CollectionUtils 【org.apache.commons.collections】

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Maven依赖

<dependency>
	<groupId>commons-collections</groupId>
	<artifactId>commons-collections</artifactId>
	<version>3.2.1</version>
</dependency>

CollectionUtils 常用 API

// 集合是否为空
boolean isEmpty(Collection coll)
boolean isNotEmpty(Collection coll)

// 集合是否为相等
boolean isEqualCollection(Collection a, Collection b)

// 并集
Collection union(Collection a, Collection b)

// 交集
Collection intersection(Collection a, Collection b)

// 交集的补集（析取）
Collection disjunction(Collection a, Collection b)

// 差集（扣除）
Collection subtract(Collection a, Collection b)

// 不可修改的集合
Collection unmodifiableCollection(Collection collection)

CollectionUtils 常用 API 详解

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
}

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]
}

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)