011—JAVA中的Collections

Collections：操作集合的工具类
Collection:集合的顶级接口

@Test
    public void test01(){
        ArrayList list = new ArrayList<>();
        //批量新增
        Collections.addAll(list,"A","B","C","D","C");
        System.out.println("list = " + list);
        //返回集合内指定元素的数量
        int count = Collections.frequency(list, "C");
        System.out.println("count = " + count);
        //洗牌
        Collections.shuffle(list);
        System.out.println("list = " + list);
        //集合元素的反转
        Collections.reverse(list);
        list.forEach(System.out::println);
    }
    @Test
    public void test02(){
        ArrayList list = new ArrayList<>();
        Collections.addAll(list, 10,9,3,2,1,23);
        System.out.println(list);         //排序
        Collections.sort(list);
        System.out.println("list = " + list);         //二分查找
        int index = Collections.binarySearch(list, 9);
        System.out.println("index = " + index);     }
    @Test
    public void test03(){
        ArrayList list = new ArrayList<>();
        Collections.addAll(list, 10,9,1,3,12,445,3,-1);
        //找最大值
        Integer max = Collections.max(list);
        System.out.println("max = " + max);
        //找最小值
        Integer min = Collections.min(list);
        System.out.println("min = " + min);         System.out.println(list);         //交换指定位置的值
        Collections.swap(list, 0, 3);
        System.out.println(list);     }
    @Test
    public void test04(){
        ArrayList list = new ArrayList<>();
        list.add("张三");
        list.add("李四");
        list.add("王五");
        ArrayList newList = new ArrayList<>();
        Collections.addAll(newList, "","","");
        目标集合的元素数量>=数据源集合的元素数量
        Collections.copy(newList, list);
        System.out.println("newList = " + newList);
        newList.add("王五");
        System.out.println("newList = " + newList);         替换集合内指定的所有元素
        Collections.replaceAll(newList, "王五", "安琪拉");
        System.out.println(newList);     }
    @Test
    public void test05(){         ArrayList list = new ArrayList<>();
        list.add("张三");
        list.add("李四");
        list.add("王五");
        //可以将线程不安全的集合变为线程安全的集合
        List newList = Collections.synchronizedList(list);     }
    @Test
    public void test06(){         ArrayList list = new ArrayList<>();         list.add("张三");
        list.add("李四");
        list.add("王五");
        //返回不能修改的集合
        List newList = Collections.unmodifiableList(list);
//        newList.add("赵六");
    }
    @Test
    public void test07() {
        ArrayList list = new ArrayList<>();         list.add("张三");
        list.add("李四");
        list.add("王五");         //集合到数组
        Object[] objects = list.toArray();
        List asList = Arrays.asList("张三", "李四", "王五");
        //由数组调用asList()转成集合之后不能在进行操作
//        asList.add("安琪拉");
        System.out.println("asList = " + asList);
    }