java常用的工具类 apache --- CollectionUtils(IT枫斗者)

今天药忘吃喽~ 2024-03-22 09:44 29阅读 0赞

## java常用的工具类—CollectionUtils(IT枫斗者) ##

**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。**

*  判断集合是否为空:
 *      CollectionUtils.isEmpty(null): true
        CollectionUtils.isEmpty(new ArrayList()): true　　
        CollectionUtils.isEmpty({a,b}): false
 *  判断集合是否不为空:
 *      CollectionUtils.isNotEmpty(null): false
        CollectionUtils.isNotEmpty(new ArrayList()): false
        CollectionUtils.isNotEmpty({a,b}): true
 *  并集
 *      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
 *      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）
 *      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）
 *      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
 *      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
 *      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)