CollectionUtils提供很多对集合的操作方法，常用的方法如下：

//并集  
Collection<String> union = CollectionUtils.union(a, b);  
//交集  
Collection<String> intersection = CollectionUtils.intersection(a, b);  
//交集的补集  
Collection<String> disjunction = CollectionUtils.disjunction(a, b);  
//集合相减 
Collection<String> subtract = CollectionUtils.subtract(a, b);  
//判断集合是否为空
Boolean boolean = CollectionUtils.isEmpty(Collection<?> collection);
//判断集合是否不为空
Boolean boolean = CollectionUtils.isNotEmpty(Collection<?> collection);

并集

@Test
public void testUnion(){
    String[] arrayA = new String[] { "A", "B", "C", "D", "E", "F" };  
    String[] arrayB = new String[] { "B", "D", "F", "G", "H", "K" };
    List<String> listA = Arrays.asList(arrayA);
    List<String> listB = Arrays.asList(arrayB);
    //2个数组取并集 
    System.out.println(ArrayUtils.toString(CollectionUtils.union(listA, listB)));
    //[A, B, C, D, E, F, G, H, K]
}

交集

@Test
public void testIntersection(){
    String[] arrayA = new String[] { "A", "B", "C", "D", "E", "F" };  
    String[] arrayB = new String[] { "B", "D", "F", "G", "H", "K" };
    List<String> listA = Arrays.asList(arrayA);
    List<String> listB = Arrays.asList(arrayB);
    //2个数组取交集 
    System.out.println(ArrayUtils.toString(CollectionUtils.intersection(listA, listB)));
    //[B, D, F]
}

交集的补集（析取）

@Test
public void testDisjunction(){
    String[] arrayA = new String[] { "A", "B", "C", "D", "E", "F" };  
    String[] arrayB = new String[] { "B", "D", "F", "G", "H", "K" };
    List<String> listA = Arrays.asList(arrayA);
    List<String> listB = Arrays.asList(arrayB);
    //2个数组取交集 的补集
    System.out.println(ArrayUtils.toString(CollectionUtils.disjunction(listA, listB)));
    //[A, C, E, G, H, K]
}

差集（扣除）

@Test
public void testSubtract(){
    String[] arrayA = new String[] { "A", "B", "C", "D", "E", "F" };  
    String[] arrayB = new String[] { "B", "D", "F", "G", "H", "K" };
    List<String> listA = Arrays.asList(arrayA);
    List<String> listB = Arrays.asList(arrayB);
    //arrayA扣除arrayB
    System.out.println(ArrayUtils.toString(CollectionUtils.subtract(listA, listB)));
    //[A, C, E]
}

集合是否为空

@Test
public void testIsEmpty(){

    class Person{}
    class Girl extends Person{}

    List<Integer> first = new ArrayList<>();
    List<Integer> second = null;
    List<Person> boy = new ArrayList<>();
    //每个男孩心里都装着一个女孩
    boy.add(new Girl());
    //判断集合是否为空
    System.out.println(CollectionUtils.isEmpty(first));   //true
    System.out.println(CollectionUtils.isEmpty(second));   //true
    System.out.println(CollectionUtils.isEmpty(boy));   //false

    //判断集合是否不为空
    System.out.println(CollectionUtils.isNotEmpty(first));   //false
    System.out.println(CollectionUtils.isNotEmpty(second));   //false
    System.out.println(CollectionUtils.isNotEmpty(boy));   //true
}

集合是否相等

@Test
public void testIsEqual(){

    class Person{}
    class Girl extends Person{
    }

    List<Integer> first = new ArrayList<>();
    List<Integer> second = new ArrayList<>();
    first.add(1);
    first.add(2);
    second.add(2);
    second.add(1);
    Girl goldGirl = new Girl();
    List<Person> boy1 = new ArrayList<>();
    //每个男孩心里都装着一个女孩
    boy1.add(new Girl());
    List<Person> boy2 = new ArrayList<>();
    //每个男孩心里都装着一个女孩
    boy2.add(new Girl());
    //比较两集合值
    System.out.println(CollectionUtils.isEqualCollection(first,second));   //true
    System.out.println(CollectionUtils.isEqualCollection(first,boy1));   //false
    System.out.println(CollectionUtils.isEqualCollection(boy1,boy2));   //false

    List<Person> boy3 = new ArrayList<>();
    //每个男孩心里都装着一个女孩
    boy3.add(goldGirl);
    List<Person> boy4 = new ArrayList<>();
    boy4.add(goldGirl);
    System.out.println(CollectionUtils.isEqualCollection(boy3,boy4));   //true
}

不可修改的集合

我们对c进行操作，s也同样获得了和c相同的内容，这样就可以避免其他人员修改这个s对象。有时候需要对它进行保护，避免返回结果被人修改。

@Test
public void testUnmodifiableCollection(){
    Collection<String> c = new ArrayList<>();
    Collection<String> s = CollectionUtils.unmodifiableCollection(c);
    c.add("boy");
    c.add("love");
    c.add("girl");
    //! s.add("have a error");
    System.out.println(s);
}

Collections.unmodifiableCollection可以得到一个集合的镜像，它的返回结果是不可直接被改变，否则会提示错误

java.lang.UnsupportedOperationException
at org.apache.commons.collections.collection.UnmodifiableCollection.add(UnmodifiableCollection.java:75)
赏

YOLO