【java开发系列】—— 集合使用方法

一、首先看一下集合的框架图：

由于collection也继承了Iterator和comparable接口，因此我们可以使用Iterator来遍历元素，也可以通过自定义compareTo函数来重新编写自己的排序。

二、代码

1、List

package testCollection;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class testList {
    public static void main(String[] args){
        List list = new ArrayList();
        list.add("test1");
        list.add("test2");
        list.add("test3");
        
        System.out.println("out by for!");
        for(Object o : list){
            System.out.println(o);
        }
        
        System.out.println("out by iterator!");
        Iterator iterator = list.iterator();
        while(iterator.hasNext()){
            String element = (String)iterator.next();
            System.out.println(element);
        }
    }
}

运行结果：

out by for!
test1
test2
test3
out by iterator!
test1
test2
test3

2、Set，如果遇到重复的元素，不会添加

package testCollection;

import java.util.HashSet;
import java.util.Set;

public class testSet {
    public static void main(String[] args){
        Set set = new HashSet();//使用set一般都是用hashset，这个会快一些
        set.add("test1");
        set.add("test2");
        if(set.add("test2")){
            System.out.println("add successful");
        }else{
            System.out.println("add failed");
        }
    }
}

运行结果：

add failed

3、Map

package testCollection;

import java.util.HashMap;
import java.util.Map;
import java.util.Set;

public class testMap {
    public static void main(String[] args){
        Map map = new HashMap();
        
        map.put(1, "test1");
        map.put(2, "test2");
        
        System.out.println("size "+map.size());
        System.out.println(map.get(1));
        
        Set keys = map.keySet();
        for(Object key : keys){
            System.out.println(key);
        }
        
        map.remove(2);
        System.out.println("size "+map.size());
    }
}

运行结果：

size 2
test1
1
2
size 1

4、自定义排序函数
person类，继承Comparable接口，重载compareTo函数

 

package testCollection;

public class Person implements Comparable{
    private String name;
    private int age;
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public Person(String name,int age){
        this.name = name;
        this.age = age;
    }
    @Override
    public int compareTo(Object person) throws ClassCastException {
        if(!(person instanceof Person)){
            throw new ClassCastException("A Person perspected!");
        }
        int age = ((Person)person).getAge();
        return this.age-age;
    }
}

测试类

 

package testCollection;

import java.util.Arrays;

public class testComparable {
    public static void main(String[] args){
        Person[] persons = new Person[4];
        persons[0] = new Person("test1",18);
        persons[1] = new Person("test2",20);
        persons[2] = new Person("test3",15);
        persons[3] = new Person("test4",19);
        
        System.out.println("before sorting!");
        
        for(Person p : persons){
            System.out.println("name: "+p.getName()+" age: "+p.getAge());
        }
        
        System.out.println("after sorting!");
        Arrays.sort(persons);
        for(Person p : persons){
            System.out.println("name: "+p.getName()+" age: "+p.getAge());
        }
    }
}

运行结果：

 

before sorting!
name: test1 age: 18
name: test2 age: 20
name: test3 age: 15
name: test4 age: 19
after sorting!
name: test3 age: 15
name: test1 age: 18
name: test4 age: 19
name: test2 age: 20