JAVA#Collections工具类'学习札记

1.Collections 是一个操作 Set、List 和 Map 等集合的工具类；

2.Method：

reverse(List)：反转 List 中元素的顺序

shuffle(List)：对 List 集合元素进行随机排序

sort(List)：根据元素的自然顺序对指定 List 集合元素按升序排序

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， int)：将指定 list 集合中的 i 处元素和 j 处元素进行交换

int frequency(Collection，Object)：返回指定集合中指定元素的出现次数

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 List 对象的所有旧值

3.e.g.

public class CollectionsClss {
    @Test
    public void MM(){
        List lt=new ArrayList();
        lt.add(23);
        lt.add(24);
        lt.add(168);
        lt.add(188);
        lt.add(996);
        lt.add(580);
        lt.add(574);
        lt.add(23);
        lt.add(23);
        System.out.println(lt);
        System.out.println("$$$$$$$$$$$$$$$$$$$$$$$$");
        Collections.reverse(lt);
        System.out.println(lt);
        System.out.println("$$$$$$$$$$$$$$$$$$$$$$$$");

        Collections.shuffle(lt);
        System.out.println(lt);
        System.out.println("$$$$$$$$$$$$$$$$$$$$$$$$");

        Collections.sort(lt);
        System.out.println(lt);
        System.out.println("$$$$$$$$$$$$$$$$$$$$$$$$");

        Collections.swap(lt, 0, 4);
        System.out.println(lt);
        System.out.println("$$$$$$$$$$$$$$$$$$$$$$$$");

        System.out.println(Collections.max(lt));
        System.out.println(Collections.min(lt));
        System.out.println("$$$$$$$$$$$$$$$$$$$$$$$$");

        int count=Collections.frequency(lt,23);
        System.out.println(count);
        System.out.println("$$$$$$$$$$$$$$$$$$$$$$$$");

        List ltbb= Arrays.asList(new Object[9]);
        Collections.copy(ltbb,lt);
        System.out.println(ltbb);

    }
}

[23, 24, 168, 188, 996, 580, 574, 23, 23]
$$$$$$$$$$$$$$$$$$$$$$$$
[23, 23, 574, 580, 996, 188, 168, 24, 23]
$$$$$$$$$$$$$$$$$$$$$$$$
[23, 996, 188, 23, 23, 168, 574, 24, 580]
$$$$$$$$$$$$$$$$$$$$$$$$
[23, 23, 23, 24, 168, 188, 574, 580, 996]
$$$$$$$$$$$$$$$$$$$$$$$$
[168, 23, 23, 24, 23, 188, 574, 580, 996]
$$$$$$$$$$$$$$$$$$$$$$$$
996
23
$$$$$$$$$$$$$$$$$$$$$$$$
3
$$$$$$$$$$$$$$$$$$$$$$$$
[168, 23, 23, 24, 23, 188, 574, 580, 996]