Java 从Set里面取出有序的记录详解及实例
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2023-12-16 09:37:28
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...
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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