java.util.Comparator中 compare(T o1, T o2) 函數，其實現決定升序降序。舉例如下：對某個對象的var類例進行排序

int compare(T o1, T o2) {return o1.var - o2.var; }

理解升序、降序的三步走：

1 明確 兩個變量o1 o2表示序列中前 后的兩個變量；位置關系：o1 在前；o2居后；

2 明確返回值表達是否交換位置，正數：交換兩者位置； 負數： 不交換；

3 判斷是降序還是升序。

升序

int compare(T o1, T o2) {return o1.var - o2.var; }

?分析如下：

1 明確o1在前， o2在后；

2 若o1.var-o2.var為正，則交換位置；為負，不交換位置；

3 若上述為正條件成立，則表明o1.var為大數，大數置后，故為升序；

降序

int compare(T o1, T o2) {return o2.var - o1.var; }

?分析如下：

1 明確o1在前， o2在后；

2 若o2.var-o1.var為正，則交換位置；為負，不交換位置；

3 若上述為正條件成立，則表明o2.var為大數，大數置前，故為降序；

默認升序/降序

int compare(T o1, T o2) {return -1;//默認不交換，表示升序return 1;//默認交換，表示降序 }

demo代碼

package bob.util; import java.lang.*; import java.util.*;class Student {int rollno;String name, address;public Student(int rollno, String name, String address) {this.rollno = rollno;this.name = name;this.address = address;}public String toString() {return this.rollno + " " + this.name + " " + this.address;} }//自定義升序 class SortbyrollAsc implements Comparator<Student> {public int compare(Student a, Student b) {return a.rollno - b.rollno;} }//自定義降序 class SortbyrollDsc implements Comparator<Student> {public int compare(Student a, Student b) {return b.rollno - a.rollno;} }//default asc //默認強制降序 class DefaultDsc implements Comparator<Student> {public int compare(Student a, Student b) {return 1;} }//default dsc //默認強制升序 class DefaultAsc implements Comparator<Student> {public int compare(Student a, Student b) {return -1;} }// Main class public class SortDemo {public static void main(String[] args) {ArrayList<Student> ar = new ArrayList<Student>();ar.add(new Student(141, "Mayank", "london"));ar.add(new Student(131, "Anshul", "nyc"));ar.add(new Student(161, "Solanki", "jaipur"));ar.add(new Student(151, "Aggarwal", "Hongkong"));// Display message on console for better readabilitySystem.out.println("初始化順序");// Iterating over entries to print themfor (int i = 0; i < ar.size(); i++)System.out.println(ar.get(i));// Sorting student entries by roll numberCollections.sort(ar, new SortbyrollAsc());// Display message on console for better readabilitySystem.out.println("\n自定義升序排序");// Again iterating over entries to print themfor (int i = 0; i < ar.size(); i++)System.out.println(ar.get(i));// Sorting student entries by roll numberCollections.sort(ar, new SortbyrollDsc());// Display message on console for better readabilitySystem.out.println("\n自定義降序排序");// Again iterating over entries to print themfor (int i = 0; i < ar.size(); i++)System.out.println(ar.get(i));Collections.sort(ar, new DefaultDsc());// Display message on console for better readabilitySystem.out.println("\n默認強制降序排序");// Again iterating over entries to print themfor (int i = 0; i < ar.size(); i++)System.out.println(ar.get(i));Collections.sort(ar, new DefaultAsc());// Display message on console for better readabilitySystem.out.println("\n默認強制升序排序");// Again iterating over entries to print themfor (int i = 0; i < ar.size(); i++)System.out.println(ar.get(i));} }

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