CollectionUtils工具类的使用

CollectionUtils在真实项目中，是一个非常好用的工具类，使用非常频繁。它可以使代码更加简洁和安全。刚好在工作中利用这个工具类重构代码，顺便总结下分享分享:

并集

@Testpublic 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

@Testpublic 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）

@Testpublic 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）

@Testpublic 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

@Testpublic 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

@Testpublic 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。

@Testpublic 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.UnsupportedOperationExceptionat org.apache.commons.collections.collection.UnmodifiableCollection.add(UnmodifiableCollection.java:75)