toString(){}就是說將Object->String）就可以用+，編譯器可以自動優(yōu)化為StringBuilder的方式，但是如果在toString上有循環(huán)你就有必要表明StringBuilder sb = new StringBuilder();，而不能去指望編譯器的優(yōu)化。
StringBuffer是線程安全的，而StringBuilder不是。
打印內(nèi)存地址不能用this,應(yīng)該用Object.toString(),解釋如下
package com.string; import java.util.*;public class InfiniteRecursion { //this->String對象，就會調(diào)用this.toString(),于是就發(fā)生了無意識遞歸 public String toString() {return " InfiniteRecursion address: " + this + "\n"; } public static void main(String[] args) {List<InfiniteRecursion> v =new ArrayList<InfiniteRecursion>();for(int i = 0; i < 10; i++)v.add(new InfiniteRecursion());System.out.println(v);//println()->InfiniteRecursion.toString(); } } ///:輸出異常格式化輸出：System.out.format("row 1: [%d %f]",x ,y),跟C語言的printf()一樣的理解
String轉(zhuǎn)為2，4，6，8，16進制經(jīng)常會用到String.format()
正則表達式
- 正則表達式用于split，可以看到正則表達式匹配的那一部分都不存在了
- 正則表達式用于替換
- Matcher和Pattern:m.find()相當(dāng)于next()返回bool型,其有個重載的m.find(i)表示從index = i的地方開始查找匹配的子串，m.group()返回當(dāng)前匹配到的字符串,m.start() 和m.end()表示匹配到的子串在原父串的起始，末尾的索引。
RTTI(類型信息)
含義：在運行時，識別一個對象的類型。代碼解釋如下：
//: typeinfo/Shapes.javaimport java.util.*;abstract class Shape {void draw() { System.out.println(this + ".draw()"); }abstract public String toString();}class Circle extends Shape {public String toString() { return "Circle"; }}class Square extends Shape {public String toString() { return "Square"; }}class Triangle extends Shape {public String toString() { return "Triangle"; }} public class Shapes {public static void main(String[] args) {List<Shape> shapeList = Arrays.asList(new Circle(), new Square(), new Triangle());for(Shape shape : shapeList)shape.draw();}} /* Output:Circle.draw()Square.draw()Triangle.draw()*///:~在編譯時，具體形狀都向上轉(zhuǎn)型為Shape類型了，方法也只能調(diào)用Shape類的方法，但在運行時，可以識別出具體的類型信息。
java程序在它開始運行之前并非完全加載，與C++這樣的靜態(tài)加載語言不同，比如Class.forName()這種方法能夠體現(xiàn)出其特點
用.class形式去創(chuàng)建對象引用時，不會像class.forName()去自動初始化Class對象，代碼解釋如下：
package com.typeinfo;import java.util.*;class Initable {static final int staticFinal = 47;static final int staticFinal2 =ClassInitialization.rand.nextInt(1000);static {System.out.println("Initializing Initable");}public void name() {System.out.println("name() is invoked");}}class Initable2 {static int staticNonFinal = 147;static {System.out.println("Initializing Initable2");}}class Initable3 {static int staticNonFinal = 74;static {System.out.println("Initializing Initable3");}}public class ClassInitialization {public static Random rand = new Random(47);public static void main(String[] args) throws Exception {// Class initable = Initable.class;//泛型語法形式，盡量使用泛化的形式// Class<Initable> initable = Initable.class;//泛型語法形式，盡量使用泛化的形式// Class<?> initable = Initable.class;//泛型語法形式，盡量使用泛化的形式Class<? extends Object> initable = Initable.class;//泛型語法形式，盡量使用泛化的形式Initable.class.newInstance().name();//實例調(diào)用,即初始化該Class對象且將對象實例化了System.out.println("After creating Initable ref");// Does not trigger initialization:System.out.println(Initable.staticFinal);// Does trigger initialization:System.out.println(Initable.staticFinal2);// Does trigger initialization:System.out.println(Initable2.staticNonFinal);Class initable3 = Class.forName("com.typeinfo.Initable3");//只初始化了Class對象，并沒有實例化System.out.println("After creating Initable3 ref");System.out.println(Initable3.staticNonFinal);}} /* Output:Initializing Initablename() is invokedAfter creating Initable ref47258Initializing Initable2147Initializing Initable3After creating Initable3 ref74*///:~泛型語法和RTTI的原理結(jié)合起來可以做出很多解耦和的事情，使代碼更加清晰漂亮，如下
package com.typeinfo;import java.util.*;class CountedInteger {private static long counter;public final long id = counter++;public String toString() { return Long.toString(id); }}public class FilledList<T> {private Class<T> type;public FilledList(Class<T> type) { this.type = type; } public List<T> create(T[] ts){List<T> result = new ArrayList<T>();for(T t : ts){result.add(t);}return result; }public List<T> create(int nElements) {List<T> result = new ArrayList<T>();try {for(int i = 0; i < nElements; i++)result.add(type.newInstance());//1.返回該對象的確切類型} catch(Exception e) {throw new RuntimeException(e);}return result;}public static void main(String[] args) {List<CountedInteger> l = new ArrayList<CountedInteger>();CountedInteger[] cis = new CountedInteger[15];//空引用,相當(dāng)于先給定了一個空間for(int i = 0; i < 15; i++){cis[i] = new CountedInteger();}FilledList<CountedInteger> fl =new FilledList<CountedInteger>(CountedInteger.class);System.out.println("create1:" + fl.create(15));System.out.println("create2:" + fl.create(cis));}} /* Output:create1:[15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29]create2:[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]*///:~在代碼中type.newInstance()產(chǎn)生的是確定的類型，因為可以表述成<? extends Object>，(?)這在某些程度上有些受限，下面代碼因為<? super FancyToy>的存在，返回的是Object類型，代碼如下：
package com.typeinfo;//: typeinfo/toys/GenericToyTest.java// Testing class Class.public class GenericToyTest {public static void main(String[] args) throws Exception {Class<FancyToy> ftClass = FancyToy.class;// Produces exact type:FancyToy fancyToy = ftClass.newInstance();Class<? super FancyToy> up = ftClass.getSuperclass();// This won't compile:// Class<Toy> up2 = ftClass.getSuperclass();// Only produces Object:Object obj = up.newInstance();}} ///:~向下轉(zhuǎn)型需要使用顯示的類型轉(zhuǎn)換如Cicle c = (Cicle)new Shape()。
instanceof即檢查某個對象的實例是否屬于某個類或接口可以簡潔的記為一個 <實例，類|接口>，如果程序中編寫了許多的instanceof表達式，就說明你的設(shè)計可能存在瑕疵，下面以一個例子說明instanceof的用法，以及里面涉及到模板設(shè)計模式,思考在ForNameCreator.java中為什么要用Class去包裹即Class<? extends Pet>而不直接用<? extends Pet>
//: typeinfo/pets/Cat.javapackage com.typeinfo.pets;public class Cat extends Pet {public Cat(String name) { super(name); }public Cat() { super(); }} ///:~//若干繼承Pet的動物,這里省略//... //: typeinfo/pets/ForNameCreator.javapackage com.typeinfo.pets;import java.util.*;public class ForNameCreator extends PetCreator {//表示了一個不確定且是Pet的導(dǎo)出類的類型的List，相當(dāng)于對類型的范圍進行了指定，可以強制讓編譯器執(zhí)行類型檢查,想一想為什么要加Class包裹rivate static List<Class<? extends Pet>> types = new ArrayList<Class<? extends Pet>>();//和上面的解釋是一樣的，但其不能添加元素，涉及到模板的知識(？)static List<? extends Pet> typ = new ArrayList<>();static Map<Person,List<? extends Pet>> petPeple = new HashMap<Person,List<? extends Pet>>();static List<Dog> pets = new ArrayList<>();// Types that you want to be randomly created:private static String[] typeNames = { "com.typeinfo.pets.Mutt","com.typeinfo.pets.Pug","com.typeinfo.pets.EgyptianMau","com.typeinfo.pets.Manx","com.typeinfo.pets.Cymric","com.typeinfo.pets.Rat","com.typeinfo.pets.Mouse","com.typeinfo.pets.Hamster"}; @SuppressWarnings("unchecked")private static void loader() {petPeple.put(new Person("d"), pets); try {for(String name : typeNames){Class<? extends Pet> c = (Class<? extends Pet>) Class.forName(name) ; // typ.add(c.newInstance());error,can not run addtypes.add((Class<? extends Pet>)Class.forName(name));//進行了類型轉(zhuǎn)換，是什么確定的類型都有可能，在運行時確定，這就是RTTI內(nèi)容之一}} catch(ClassNotFoundException e) {throw new RuntimeException(e);}}static { loader(); }//直接在new ForNameCreator()中執(zhí)行l(wèi)oader()，或者直接在構(gòu)造器中調(diào)用loader()public List<Class<? extends Pet>> types() {return types;}} ///:~ //: typeinfo/pets/PetCreator.java// Creates random sequences of Pets.package com.typeinfo.pets;import java.util.*;public abstract class PetCreator {private Random rand = new Random(47);// The List of the different types of Pet to createpublic abstract List<Class<? extends Pet>> types();public Pet randomPet() { // Create one random Petint n = rand.nextInt(types().size());try {return types().get(n).newInstance();//產(chǎn)生了確切的實例，因為<? extends Pet>} catch(InstantiationException e) {throw new RuntimeException(e);} catch(IllegalAccessException e) {throw new RuntimeException(e);}} public Pet[] createArray(int size) {Pet[] result = new Pet[size];for(int i = 0; i < size; i++)result[i] = randomPet();return result;}public ArrayList<Pet> arrayList(int size) {ArrayList<Pet> result = new ArrayList<Pet>();Collections.addAll(result, createArray(size));return result;}} ///:~ package com.typeinfo;// Using instanceof.import java.util.*;import com.typeinfo.pets.*;public class PetCount {static class PetCounter extends HashMap<String,Integer> {public void count(String type) {Integer quantity = get(type);if(quantity == null)put(type, 1);elseput(type, quantity + 1);}} public static voidcountPets(PetCreator creator) {PetCounter counter= new PetCounter();for(Pet pet : creator.createArray(20)) {// List each individual pet:System.out.println(pet.getClass().getSimpleName() + " ");if(pet instanceof Pet)counter.count("Pet");if(pet instanceof Dog)counter.count("Dog");if(pet instanceof Mutt)counter.count("Mutt");if(pet instanceof Pug)counter.count("Pug");if(pet instanceof Cat)counter.count("Cat");if(pet instanceof Manx)counter.count("EgyptianMau");if(pet instanceof Manx)counter.count("Manx");if(pet instanceof Manx)counter.count("Cymric");if(pet instanceof Rodent)counter.count("Rodent");if(pet instanceof Rat)counter.count("Rat");if(pet instanceof Mouse)counter.count("Mouse");if(pet instanceof Hamster)counter.count("Hamster");}// Show the counts:System.out.println();System.out.println(counter);} public static void main(String[] args) {countPets(new ForNameCreator());}} /* Output:Rat Manx Cymric Mutt Pug Cymric Pug Manx Cymric Rat EgyptianMau Hamster EgyptianMau Mutt Mutt Cymric Mouse Pug Mouse Cymric{Pug=3, Cat=9, Hamster=1, Cymric=7, Mouse=2, Mutt=3, Rodent=5, Pet=20, Manx=7, EgyptianMau=7, Dog=6, Rat=2}*///:~ 用動態(tài)的`instanceof`即isInstance()去計數(shù),該計數(shù)的原理如下圖：ABCDEF B C B C A D 找出第二列出現(xiàn)B的次數(shù)，因為BCAD全都繼承于Pet,所以要用isInstance()去判定是否 屬于某個具體類型
//: typeinfo/PetCount3.java// Using isInstance()import typeinfo.pets.*;import java.util.*;import net.mindview.util.*;import static net.mindview.util.Print.*;public class PetCount3 {static class PetCounterextends LinkedHashMap<Class<? extends Pet>,Integer> {public PetCounter() {super(MapData.map(LiteralPetCreator.allTypes, 0));}public void count(Pet pet) {// Class.isInstance() eliminates instanceofs:for(Map.Entry<Class<? extends Pet>,Integer> pair: this.entrySet())if(pair.getKey().isInstance(pet))put(pair.getKey(), pair.getValue() + 1);} public String toString() {StringBuilder result = new StringBuilder("{");for(Map.Entry<Class<? extends Pet>,Integer> pair: entrySet()) {result.append(pair.getKey().getSimpleName());result.append("=");result.append(pair.getValue());result.append(", ");}result.delete(result.length()-2, result.length());result.append("}");return result.toString();}} public static void main(String[] args) {PetCounter petCount = new PetCounter();for(Pet pet : Pets.createArray(20)) {printnb(pet.getClass().getSimpleName() + " ");petCount.count(pet);}print();print(petCount);}} /* Output:Rat Manx Cymric Mutt Pug Cymric Pug Manx Cymric Rat EgyptianMau Hamster EgyptianMau Mutt Mutt Cymric Mouse Pug Mouse Cymric{Pet=20, Dog=6, Cat=9, Rodent=5, Mutt=3, Pug=3, EgyptianMau=2, Manx=7, Cymric=5, Rat=2, Mouse=2, Hamster=1}*///:~還可以用Class.isAssignableFrom()來計數(shù)，其是用來判斷一個類Class1和另一個類Class2是否相同或是另一個類的超類或接口。下面代碼對基類型和確切類型都進行了計數(shù)比如當(dāng)遍歷到dog的時候，dog的count++,pet的count++：
package net.mindview.util;import java.util.*;public class TypeCounter extends HashMap<Class<?>,Integer>{private Class<?> baseType;public TypeCounter(Class<?> baseType) {this.baseType = baseType;}public void count(Object obj) {Class<?> type = obj.getClass();if(!baseType.isAssignableFrom(type))throw new RuntimeException(obj + " incorrect type: "type + ", should be type or subtype of "baseType);countClass(type);} private void countClass(Class<?> type) {Integer quantity = get(type);put(type, quantity == null ? 1 : quantity + 1);Class<?> superClass = type.getSuperclass();if(superClass != null &&baseType.isAssignableFrom(superClass))countClass(superClass);}public String toString() {StringBuilder result = new StringBuilder("{");for(Map.Entry<Class<?>,Integer> pair : entrySet()) {result.append(pair.getKey().getSimpleName());result.append("=");result.append(pair.getValue());result.append(", ");}result.delete(result.length()-2, result.length());result.append("}");return result.toString();}} ///:~ //: typeinfo/PetCount4.javaimport typeinfo.pets.*;import net.mindview.util.*;import static net.mindview.util.Print.*;public class PetCount4 {public static void main(String[] args) {TypeCounter counter = new TypeCounter(Pet.class);for(Pet pet : Pets.createArray(20)) {printnb(pet.getClass().getSimpleName() + " ");counter.count(pet);}print();print(counter);}} /* Output: (Sample)Rat Manx Cymric Mutt Pug Cymric Pug Manx Cymric Rat EgyptianMau Hamster EgyptianMau Mutt Mutt Cymric Mouse Pug Mouse Cymric{Mouse=2, Dog=6, Manx=7, EgyptianMau=2, Rodent=5, Pug=3, Mutt=3, Cymric=5, Cat=9, Hamster=1, Pet=20, Rat=2}*///:~注冊工廠，可以用模板的方法+構(gòu)造器的方式，結(jié)合上面所講可以用newInstance去代替構(gòu)造器，兩種方法如下
package com.typeinfo;//: typeinfo/RegisteredFactories.java// Registering Class Factories in the base class.import java.util.*;import com.typeinfo.Factory;interface Factory<T> { T create(); } ///:~class Part {/*** 模板+RTTI方法*/public String toString() {return getClass().getSimpleName();}static List<Class<? extends Part>> partClasses = new ArrayList<Class<? extends Part>>();static {//創(chuàng)建類字面變量，利用newInstance()方法使其不用再類里面顯示調(diào)用構(gòu)造器partClasses.add(FuelFilter.class);partClasses.add(AirFilter.class);partClasses.add(CabinAirFilter.class);partClasses.add(OilFilter.class);partClasses.add(FanBelt.class);partClasses.add(PowerSteeringBelt.class);partClasses.add(GeneratorBelt.class);}private static Random rand = new Random();public static Part createRandom() {int n = rand.nextInt(partClasses.size());try {return partClasses.get(n).newInstance();} catch(InstantiationException e) {throw new RuntimeException(e);} catch(IllegalAccessException e) {throw new RuntimeException(e);}} /*** 模板+構(gòu)造器方法*/// public String toString() {// return getClass().getSimpleName();// }// static List<Factory<? extends Part>> partFactories =// new ArrayList<Factory<? extends Part>>(); // static {// // Collections.addAll() gives an "unchecked generic// // array creation ... for varargs parameter" warning.// partFactories.add(new FuelFilter.Factory());// partFactories.add(new AirFilter.Factory());// partFactories.add(new CabinAirFilter.Factory());// partFactories.add(new OilFilter.Factory());// partFactories.add(new FanBelt.Factory());// partFactories.add(new PowerSteeringBelt.Factory());// partFactories.add(new GeneratorBelt.Factory());// }// private static Random rand = new Random(47);// public static Part createRandom() {// int n = rand.nextInt(partFactories.size());// return partFactories.get(n).create();// }} class Filter extends Part {}class FuelFilter extends Filter {// Create a Class Factory for each specific type:public static class Factoryimplements com.typeinfo.Factory<FuelFilter> {public FuelFilter create() { return new FuelFilter(); }}}class AirFilter extends Filter {public static class Factoryimplements com.typeinfo.Factory<AirFilter> {public AirFilter create() { return new AirFilter(); }}} class CabinAirFilter extends Filter {public static class Factoryimplements com.typeinfo.Factory<CabinAirFilter>{public CabinAirFilter create() {return new CabinAirFilter();}}}class OilFilter extends Filter {public static class Factoryimplements com.typeinfo.Factory<OilFilter> {public OilFilter create() { return new OilFilter(); }}} class Belt extends Part {}class FanBelt extends Belt {public static class Factoryimplements com.typeinfo.Factory<FanBelt> {public FanBelt create() { return new FanBelt(); }}}class GeneratorBelt extends Belt {public static class Factoryimplements com.typeinfo.Factory<GeneratorBelt> {public GeneratorBelt create() {return new GeneratorBelt();}}} class PowerSteeringBelt extends Belt {public static class Factoryimplements com.typeinfo.Factory<PowerSteeringBelt> {public PowerSteeringBelt create() {return new PowerSteeringBelt();}}} public class RegisteredFactories {public static void main(String[] args) {for(int i = 0; i < 10; i++)System.out.println(Part.createRandom());}} /* Output:GeneratorBeltCabinAirFilterGeneratorBeltAirFilterPowerSteeringBeltCabinAirFilterFuelFilterPowerSteeringBeltPowerSteeringBeltFuelFilter*///:~- 反射其中之一的功能是在運行時創(chuàng)建類，但是其在運行時也必須要有.class文件（提前設(shè)置在本地或者網(wǎng)絡(luò)），所以它和RTTI的核心的區(qū)別只在于一個在運行時需要.calss文件，一個在編譯時需要.class文件
在反射中一般用到的并不是運行時創(chuàng)建類，而是利用forName()得到的一個未可知的對象在運行時進行方法調(diào)用，其常用的方法有g(shù)etMethods(),getDeclaredMethod(),invoke(),getConstructors()，下面代碼給出了實例：
public class TestRef {public staticvoid main(String args[]) throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {Foo foo = new Foo("這個一個Foo對象！");Class clazz = foo.getClass(); Method m1 = clazz.getDeclaredMethod("outInfo");Method m2 = clazz.getDeclaredMethod("setMsg", String.class);Method m3 = clazz.getDeclaredMethod("getMsg");m1.invoke(foo); m2.invoke(foo, "重新設(shè)置msg信息！"); String msg = (String) m3.invoke(foo); System.out.println(msg); } } class Foo { private String msg; public Foo(String msg) { this.msg = msg; } public void setMsg(String msg) {this.msg = msg; } public String getMsg() { return msg; } public void outInfo() {System.out.println("這是測試Java反射的測試類"); } }代理模式是基本的設(shè)計模式之一，代理通常充當(dāng)著中間人的作用，其具體分為靜態(tài)代理與動態(tài)代理，靜態(tài)代理很簡單，代碼如下：
package com.typeinfo;//: typeinfo/SimpleProxyDemo.javainterface Interface {void doSomething();void somethingElse(String arg);void doLastThing();}class RealObject implements Interface {public void doSomething() { System.out.println("doSomething"); }public void somethingElse(String arg) {System.out.println("somethingElse " + arg);}public void doLastThing(){System.out.println("doLastThing");}} class SimpleProxy implements Interface {private Interface proxied;public SimpleProxy(Interface proxied) {this.proxied = proxied;}public void doSomething() {System.out.println("SimpleProxy doSomething");proxied.doSomething();}public void somethingElse(String arg) {System.out.println("SimpleProxy somethingElse " + arg);proxied.somethingElse(arg);}@Overridepublic void doLastThing() {// TODO Auto-generated method stub}} class SimpleProxyDemo {public static void consumer(Interface iface) {iface.doSomething();iface.somethingElse("bonobo");}public static void main(String[] args) {consumer(new RealObject());consumer(new SimpleProxy(new RealObject()));}} /* Output:doSomethingsomethingElse bonoboSimpleProxy doSomethingdoSomethingSimpleProxy somethingElse bonobosomethingElse bonobo*///:~而動態(tài)代理是指代理類在程序運行前不存在、運行時由程序動態(tài)生成的代理方式,即proxy代碼在運行時，jvm動態(tài)產(chǎn)生代理類(proxy)代碼，因為使用反射和RTTI的特性，所以在性能上存在缺陷，通常代理模式用于java web而不用于前端，如Spring中大量使用代理模式，我們稱之為AOP（面向切面編程）。但是在代碼結(jié)構(gòu)和耦合性來看具有無可比擬的優(yōu)勢。動態(tài)代理的簡單代碼如下
package com.typeinfo;//: typeinfo/SimpleDynamicProxy.javaimport java.lang.reflect.*;class DynamicProxyHandler implements InvocationHandler {private Object proxied;public DynamicProxyHandler(Object proxied) {this.proxied = proxied;}public Objectinvoke(Object proxy, Method method, Object[] args)throws Throwable {System.out.println("**** proxy: " + proxy.getClass() +", method: " + method + ", args: " + args);//可以過濾方法if(method.getName().equals("doLastThing")){return null;}if(args != null)for(Object arg : args)System.out.println(" " + arg);return method.invoke(proxied, args);}} class SimpleDynamicProxy {public static void consumer(Interface iface) {iface.doSomething();iface.somethingElse("bonobo");iface.doLastThing();}public static void main(String[] args) {RealObject real = new RealObject();//consumer(real);// 動態(tài)代理模式的關(guān)鍵點1:后面兩個參數(shù)說明了如何進行動態(tài)代理Interface proxy = (Interface)Proxy.newProxyInstance(Interface.class.getClassLoader(),new Class[]{ Interface.class },new DynamicProxyHandler(real));consumer(proxy);}} /* Output: (95% match) doSomethingsomethingElse bonobo**** proxy: class $Proxy0, method: public abstract void Interface.doSomething(), args: nulldoSomething**** proxy: class $Proxy0, method: public abstract void Interface.somethingElse(java.lang.String), args: [Ljava.lang.Object;@42e816bonobosomethingElse bonobo*///:~空對象模式（就是把NULL用一個類去替代）中使用動態(tài)代理去自動創(chuàng)建空對象 ，首先看空對象模式，因為這個比較簡單，直接上代碼：
interface Book {// 判斷Book對象是否為空對象（Null Object）public boolean isNull();// 展示Book對象的信息內(nèi)容。public void show();}public class NullBook implements Book {public boolean isNull() {return true;}public void show() {}}public class ConcreteBook implements Book{private int ID;private String name;private String author;// 構(gòu)造函數(shù)public ConcreteBook(int ID, String name, String author) {this.ID = ID;this.name = name;this.author = author;}/***Description About show*展示圖書的相關(guān)信息*/public void show() {System.out.println(ID + "**" + name + "**" + author);}public boolean isNull(){return false;}} public class BookFactory {/***根據(jù)ConcreteBook的ID，獲取圖書對象。*@param ID 圖書的ID*@return 圖書對象*/public Book getBook(int ID) {Book book;//將原來的ConcreteBook改為Bookswitch (ID) {case 1:book = new ConcreteBook(ID, "設(shè)計模式", "GoF");break;case 2:book = new ConcreteBook(ID, "被遺忘的設(shè)計模式", "Null Object Pattern");break;default:book = new NullBook();//創(chuàng)建一個NullBook對象break;}return book;}} public static void main(String[] args) {BookFactory bookFactory = new BookFactory();Book book = bookFactory.getBook(-1);book.show();}（？）接著是利用動態(tài)代理模式+對象模式的改版，但這個例子不能讓我明白它的好處,例子如下：
package com.typeinfo;import java.lang.reflect.InvocationHandler;import java.lang.reflect.Method;import java.lang.reflect.Proxy;interface Book {// 判斷Book對象是否為空對象（Null Object）public boolean isNull();// 展示Book對象的信息內(nèi)容。public void show();}class NullBookProxyHandler implements InvocationHandler{private Class<? extends Book> mType;private Book proxyied = new NullBook();//指定了動態(tài)產(chǎn)生的代理實例public NullBookProxyHandler(Class<? extends Book> type){mType = type;}@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {// TODO Auto-generated method stubSystem.out.println(mType);return method.invoke(proxyied, args);}public class NullBook implements Book {public boolean isNull() {return true;}public void show() {System.out.println("對不起，查之無物");}}}public class NullPattern {public static Book newNullBook(Class<? extends Book> type){return (Book)Proxy.newProxyInstance(NullPattern.class.getClassLoader(), new Class[]{Book.class}, new NullBookProxyHandler(type)); }public class ConcreteBook implements Book{private int ID;private String name;private String author;// 構(gòu)造函數(shù)public ConcreteBook(int ID, String name, String author) {this.ID = ID;this.name = name;this.author = author;}public void show() {System.out.println(ID + "**" + name + "**" + author);}public boolean isNull(){return false;}}public class BookFactory {public Book getBook(int ID) {Book book = newNullBook(ConcreteBook.class);//將原來的ConcreteBook改為Bookswitch (ID) {case 1:book = new ConcreteBook(ID, "設(shè)計模式", "GoF");break;case 2:book = new ConcreteBook(ID, "被遺忘的設(shè)計模式", "Null Object Pattern");break;default://book = ;break;}return book;}}public BookFactory create(){return new BookFactory();}public static void main(String[] args) {NullPattern np = new NullPattern();BookFactory bookFactory = np.create();Book book = bookFactory.getBook(-1);book.show();}}動態(tài)代理模式整體框步驟如下：以SimpleDynamicProxy.java為例說明
//1.建立一個接口，減少耦合性interface Interface {void doSomething();void somethingElse(String arg);void doLastThing();}//2.需要代理的接口的實現(xiàn)class RealObject implements Interface {public void doSomething() { System.out.println("doSomething"); }public void somethingElse(String arg) {System.out.println("somethingElse " + arg);}public void doLastThing(){System.out.println("doLastThing");}} //3.建立一個繼承自InvocationHandler的導(dǎo)出類class DynamicProxyHandler implements InvocationHandler {private Object proxied;//3.1 你要代理的對象public DynamicProxyHandler(Object proxied) {this.proxied = proxied;}public Objectinvoke(Object proxy, Method method, Object[] args)throws Throwable {System.out.println("**** proxy: " + proxy.getClass() +", method: " + method + ", args: " + args);//可以過濾方法if(method.getName().equals("doLastThing")){return null;}if(args != null)for(Object arg : args)System.out.println(" " + arg);//6.RTTI即反射得知proxied的確切類型是RealObject，調(diào)用客戶端在第5步制定的方法return method.invoke(proxied, args);}} class SimpleDynamicProxy {public static void consumer(Interface iface) {iface.doSomething();iface.somethingElse("bonobo");iface.doLastThing();}public static void main(String[] args) {RealObject real = new RealObject();//consumer(real);// 4.具體指明你需要代理的對象，比如這里就是RealObject，因為在處理器內(nèi)部是Object，所以這是在編譯器無法知曉的，只能在運行時知道具體的類型信息。Interface proxy = (Interface)Proxy.newProxyInstance(Interface.class.getClassLoader(),new Class[]{ Interface.class },new DynamicProxyHandler(real));//5.調(diào)用接口方法consumer(proxy);}}通過反射甚至可以訪問private修飾的字段，自己感覺反編譯和發(fā)射有莫大的關(guān)系。
泛型
對象和實例是一個意思，類與對象的關(guān)系就像數(shù)據(jù)類型和變量一樣。
泛型的主要目的之一就是用來指定類（如：容器）要持有什么類型的對象，代碼解釋如下：
public class Holder3<T> {private T a;//持有了T的對象,此處可以持有任何對象，可以用Object代替但是要向下轉(zhuǎn)型public Holder3(T a) { this.a = a; }public void set(T a) { this.a = a; }public T get() { return a; }public static void main(String[] args) {Holder3<Automobile> h3 =new Holder3<Automobile>(new Automobile());Automobile a = h3.get(); // No cast needed// h3.set("Not an Automobile"); // Error// h3.set(1); // Error} } ///:~在有些場景中會有一個方法返回多個對象，你可以使用創(chuàng)建類用它來持有返回的多個對象，如果再 加上泛型技術(shù)就會在編譯期確保類型安全。代碼解釋如下：
//: net/mindview/util/TwoTuple.java package net.mindview.util;public class TwoTuple<A,B> {public final A first;public final B second;public TwoTuple(A a, B b) { first = a; second = b; }public String toString() {return "(" + first + ", " + second + ")";} } ///:~如果泛型用得好，基本上不用強制性轉(zhuǎn)換
泛型也可以應(yīng)用于接口，比如public interface Generator<T>,在寫繼承的時候T可以寫成任意類型，比如構(gòu)造一個咖啡工廠public class CoffeeGenerator implements Generator<Coffee>,構(gòu)造一個生成Fibonacci數(shù)列的類public class Fibonacci implements Generator<Integer>，咖啡代碼如下：
//... 省略處為一些簡單類，如Latte,Mocha,Cappuccino,Americano,Breve這些類都繼承于coffee,coffee.java如下 package com.generics.coffee; public class Coffee {private static long counter = 0;private final long id = counter++;public String toString() {return getClass().getSimpleName() + " " + id;} } ///:~//: generics/coffee/CoffeeGenerator.java // Generate different types of Coffee: package com.generics.coffee; import java.util.*; import net.mindview.util.*;public class CoffeeGenerator implements Generator<Coffee> ,Iterable<Coffee> {@SuppressWarnings("rawtypes")private Class[] types = { Latte.class, Mocha.class,Cappuccino.class, Americano.class, Breve.class, };private static Random rand = new Random(47);public CoffeeGenerator() {}// For iteration:private int size = 0;public CoffeeGenerator(int sz) { size = sz; } public Coffee next() {try {return (Coffee)types[rand.nextInt(types.length)].newInstance();// Report programmer errors at run time:} catch(Exception e) {throw new RuntimeException(e);}}//解釋：內(nèi)部類實現(xiàn)迭代器，該實現(xiàn)了Iterator而CoffeeGenerator實現(xiàn)的是Iterable,要實現(xiàn)foreach循環(huán)必須實現(xiàn)這兩個接口, //從代碼看起來foreach循環(huán)是看出來了，要理解其本質(zhì)的原理需要看jvm里面的字節(jié)碼，new CoffeeGenerator(5)調(diào)用后，首先產(chǎn)生 //CoffeeIterator的實例，執(zhí)行hasNext()->next() //此處可以也可以用匿名內(nèi)部類class CoffeeIterator implements Iterator<Coffee> {int count = size;public boolean hasNext() { return count > 0; }public Coffee next() {count--;return CoffeeGenerator.this.next();}public void remove() { // Not implementedthrow new UnsupportedOperationException();}}; public Iterator<Coffee> iterator() {return new CoffeeIterator();}public static void main(String[] args) {CoffeeGenerator gen = new CoffeeGenerator();for(int i = 0; i < 5; i++)System.out.println(gen.next());for(Coffee c : new CoffeeGenerator(5))System.out.println(c);} } /* Output: Americano 0 Latte 1 Americano 2 Mocha 3 Mocha 4 Breve 5 Americano 6 Latte 7 Cappuccino 8 Cappuccino 9 *///:~Fibonacci數(shù)列的代碼如下：
package com.generics;import net.mindview.util.*;public class Fibonacci implements Generator<Integer> {private int count = 0;public Integer next() { return fib(count++); }private int fib(int n) {if(n < 2) return 1;return fib(n-2) + fib(n-1);}public static void main(String[] args) {Fibonacci gen = new Fibonacci();for(int i = 0; i < 18; i++)System.out.print(gen.next() + " ");}} /* Output:1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584*///:~如果想要實現(xiàn)迭代，而且要不用內(nèi)部類的方式（CoffeeGenerator.java使用的是內(nèi)部類實現(xiàn)的迭代器模式），用適配器模式實現(xiàn)，適配器模式即把兩個互不相關(guān)的接口或者類相連接，所以可以使用繼承或者組合，UML如下:
迭代如下：
package com.generics;// Adapt the Fibonacci class to make it Iterable.import java.util.*;//組合來創(chuàng)建適配器public class IterableFibonacci implements Iterable<Integer> {private Fibonacci fibonacci = new Fibonacci();private int n;public IterableFibonacci(int count) { n = count; }public Iterator<Integer> iterator() {//匿名內(nèi)部類的形式return new Iterator<Integer>() {@Overridepublic Integer next() {// TODO Auto-generated method stubn--;return fibonacci.next();//invoke next() in Fibonacci,for this extends Fibonacci}@Overridepublic boolean hasNext() {// TODO Auto-generated method stubreturn n > 0; }public void remove() { // Not implementedthrow new UnsupportedOperationException();}};} public static void main(String[] args) {for(int i : new IterableFibonacci(18))System.out.print(i + " ");}} /* Output:1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584*///:~泛型應(yīng)用于方法
應(yīng)用于方法public <T> void f(T x){}，其中一定要寫，不然編譯器是無法識別出參數(shù)的T
當(dāng)可變參數(shù)與方法結(jié)合：
package com.generics;//: generics/GenericVarargs.java import java.util.*;public class GenericVarargs {//此處的makeList就像是java.util.Arrays里面的asList(T... args);public static <T> List<T> makeList(T... args) {List<T> result = new ArrayList<T>();for(T item : args)result.add(item);return result;}public static void main(String[] args) {List<String> ls = makeList("A");System.out.println(ls);ls = makeList("A", "B", "C");System.out.println(ls);ls = makeList("ABCDEFFHIJKLMNOPQRSTUVWXYZ".split(""));System.out.println(ls);} } /* Output:[A][A, B, C][, A, B, C, D, E, F, F, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z]*///:~從上面代碼注釋可以看到makeList和asList方法很像，下面來看看asList的源碼分析
public static <T> List<T> asList(T... a) {return new ArrayList<>(a);//此處的ArrayList不是你想的java.util.ArrayList,他是Arrays里面的一個靜態(tài)內(nèi)部類}//此處是靜態(tài)內(nèi)部類的構(gòu)造器，返回一個數(shù)組，需要說明的是該內(nèi)部類并沒有實現(xiàn)add,remove等方法，因為asList()方法在大多數(shù)使用場景中是不用改變的，所以要構(gòu)造一個可編輯的ArrayList()用類似下面的代碼即可List<WaiterLevel> levelList = new ArrayList<WaiterLevel>(Arrays.asList("a", "b", "c")); ArrayList(E[] array) {a = Objects.requireNonNull(array);//判斷array是否為空}
利用泛型方法對前一章的生成器進行更一步的抽象，代碼如下：
//: net/mindview/util/BasicGenerator.java // Automatically create a Generator, given a class // with a default (no-arg) constructor. package net.mindview.util; //this class can generate any Class which have default constructor by create() function,but there is a limit which is that constructor cannot pass argument(傳參) public class BasicGenerator<T> implements Generator<T> {private Class<T> type; public BasicGenerator(Class<T> type){ this.type = type; }public T next() {try {// Assumes type is a public class:return type.newInstance();} catch(Exception e) {throw new RuntimeException(e);}}// Produce a Default generator given a type token:public static <T> Generator<T> create(Class<T> type) {return new BasicGenerator<T>(type);} } ///:~更多的，我們可以對前面提到的元組進行進一步的抽象
//: net/mindview/util/Tuple.java // Tuple library using type argument inference. package net.mindview.util;public class Tuple {public static <A,B> TwoTuple<A,B> tuple(A a, B b) {return new TwoTuple<A,B>(a, b);}public static <A,B,C> ThreeTuple<A,B,C>tuple(A a, B b, C c) {return new ThreeTuple<A,B,C>(a, b, c);}public static <A,B,C,D> FourTuple<A,B,C,D>tuple(A a, B b, C c, D d) {return new FourTuple<A,B,C,D>(a, b, c, d);}public static <A,B,C,D,E>FiveTuple<A,B,C,D,E> tuple(A a, B b, C c, D d, E e) {return new FiveTuple<A,B,C,D,E>(a, b, c, d, e);} } ///:~java對泛型的擦除有四句話
- 泛型類型在運行時都是Object類型
- 模板只在編譯階段有效是為了提供編譯期的類型安全，通過反射操作可以繞過編譯階段
- 在編譯期就可以知道的類型信息是可以操作的
- 所有在運行時才能知道類型信息的操作都將無法工作
不能創(chuàng)建泛型類型數(shù)組的,一般的解決方案是在任何想要創(chuàng)建泛型數(shù)組的地方都是用ArrayList去創(chuàng)建。
泛型的主要目標之一是將錯誤檢測移入到編譯期
編譯器直接拒絕對參數(shù)列表中涉及到的通配符的方法，即add(? extends fruit)如果變成了這樣結(jié)果如下圖
?
下面代碼能夠?qū)崿F(xiàn)只能存水果的集合，且在編譯期內(nèi)就能檢查類型信息,其中第二種方式稱之為逆變。為什么逆變的方式可以實現(xiàn)？答：super關(guān)鍵字表示下界，List<? super Apple> fruit = new ArrayList<>();，而?必須要表示一個確切的類型，準確來講應(yīng)該是這樣聲明一個實例即：List<? super Apple> fruit = new ArrayList<在這個括號內(nèi)部必須是Apple的父類>();即在比如List<? super Apple> fruit = new ArrayList<Fruit>()，所以當(dāng)add()的時候，可以插入Apple的子類，同樣的道理分析List<? extends Apple> flist2 = new ArrayList<這里面要插入的是Apple的子類>();所以當(dāng)add(new Apple())時候，會失敗，比如List<? extends Apple> flist2 = new ArrayList<Jonathan>();Jonathan = new Apple()//error;
//1.想要實現(xiàn)一個集合里面能裝所有類型的水果，但是在編譯期不允許裝除了水果以外的其他對象 List<Fruit> flist3 = new ArrayList<>(); flist3.addAll(Arrays.asList(new Apple())); flist3.addAll(Arrays.asList(new Orange())); System.out.println(flist3.get(1)); //2.第一種方式太復(fù)雜,下面用逆變的方式實現(xiàn) List<? super Fruit> fruit = new ArrayList<>(); fruit.add(new Apple()); fruit.add(new Orange());混型即Timestamped<Serialnumbered<Basic>> mixin其中mixin能夠調(diào)用基類的所有函數(shù)，在C++中，這是顯然的，但是在java中可以這樣聲明，但不能調(diào)用基類的任何函數(shù)只能調(diào)用Timestamped類中的函數(shù),所以必須使用有些設(shè)計模式來代替，其中涉及到裝飾器模式，和用動態(tài)代理(即我們可以動態(tài)注入類方法)來實現(xiàn)混合，但是結(jié)果都沒有C++中方便直接。
implements和extends關(guān)鍵字實現(xiàn)：
package com.generics; //: generics/Mixins.java import java.util.*;interface TimeStamped { long getStamp(); }class TimeStampedImp implements TimeStamped {private final long timeStamp;public TimeStampedImp() {timeStamp = new Date().getTime();}public long getStamp() { return timeStamp; } }interface SerialNumbered { long getSerialNumber(); }class SerialNumberedImp implements SerialNumbered {private static long counter = 1;private final long serialNumber = counter++;public long getSerialNumber() { return serialNumber; } }interface Basic {public void set(String val);public String get(); }class BasicImp implements Basic {private String value;public void set(String val) { value = val; }public String get() { return value; } } // for Mixin2.java,Timestamped<Serialnumbered<Basic>> mixin = new Timestamped();,mixin can not invoke set() function of Basic,but c++ is capable to do it. //so in java, use implements and extends keywords to realize it. class Mixin extends BasicImp implements TimeStamped, SerialNumbered {//if use this,you must have a instance of response to interfaceprivate TimeStamped timeStamp = new TimeStampedImp();private SerialNumbered serialNumber =new SerialNumberedImp();public long getStamp() { return timeStamp.getStamp(); }public long getSerialNumber() {return serialNumber.getSerialNumber();} }public class Mixins {public static void main(String[] args) {Mixin mixin1 = new Mixin(), mixin2 = new Mixin();mixin1.set("test string 1");mixin2.set("test string 2");System.out.println(mixin1.get() + " " +mixin1.getStamp() + " " + mixin1.getSerialNumber());System.out.println(mixin2.get() + " " +mixin2.getStamp() + " " + mixin2.getSerialNumber());} } /* Output: (Sample) test string 1 1132437151359 1 test string 2 1132437151359 2 *///:~裝飾器模式實現(xiàn)（并沒有完全實現(xiàn)）：
package com.generics.decorator;//: generics/decorator/Decoration.javaimport java.util.*;class Basic {private String value;public void set(String val) { value = val; }public String get() { return value; } }class Decorator extends Basic {protected Basic basic;public Decorator(Basic basic) { this.basic = basic; }public void set(String val) { basic.set(val); }public String get() { return basic.get(); } } class TimeStamped extends Decorator {private final long timeStamp;public TimeStamped(Basic basic) {super(basic);timeStamp = new Date().getTime();}public long getStamp() { return timeStamp; } }class SerialNumbered extends Decorator {private static long counter = 1;private final long serialNumber = counter++;public SerialNumbered(Basic basic) { super(basic); }public long getSerialNumber() { return serialNumber; } } //this is decoration design patternspublic class Decoration {public static void main(String[] args) {TimeStamped t = new TimeStamped(new Basic());// because timestamped extends Basict.set("fasdfa");//realize such as TimeStamped<SerialNumbered<Basic>> mixin1, mixin2TimeStamped t2 = new TimeStamped(new SerialNumbered(new Basic()));//! t2.getSerialNumber(); // Not available, obviouslySerialNumbered s = new SerialNumbered(new Basic());SerialNumbered s2 = new SerialNumbered(new TimeStamped(new Basic()));//! s2.getStamp(); // Not available} } ///:~動態(tài)代理模式實現(xiàn):
package com.generics;//: generics/DynamicProxyMixin.java import java.lang.reflect.*; import java.util.*; import net.mindview.util.*; import static net.mindview.util.Tuple.*;class MixinProxy implements InvocationHandler {Map<String,Object> delegatesByMethod;public MixinProxy(TwoTuple<Object,Class<?>>... pairs) {delegatesByMethod = new HashMap<String,Object>();for(TwoTuple<Object,Class<?>> pair : pairs) {for(Method method : pair.second.getMethods()) {String methodName = method.getName();System.out.println(methodName + "()");// The first interface in the map// implements the method.if (!delegatesByMethod.containsKey(methodName))delegatesByMethod.put(methodName, pair.first);// this is the most important, because this inject all functions of pairs}}} public Object invoke(Object proxy, Method method,Object[] args) throws Throwable {System.out.println("invoke() is invoked"); String methodName = method.getName();Object delegate = delegatesByMethod.get(methodName);return method.invoke(delegate, args);}@SuppressWarnings("unchecked")public static Object newInstance(TwoTuple... pairs) {Class[] interfaces = new Class[pairs.length];for(int i = 0; i < pairs.length; i++) {interfaces[i] = (Class)pairs[i].second;//second represent XXX.class}ClassLoader cl =pairs[0].first.getClass().getClassLoader();return Proxy.newProxyInstance(cl, interfaces, new MixinProxy(pairs));} } public class DynamicProxyMixin {public static void main(String[] args) {Object mixin = MixinProxy.newInstance(tuple(new BasicImp(), Basic.class),tuple(new TimeStampedImp(), TimeStamped.class),tuple(new SerialNumberedImp(),SerialNumbered.class));//Basic b = (Basic)mixin;TimeStamped t = (TimeStamped)mixin;SerialNumbered s = (SerialNumbered)mixin;b.set("Hello");System.out.println(b.get());System.out.println(t.getStamp());System.out.println(s.getSerialNumber());} } /* get() set() getStamp() getSerialNumber() invoke() is invoked invoke() is invoked Hello invoke() is invoked 1489219456567 invoke() is invoked 1 *///:~靜態(tài)類型檢查即在程序沒有運行時就能夠通過檢查源代碼確定類型安全，與動態(tài)類型相對應(yīng)
潛在類型機制即直接可以用模板T，而不用指定該模板屬于哪個基類，比如在C++里面就可以直接定義
template<class T> void perform(T anything) {anything.speak();anything.sit(); }而在java中必須要指明邊界
class Communicate { //must specify the bounds of generic type,but C++ is not necessary public static <T extends Performs> void perform(T performer) { performer.speak(); performer.sit(); } }java對潛在類型機制的補償?shù)囊环N方式是反射，如下
class CommunicateReflectively { //接受一個Object，然后看是哪個類 public static void perform(Object speaker) { Class<?> spkr = speaker.getClass(); try { try { Method speak = spkr.getMethod("speak"); speak.invoke(speaker); } catch(NoSuchMethodException e) { print(speaker + " cannot speak"); } try { Method sit = spkr.getMethod("sit"); sit.invoke(speaker); } catch(NoSuchMethodException e) { print(speaker + " cannot sit"); } } catch(Exception e) { throw new RuntimeException(speaker.toString(), e); } } }15.17中15.17.2與15.17.3,15.17.4沒理解
應(yīng)用于序列的泛型技術(shù)很多都會涉及到Iterable接口
數(shù)組
在java中數(shù)組是一種效率最高的存儲和隨機訪問對象應(yīng)用序列的方式
Comparable接口和Comaprator接口用于排序，jdk中運用策略設(shè)計模式將“保持不變的事物與會發(fā)生改變的事物相分離”，代碼如下：
//Comparable class Student implements Comparable<Student>{private String name;private int age;private float score;public Student(String name, int age, float score) {this.name = name;this.age = age;this.score = score;}public String toString(){return name+"\t\t"+age+"\t\t"+score;}@Overridepublic int compareTo(Student o) {// TODO Auto-generated method stubif(this.score>o.score)//score是private的，為什么能夠直接調(diào)用,這是因為在Student類內(nèi)部return -1;//由高到底排序else if(this.score<o.score)return 1;else{if(this.age>o.age)return 1;//由底到高排序else if(this.age<o.age)return -1;elsereturn 0;}} }public class ComparableDemo01 {public static void main(String[] args) {// TODO Auto-generated method stubStudent stu[]={new Student("zhangsan",20,90.0f),new Student("lisi",22,90.0f),new Student("wangwu",20,99.0f),new Student("sunliu",22,100.0f)};java.util.Arrays.sort(stu);for(Student s:stu){System.out.println(s);}} }//Comparator package edu.sjtu.ist.comutil;import java.util.Comparator;class Student {private String name;private int age;private float score;public Student(String name, int age, float score) {this.name = name;this.age = age;this.score = score;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}public float getScore() {return score;}public void setScore(float score) {this.score = score;}public String toString(){return name+"\t\t"+age+"\t\t"+score;}}class StudentComparator implements Comparator<Student>{@Overridepublic int compare(Student o1, Student o2) {// TODO Auto-generated method stubif(o1.getScore()>o2.getScore())return -1;else if(o1.getScore()<o2.getScore())return 1;else{if(o1.getAge()>o2.getAge())return 1;else if(o1.getAge()<o2.getAge())return -1;else return 0;}}} public class ComparableDemo02 {public static void main(String[] args) {// TODO Auto-generated method stubStudent stu[]={new Student("zhangsan",20,90.0f),new Student("lisi",22,90.0f),new Student("wangwu",20,99.0f),new Student("sunliu",22,100.0f)};java.util.Arrays.sort(stu,new StudentComparator());for(Student s:stu){System.out.println(s);}}}當(dāng)你使用最近的java版本編程時，應(yīng)該優(yōu)先選擇容器而不是數(shù)組，只有在證明性能成為問題時，你才應(yīng)該講程序重構(gòu)為使用數(shù)組
容器源碼解讀
繼承結(jié)構(gòu)代碼如下：
public interface Iterable<T>{...}public interface Collection<E> extends Iterable<E>{...} public interface Set<E> extends Collection<E>{...} public interface SortedSet<E> extends Set<E>{ Comparator<? super E> comparator();} public interface List<E> extends Collection<E>{...}- 抽象類實現(xiàn)接口，可以不用實現(xiàn)其全部的方法即可以篩選一些方法來實現(xiàn)，比如：
java
//in this abstract class, the equals() funtion is not implemented
public abstract class AbstractCollection<E> implements Collection<E> {...}
//next eg:
interface test{
void m();
void f();
}
abstract class test2 implements test{
@Override
public void m() {
// TODO Auto-generated method stub
}
}
?
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