Java中取两个集合的并集、交加和差集

 java如何求两个集合的交集和并集呢？？

其实java的API中已经封装了方法。今天写个简单的例子测试一下：（例子中以java.util.ArrayList为例）

package org.suk;

import java.text.MessageFormat;
import java.util.ArrayList;
import java.util.List;

/**
 * Description: <br/>
 * @version V1.0  by 石冬冬-Heil Hitler on  2017/4/7 13:08
 */
public class Test {
    /**
     * Description: 并集<br>
     * @version V1.0 2017/4/7 13:09  by 石冬冬-Heil Hilter（dd.shi02@zuche.com)
     * @return
     */
    public static void union(){
        List<String> list1 = new ArrayList<String>(){{
            add("A");
            add("B");
            add("C");
        }};
        List<String> list2 = new ArrayList<String>(){{
            add("C");
            add("D");
            add("B");
        }};
        //求出并集
        list1.addAll(list2);
        System.out.println(list1);
    }

    /**
     * Description: 交集<br>
     * @version V1.0 2017/4/7 13:09  by 石冬冬-Heil Hilter（dd.shi02@zuche.com)
     * @return
     */
    public static void intersection(){
        List<String> list1 = new ArrayList<String>(){{
            add("A");
            add("B");
            add("C");
        }};
        List<String> list2 = new ArrayList<String>(){{
            add("C");
            add("D");
            add("B");
        }};
        //求出交集
        list1.retainAll(list2);
        System.out.println(list1);
    }

    /**
     * Description: 差集<br>
     * @version V1.0 2017/4/7 13:09  by 石冬冬-Heil Hilter（dd.shi02@zuche.com)
     * @return
     */
    public static void diff(){
        List<String> list1 = new ArrayList<String>(){{
            add("A");
            add("B");
            add("C");
        }};
        List<String> list2 = new ArrayList<String>(){{
            add("C");
            add("D");
            add("B");
        }};
        //求出差集
        list1.removeAll(list2);
        System.out.println(list1);
    }
    /**
     * Description: 对象集合的交集<br>
     * @version V1.0 2017/4/7 13:21  by 石冬冬-Heil Hilter（dd.shi02@zuche.com)
     * @param null
     * @return
     */
    public static void test(){
        List<Student> list1 = new ArrayList<Student>(){{
            add(new Student("Linda",20));
            add(new Student("Bruce",20));
            add(new Student("Linda",21));
        }};
        List<Student> list2 = new ArrayList<Student>(){{
            add(new Student("Linda",21));
            add(new Student("Bruce",22));
            add(new Student("Jack",21));
        }};
        //求出交集
        list1.retainAll(list2);
        System.out.println(list1);
    }

    public static void main(String[] args) {
        union();
        intersection();
        diff();
        test();
    }
    /**
     * Description: 内部学生类<br>
     * @version V1.0 2017/4/7 13:20  by 石冬冬-Heil Hilter（dd.shi02@zuche.com)
     * @param null
     * @return 
     */
    public static class Student{
        private String name;
        private int age;

        public Student(String name, int age) {
            this.name = name;
            this.age = 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;
        }

        @Override
        public int hashCode() {
            return (this.getName() + String.valueOf(this.getAge())).hashCode();
        }

        @Override
        public boolean equals(Object obj) {
            if(null == obj)
                return false;
            if(obj instanceof Student){
                Student other = (Student)obj;
                return this.getName().equals(other.getName()) && this.getAge() == other.getAge();
            }
            return false;
        }

        @Override
        public String toString() {
            return MessageFormat.format("[name:{0},age:{1}]",this.getName(),this.getAge());
        }
    }
}

输出的结果依次是：

[A, B, C, C, D, B]
[B, C]
[A]
[[name:Linda,age:21]]

最后

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