Java 从Set里面取出有序的记录详解及实例
Set里面的记录是无序的，如果想使用Set，然后又想里面的记录是有序的，就可以使用TreeSet，而不是HashSet，在使用TreeSet的时候，里面的元素必须是实现了Comparable接口的，TreeSet在进行排序的时候就是通过比较它们的Comparable接口的实现！
下面是HashSet的无序和TreeSet的有序的比较：
Test类：
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默认是无序的，一般从Set里面拿出来的数据每次的顺序都会是不一样的,如果想里面的顺序一样
* 就使用TreeSet
*/
public static void useHashSet() {
System.out.println("-----------------HashSet Start------------------");
Set<User> set = new HashSet<User>();
for (int i = 0; i < 10; i++) {
User user = new User((i + 1), "uname" + (i + 1), "pswd" + (i + 1));
set.add(user);
}
Iterator<User> iter = set.iterator();
while (iter.hasNext())
System.out.println(iter.next());
System.out.println("------------------HashSet End----------------------");
}
/**
* TreeSet是有序的，TreeSet在给里面的元素排序是通过它们的Comparable接口的实现来比较的，所以，
* 如果里面的对象没有实现Comparable接口，则TreeSet在运行时就会报错，所以如果想从Set里面拿出来的数据是
* 有序的就得使里面的对象实现Comparable接口,User2是实现了Comparable接口的，并对它们的id进行比较，id大
* 的就会排在后面
*/
public static void useTreeSet() {
System.out.println("-----------------TreeSet Start------------------");
Set<User2> set = new TreeSet<User2>();
for (int i = 0; i < 10; i++) {
User2 user = new User2((i + 1), "uname" + (i + 1), "pswd" + (i + 1));
set.add(user);
}
Iterator<User2> iter = set.iterator();
while (iter.hasNext())
System.out.println(iter.next());
System.out.println("------------------TreeSet End----------------------");
}
}
User类：
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类：
public class User2 implements Comparable<User2> {
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
//这里我的实现是按照id进行排序
if (user == null)
return 1;
if (id > user.getId())
return 1;
else if (id == user.getId())
return 0;
else
return -1;
}
}
运行结果：
-----------------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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