java 有序set_Java 从Set里面取出有序的记录详解及实例
Java 從Set里面取出有序的記錄詳解及實例
Set里面的記錄是無序的，如果想使用Set，然后又想里面的記錄是有序的，就可以使用TreeSet，而不是HashSet，在使用TreeSet的時候，里面的元素必須是實現了Comparable接口的，TreeSet在進行排序的時候就是通過比較它們的Comparable接口的實現！
下面是HashSet的無序和TreeSet的有序的比較：
Test類：
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;
public class Test {
public static void main(String args[]) {
useHashSet();
useTreeSet();
}
/**
* Set默認是無序的，一般從Set里面拿出來的數據每次的順序都會是不一樣的,如果想里面的順序一樣
* 就使用TreeSet
*/
public static void useHashSet() {
System.out.println("-----------------HashSet Start------------------");
Set set = new HashSet();
for (int i = 0; i < 10; i++) {
User user = new User((i + 1), "uname" + (i + 1), "pswd" + (i + 1));
set.add(user);
}
Iterator iter = set.iterator();
while (iter.hasNext())
System.out.println(iter.next());
System.out.println("------------------HashSet End----------------------");
}
/**
* TreeSet是有序的，TreeSet在給里面的元素排序是通過它們的Comparable接口的實現來比較的，所以，
* 如果里面的對象沒有實現Comparable接口，則TreeSet在運行時就會報錯，所以如果想從Set里面拿出來的數據是
* 有序的就得使里面的對象實現Comparable接口,User2是實現了Comparable接口的，并對它們的id進行比較，id大
* 的就會排在后面
*/
public static void useTreeSet() {
System.out.println("-----------------TreeSet Start------------------");
Set set = new TreeSet();
for (int i = 0; i < 10; i++) {
User2 user = new User2((i + 1), "uname" + (i + 1), "pswd" + (i + 1));
set.add(user);
}
Iterator iter = set.iterator();
while (iter.hasNext())
System.out.println(iter.next());
System.out.println("------------------TreeSet End----------------------");
}
}
User類：
public class User {
private int id;
private String username;
private String password;
public User() {
}
public User(int id, String username, String password) {
this.id = id;
this.username = username;
this.password = password;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + id;
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
User other = (User) obj;
if (id != other.id)
return false;
return true;
}
@Override
public String toString() {
return " id = " + id + ", \r\n username = " + username
+ ", \r\n password = " + password;
}
}
User2類：
public class User2 implements Comparable {
private int id;
private String username;
private String password;
public User2() {
}
public User2(int id, String username, String password) {
this.id = id;
this.username = username;
this.password = password;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
@Override
public int hashCode() {
final int prime = 31;
int result = 1;
result = prime * result + id;
return result;
}
@Override
public boolean equals(Object obj) {
if (this == obj)
return true;
if (obj == null)
return false;
if (getClass() != obj.getClass())
return false;
User2 other = (User2) obj;
if (id != other.id)
return false;
return true;
}
@Override
public String toString() {
return " id = " + id + ", \r\n username = " + username
+ ", \r\n password = " + password;
}
@Override
public int compareTo(User2 user) {
// TODO Auto-generated method stub
//這里我的實現是按照id進行排序
if (user == null)
return 1;
if (id > user.getId())
return 1;
else if (id == user.getId())
return 0;
else
return -1;
}
}
運行結果：
-----------------HashSet Start------------------
id = 3,
username = uname3,
password = pswd3
id = 4,
username = uname4,
password = pswd4
id = 1,
username = uname1,
password = pswd1
id = 2,
username = uname2,
password = pswd2
id = 7,
username = uname7,
password = pswd7
id = 8,
username = uname8,
password = pswd8
id = 5,
username = uname5,
password = pswd5
id = 6,
username = uname6,
password = pswd6
id = 9,
username = uname9,
password = pswd9
id = 10,
username = uname10,
password = pswd10
------------------HashSet End----------------------
-----------------TreeSet Start------------------
id = 1,
username = uname1,
password = pswd1
id = 2,
username = uname2,
password = pswd2
id = 3,
username = uname3,
password = pswd3
id = 4,
username = uname4,
password = pswd4
id = 5,
username = uname5,
password = pswd5
id = 6,
username = uname6,
password = pswd6
id = 7,
username = uname7,
password = pswd7
id = 8,
username = uname8,
password = pswd8
id = 9,
username = uname9,
password = pswd9
id = 10,
username = uname10,
password = pswd10
------------------TreeSet End----------------------
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