Java 從Set里面取出有序的記錄詳解及實(shí)例
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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(shí)現(xiàn)了Comparable接口的，TreeSet在進(jìn)行排序的時候就是通過比較它們的Comparable接口的實(shí)現(xiàn)！
下面是HashSet的無序和TreeSet的有序的比較：
Test類：
重慶分公司
重慶分公司
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(rèn)是無序的，一般從Set里面拿出來的數(shù)據(jù)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(shí)現(xiàn)來比較的，所以，
* 如果里面的對象沒有實(shí)現(xiàn)Comparable接口，則TreeSet在運(yùn)行時就會報錯，所以如果想從Set里面拿出來的數(shù)據(jù)是
* 有序的就得使里面的對象實(shí)現(xiàn)Comparable接口,User2是實(shí)現(xiàn)了Comparable接口的，并對它們的id進(jìn)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(shí)現(xiàn)是按照id進(jìn)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(yùn)行結(jié)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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