Java 集合运算（并集、交集与差集）

快来打我* 2022-05-24 12:47 280阅读 0赞

**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 */

说明：

*  split() 方法根据匹配给定的正则表达式来拆分字符串。
 *  `.` `、` `|` 和 `*` 等转义字符，必须得加 \\ ；
 *  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][https_blog.csdn.net_CR_fun_article_details_79294541]

[https_blog.csdn.net_CR_fun_article_details_79294541]: https://blog.csdn.net/CR_fun/article/details/79294541