一、Collections

提供了对容器操作的工具方法，与 Arrays 使用差不多。

1、常用方法

List
   
     list = new ArrayList
    
     ();	list.add(50);	list.add(33);	list.add(52);	list.add(45);	System.out.println(list);		Collections.sort(list);		// 排序、默认按照升序排序	System.out.println(list);		System.out.println(Collections.binarySearch(list, 50));	// 二分查找（前提必须是有序的）		Collections.reverse(list);	// 反转	System.out.println(list);		Collections.shuffle(list);	// 乱序	System.out.println(list);		Collections.fill(list, 0);	// 填充，填充为指定值	System.out.println(list);

运行结果
[50, 33, 52, 45] [33, 45, 50, 52] 2 [52, 50, 45, 33] [33, 45, 52, 50] [0, 0, 0, 0]

二、Comparable

通过实现Comparable接口进行排序

public class Student implements Comparable
   
    {
   	private String name;	private int age;	public Student() {
   		super();	}	public Student(String name, int age) {
   		super();		this.name = name;		this.age = age;	}	public String getName() {
   		return name;	}	public void setName(String name) {
   		this.name = name;	}	public int getAge() {
   		return age;	}	public void setAge(int age) {
   		this.age = age;	}	@Override	public int hashCode() {
   		final int prime = 31;		int result = 1;		result = prime * result + age;		result = prime * result + ((name == null) ? 0 : name.hashCode());		return result;	}	@Override	public boolean equals(Object obj) {
   		if (this == obj)			return true;		if (obj == null)			return false;		if (getClass() != obj.getClass())			return false;		Student other = (Student) obj;		if (age != other.age)			return false;		if (name == null) {
   			if (other.name != null)				return false;		} else if (!name.equals(other.name))			return false;		return true;	}	@Override	public String toString() {
   		return "Student [name=" + name + ", age=" + age + "]";	}	@Override	public int compareTo(Student o) {
   		// TODO Auto-generated method stub		// 0 不变    负数小于 正数大于		return this.age-o.getAge();	}}

public class Test2 {
   	public static void main(String[] args) {
   		List
   
     list = new ArrayList
    
     ();		list.add(new Student("张三",20));		list.add(new Student("李四",22));		list.add(new Student("王二",24));		list.add(new Student("麻子",19));		Collections.sort(list);		// 排序		for(Student s:list) {
   			System.out.println(s);		}	}}

运行接口
Student [name=麻子, age=19] Student [name=张三, age=20] Student [name=李四, age=22] Student [name=王二, age=24]

三、Comparator

Comparator比较器，可以根据需要定制特定的比较规则。

public class Test3 {
   	public static void main(String[] args) {
   		List
   
     list = new ArrayList
    
     ();		list.add(50);		list.add(33);		list.add(52);		list.add(45);		System.out.println(list);		// Comparator 内部类排序		Collections.sort(list, new Comparator
     
      () {
   			@Override			public int compare(Integer o1, Integer o2) {
   				// TODO Auto-generated method stub				return o1.compareTo(o2);		// 使用Integer提供的比较规则			}		});		System.out.println(list);		System.out.println("------------");		Collections.shuffle(list);		// 随机排序		System.out.println(list);		System.out.println("------------");		Collections.sort(list,(o1,o2)->o1-o2);		// lambda排序		System.out.println(list);		System.out.println("------------");		Collections.sort(list,Test3::a1);		System.out.println(list);		}	static int a1(int o1,int o2) {
   		return -(o1-o2);	}}

运行结果
[50, 33, 52, 45] [33, 45, 50, 52] ------------ [52, 45, 33, 50] ------------ [33, 45, 50, 52] ------------ [52, 50, 45, 33]

四、Stream

1、创建源

public class Test4 {
   	public static void main(String[] args) {
   		List
   
     list = new ArrayList
    
     ();		list.add(50);		list.add(33);		list.add(52);		list.add(45);				// 方式一		Stream
     
       stream1 = list.stream();				// 方式二		Integer[] ints = {
   1,2,3,4,5,6,7,8,9};		Stream
      
        stream2 = Arrays.stream(ints);				// 方式三		Stream
       
         stream3 = Stream.of(1,2,3);		stream3 =  Stream.of(ints);	}}

2、中间操作

2.1、切片和筛选

public class Test5 {
   	public static void main(String[] args) {
   		List
   
     list = new ArrayList
    
     ();		list.add(new Student("张三",20));		list.add(new Student("李四",22));		list.add(new Student("王二",24));		list.add(new Student("麻子",19));		list.add(new Student("麻子",19));				// 过滤，年龄大于20的		list.stream().filter(x->x.getAge()>20).forEach(System.out::println);		System.out.println("---------");		// 截取元素，获取前两个		list.stream().limit(2).forEach(System.out::println);		System.out.println("---------");		// 跳过元素，敲过前两个		list.stream().skip(2).forEach(System.out::println);		System.out.println("---------");		// 筛选去重，去掉重复的元素		list.stream().distinct().forEach(System.out::println);	}}

运行结果
Student [name=李四, age=22] Student [name=王二, age=24] --------- Student [name=张三, age=20] Student [name=李四, age=22] --------- Student [name=王二, age=24] Student [name=麻子, age=19] Student [name=麻子, age=19] --------- Student [name=张三, age=20] Student [name=李四, age=22] Student [name=王二, age=24] Student [name=麻子, age=19]

2.2、排序

public class Test6 {
   	public static void main(String[] args) {
   		List
   
     list = new ArrayList
    
     ();		list.add("aa");		list.add("cc");		list.add("bb");		list.add("dd");				// 自然排序		list.stream().sorted().forEach(System.out::println);		System.out.println();		// 定制排序		list.stream().sorted((o1,o2)->o1.compareTo(o2)).forEach(System.out::println);;	}}

运行结果
aa bb cc dd aa bb cc dd

3、终止

3.1、查找匹配

public class Test7 {
   	public static void main(String[] args) {
   		List
   
     list = new ArrayList
    
     ();		list.add(50);		list.add(33);		list.add(52);		list.add(45);				// 是否匹配所有元素		boolean flag = list.stream().allMatch(x->x>30);		System.out.println(flag);				// 至少匹配一个元素		flag = list.stream().anyMatch(x->x>50);		System.out.println(flag);				// 没有匹配所有元素		flag = list.stream().noneMatch(x->x>50);		System.out.println(flag);				// 返回第一个元素		Optional
     
       opt = list.stream().findFirst();		System.out.println(opt);				// 返回任意一个元素		opt = list.stream().findAny();		System.out.println(opt);				// 返回流中的元素		System.out.println(list.stream().count());				// 返回流中的最大值		opt = list.stream().max((x,y)->x-y);		System.out.println(opt);				// 返回流中的阿最小值		opt= list.stream().min((x,y)->x-y);		System.out.println(opt);			}}

运行结果
true true false Optional[50] Optional[50] 4 Optional[52] Optional[33]

3.2、收集

public class Test8 {
   	public static void main(String[] args) {
   		List
   
     list = new ArrayList
    
     ();		list.add(50);		list.add(33);		list.add(52);		list.add(45);//		list.add(45);				// 返回一个List		List
     
       list1 = list.stream().limit(2).collect(Collectors.toList());		System.out.println(list1);				// 返回Set，去除重复		Set
      
        set = list.stream().collect(Collectors.toSet());		System.out.println(set);				// 返回Map		Map
       
         map = list.stream().collect(Collectors.toMap((x)->{
   return x;}, (x)->{
   return x;}));		System.out.println(map);	}}

运行结果
[50, 33] [33, 50, 52, 45] {33=33, 50=50, 52=52, 45=45}

public class Test10 {
   	public static void main(String[] args) {
   		List
   
     list = new ArrayList
    
     ();		list.add(50);		list.add(33);		list.add(52);		list.add(45);				System.out.println(list.stream().collect(Collectors.counting()));	// 求总数		System.out.println(list.stream().collect(Collectors.averagingInt(x->x)));	// 求平均值		System.out.println(list.stream().collect(Collectors.summingInt(x->x)));	// 求总和		System.out.println(list.stream().collect(Collectors.maxBy((x,y)->Integer.compare(x, y))));	// 求最大值	}}