Java 集合运算

Java 集合运算（交集、并集和差集）

参考：https://blog.csdn.net/lsp1991/article/details/46931151

1、用法

[List obj1].retainAll([List obj2]); //交集
[List obj1].addAll([List obj2]); //并集
[List obj1].removeAll([List obj2]); //差集,obj1-obj2

集合运算的返回值是一个集合
集合运算时一般先把目标集合拷贝备份，不然运算后集合元素将被更改，如：

private List union(List l1,List l2) {
        List list =new ArrayList(Arrays.asList(new Object[l1.size()]));
        Collections.copy(list, l1); //将l1拷贝到list中
        list.addAll(l2);

        return list;

    }

在并集运算中，使用 [List obj].addAll([List obj])的方法得到的结果是两个集合中的所有元素，有可能有重复的元素，跟数学上意义上的并集有所差别，欲求数学意义上的并集则需进行转化(因此，一般将元素存储到HashSet或TreeSet中，能保证集合运算结果中元素的唯一性)，例如：

        /*实际的集合运算结果*/
        //print the union of the two list
        System.out.println("\n");
        System.out.println("Union: ");

        list=opt.union(l1, l2);
        for(i=0;i<list.size();i++) {
            System.out.print(list.get(i)+" ");
        }

        /*运算结果
            Union: 
            1 2 3 4 6 8 2 4 7 9
        */

转换成数学意义上的并集结果

/*转换后变成数学意义上的运算结果*/
//print the union of the two list
        System.out.println("\n");
        System.out.println("Union: ");

        list=opt.union(l1, l2);
        for(i=0;i<list.size();i++) {
            System.out.print(list.get(i)+" ");
        }

                //The real union of the two list....
                System.out.println("\n");
                HashSet unionlist=new HashSet();
                for(i=0;i<list.size();i++) {
                    unionlist.add(list.get(i));
                }

                System.out.println("The real union of the two list is : ");
                Iterator ite=unionlist.iterator();
                while(ite.hasNext()) {
                    System.out.print(ite.next()+" ");
                }

/*
Union: 
1 2 3 4 6 8 2 4 7 9 

The real union of the two list is : 
1 2 3 4 6 7 8 9 
*/

*注：

split() 方法根据匹配给定的正则表达式来拆分字符串。
注意： . 、 | 和 * 等转义字符，必须得加 \ 。
注意：多个分隔符，可以用 | 作为连字符。

2、集合运算实例：

import java.util.*;
public class setOpt {

    //The operation of Intersect
    private List intersetct(List l1,List l2) {
        List list=new ArrayList(Arrays.asList(new Object[l1.size()]));
        Collections.copy(list, l1);
        list.retainAll(l2);

        return list;
    }

    //The operation of Union
    private List union(List l1,List l2) {
        List list =new ArrayList(Arrays.asList(new Object[l1.size()]));
        Collections.copy(list, l1);
        list.addAll(l2);

        return list;

    }

    //The operation of Diff
    private List diff(List l1,List l2) {
        List list =new ArrayList(Arrays.asList(new Object[l1.size()]));
        Collections.copy(list, l1);
        list.removeAll(l2);

        return list;
    }

    public static void main(String[] args) {

        String str1[]="1;2;3;4;6;8".split(";");
        String str2[]="2;4;7;9".split(";");
        int i,j;

        setOpt opt=new setOpt();

        List l1=new ArrayList();
        List l2=new ArrayList();
        List list;

        for(i=0;i<str1.length;i++) {
            l1.add(str1[i]);
        }

        for(j=0;j<str2.length;j++) {
            l2.add(str2[j]);
        }

        //print the intersection of the two list
        System.out.println("Intersection: ");
        list=opt.intersetct(l1, l2);
        for(i=0;i<list.size();i++) {
            System.out.print(list.get(i)+" ");
        }

        //print the union of the two list
        System.out.println("\n");
        System.out.println("Union: ");

        list=opt.union(l1, l2);
        for(i=0;i<list.size();i++) {
            System.out.print(list.get(i)+" ");
        }

                //The real union of the two list....
                System.out.println("\n");
                HashSet unionlist=new HashSet();
                for(i=0;i<list.size();i++) {
                    unionlist.add(list.get(i));
                }

                System.out.println("The real union of the two list is : ");
                Iterator ite=unionlist.iterator();
                while(ite.hasNext()) {
                    System.out.print(ite.next()+" ");
                }


        //print the diff of the two list
        System.out.println("\n");
        System.out.println("Diff: ");
        list=opt.diff(l1, l2);
        for(i=0;i<list.size();i++) {
            System.out.print(list.get(i)+" ");
        }



    }

}

/*运行结果：
Intersection: 
2 4 

Union: 
1 2 3 4 6 8 2 4 7 9 

The real union of the two list is : 
1 2 3 4 6 7 8 9 

Diff: 
1 3 6 8 
*/

附：
蓝桥杯集合运算算法题（TreeSet存储元素的唯一性求解法）：
https://blog.csdn.net/CR_fun/article/details/79294541

Java 集合运算（交集、并集和差集）

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