取得集合list的交集并集（javaAPI或者apache的API）

javaAPI方式----------->

/**
 * 求ls对ls2的差集,即ls中有，但ls2中没有的
 *
 * @param ls
 * @param ls2
 * @return
 */
public static List diff(List ls, List ls2) {
    List list = new ArrayList(Arrays.asList(new Object[ls.size()]));
    Collections.copy(list, ls);
    list.removeAll(ls2);
    return list;
}

/**
 * 求2个集合的交集
 *
 * @param ls
 * @param ls2
 * @return
 */
public static List intersect(List ls, List ls2) {
    List list = new ArrayList(Arrays.asList(new Object[ls.size()]));
    Collections.copy(list, ls);
    list.retainAll(ls2);
    return list;
}

/**
 * 求2个集合的并集
 *
 * @param ls
 * @param ls2
 * @return
 */
public static List union(List ls, List ls2) {
    List list = new ArrayList(Arrays.asList(new Object[ls.size()]));
    Collections.copy(list, ls);//将ls的值拷贝一份到list中
    list.removeAll(ls2);
    list.addAll(ls2);
    return list;
}

apacheAPI方式---------->

使用 CollectionUtils 中四个方法之一执行集合操作.这四种分别是 union(),intersection();disjunction(); subtract(); 下列例子就是演示了如何使用上述四个方法处理两个 Collection; 注: 这些方法都是数学的集合算法

import  java.util.Arrays;
import  java.util.Collection;
import  java.util.Collections;
import  java.util.List;

import  org.apache.commons.collections.CollectionUtils;
import  org.apache.commons.lang.ArrayUtils;

public  class  CollectionUtilsIntro {
 @SuppressWarnings ( "unchecked" )
 public  static  void  main(String[] args) {
  String[] arrayA =  new  String[] {  "1" ,  "2" ,  "3" ,  "3" ,  "4" ,  "5"  };
  String[] arrayB =  new  String[] {  "3" ,  "4" ,  "4" ,  "5" ,  "6" ,  "7"  };

  List a = Arrays.asList(arrayA);
  List b = Arrays.asList(arrayB);
  //并集
  Collection union = CollectionUtils.union(a, b);
  //交集
  Collection intersection = CollectionUtils.intersection(a, b);
  //交集的补集
  Collection disjunction = CollectionUtils.disjunction(a, b);
  //集合相减
  Collection subtract = CollectionUtils.subtract(a, b);

  Collections.sort((List) union);
  Collections.sort((List) intersection);
  Collections.sort((List) disjunction);
  Collections.sort((List) subtract);

  System.out.println( "A: "  + ArrayUtils.toString(a.toArray()));
  System.out.println( "B: "  + ArrayUtils.toString(b.toArray()));
  System.out.println( "--------------------------------------------" );
  System.out.println( "Union(A, B): "  + ArrayUtils.toString(union.toArray()));
  System.out.println( "Intersection(A, B): "  + ArrayUtils.toString(intersection.toArray()));
  System.out.println( "Disjunction(A, B): "  + ArrayUtils.toString(disjunction.toArray()));
  System.out.println( "Subtract(A, B): "  + ArrayUtils.toString(subtract.toArray()));
 }
}

输出如下: A: {1,2,3,3,4,5} B: {3,4,4,5,6,7} -------------------------------------------- Union(A, B): {1,2,3,3,4,4,5,6,7} Intersection(A, B): {3,4,5} Disjunction(A, B): {1,2,3,4,6,7} Subtract(A, B): {1,2,3}