十一(三)、集合之Collections工具类

一、概述：

• Collections是一个操作Set List Map的工具类
• 提供了一系列静态的方法对集合元素进行排序、查询和修改等操作，
• 还提供了对计划对象设置不可变、对计划对象实现同步控制等方法

二、Collections常用方法

1、排序方法：（均为Static）

• reverse(List),反转List中元素的顺序；
• shuffle(List):对List重的元素随机排序
• sort(List) 根据元素的自然顺序对指定的List集合元素升序排序；
• sort(List,Comparator):根据指定的Comparator产生的顺序对List集合元素进行排序
• Swap(List,int i,int j)将指定List集合中的i处元素和j处元素进行交换；

2、取最值方法： 

 

Object max(Collection):根据元素的自然顺、返回给定集合中最大元素；

Object max(Collection,Comparator):根据Comparator指定的顺序，返回给定集合中最大的元素

Object min(Collection)

Object min(Collection,Comparator):

3、其他方法

• int frequnency(Collection,Object),返回集合中元素出现的次数；
• void copy(List dest,List src),将src中的内容复制到dest中
• boolean replaceAll(List list,Object oldVal,Object newVal)使用新值，替换老的值
• Collections 类中提供了多个synchronizedXxx()方法；

　　　　该方法可以使得指定集合包装成线程同步的集合，从而可以解决多线程并发访问集合是的线程安全问题

4、测试方法代码：

@Test
    public void test() {
        ArrayList arrayList = new ArrayList();
        arrayList.add(123);
        arrayList.add(456);
        arrayList.add("AA");
        arrayList.add(new Person("Tom",1));
        arrayList.add(456);
        System.out.println(arrayList.toString());
        
        Collections.reverse(arrayList);
        System.out.println("Collections的 reverse方法"+arrayList);
        
        
    }
    @Test
    public void test1() {
        ArrayList arrayList = new ArrayList();
        arrayList.add(123);
        arrayList.add(456);
        arrayList.add("AA");
        arrayList.add(new Person("Tom",1));
        arrayList.add(456);
        System.out.println(arrayList.toString());
        
        Collections.shuffle(arrayList);
        System.out.println("Collections的 shuffle方法"+arrayList);
        
        
    }
    
    @Test
    public void test2() {
        ArrayList arrayList = new ArrayList();
        arrayList.add(new Person("Tom",12));
        arrayList.add(new Person("joy",13));
        arrayList.add(new Person("jerry",15));
        arrayList.add(new Person("lucy",1));
        System.out.println(arrayList.toString());
        
        Collections.sort(arrayList);
        System.out.println("Collections的 sort的自然排序方法"+arrayList);
        
        Collections.sort(arrayList,new Comparator<Person>() {

            @Override
            public int compare(Person o1, Person o2) {
                if(o1.getAge()-o2.getAge()>1) {
                    return 1;
                }
                return -1;
            }
        });
        System.out.println("Collections的 sort和 comparator方法"+arrayList);
        
        
    }
    
    @Test
    public void test3() {
        ArrayList arrayList = new ArrayList();
        arrayList.add(new Person("Tom",12));
        arrayList.add(new Person("joy",13));
        arrayList.add(new Person("jerry",15));
        arrayList.add(new Person("lucy",1));
        System.out.println(arrayList.toString());
        Collections.swap(arrayList, 0, 1);
        System.out.println("Collections的 swap方法"+arrayList);
    }
    
    @Test
    public void test4() {
        ArrayList arrayList = new ArrayList();
        arrayList.add(new Person("Tom",12));
        arrayList.add(new Person("joy",13));
        arrayList.add(new Person("jerry",15));
        arrayList.add(new Person("lucy",1));
        System.out.println(arrayList.toString());
        Person person = Collections.max(arrayList,new Comparator<Person>() {

            @Override
            public int compare(Person o1, Person o2) {
                if(o1.getAge()-o2.getAge()>1) {
                    return 1;
                }
                return -1;            }
        });
        System.out.println("Collections的 max方法"+person);
    }
    
    @Test
    public void test5() {
        ArrayList arrayList = new ArrayList();
        arrayList.add(new Person("Tom",12));
        arrayList.add(new Person("joy",13));
        arrayList.add(new Person("jerry",15));
        arrayList.add(new Person("lucy",1));
        arrayList.add(new Person("lucy",1));
        System.out.println(arrayList.toString());
        int times = Collections.frequency(arrayList, new Person("lucy",1));
        System.out.println("Collections的 frequency方法"+times);
    }
    
    @Test
    public void test6() {
        ArrayList arrayList = new ArrayList();
        arrayList.add(new Person("Tom",12));
        arrayList.add(new Person("joy",13));
        arrayList.add(new Person("jerry",15));
        arrayList.add(new Person("lucy",1));
        arrayList.add(new Person("lucy",1));
        
        //错误
//        ArrayList arrayList1 = new ArrayList();
//        Collections.copy(arrayList1, arrayList);;
        List arrayList1 = Arrays.asList(new Object[arrayList.size()]);
        Collections.copy(arrayList1, arrayList);;
        System.out.println("Collections的 copy方法"+arrayList1);
    }
    
    @Test
    public void test7() {
        ArrayList arrayList = new ArrayList();
        arrayList.add(new Person("Tom",12));
        arrayList.add(new Person("joy",13));
        arrayList.add(new Person("jerry",15));
        arrayList.add(new Person("lucy",1));
        arrayList.add(new Person("lucy",1));
        
        Collections.replaceAll(arrayList, new Person("lucy",1), new Person("lucy",3));
        System.out.println("Collections的 replaceAll方法"+arrayList);
    }
    
    @Test
    public void test8() {
        ArrayList arrayList = new ArrayList();
        arrayList.add(new Person("Tom",12));
        arrayList.add(new Person("joy",13));
        arrayList.add(new Person("jerry",15));
        arrayList.add(new Person("lucy",1));
        arrayList.add(new Person("lucy",1));
        
        //返回线程安全的 list
        List list = Collections.synchronizedList(arrayList);
        System.out.println("Collections的 synchronizedList方法"+list);
    }