【Java基础】Java8特性——Stream API，Optional

小咪咪 2024-03-26 12:22 23阅读 0赞

## Stream ##

Stream关注的数据的运算，与cpu打交道  
集合关注数据的存储，与内存打交道

![在这里插入图片描述][1107c10097f5432bb415b32065101abc.png]

### 实例化 ###

//方式一:容器  
    //顺序流  
    Stream<Integer> stream = list.stream();  
    //并行流  
    Stream<Integer> parallelStream = list.parallelStream();  
      
    //方式二：数组  
    IntStream stream1 = Arrays.stream(new int[]{
        1, 2, 3});  
      
    //方式三：Stream.of  
    Stream<Integer> integerStream = Stream.of(1, 2, 4);  
    //方式四：Stream.iterator 创建无限流  （使用较少）
    Stream.iterate(0,t->t+2).limit(10).forEach(System.out::println);  
      
    Stream.generate(Math::random).limit(10).forEach(System.out::println);

### 中间操作 ###

ArrayList<Integer> list=new ArrayList<>();  
    list.add(1);  
    list.add(9);  
    list.add(3);  
    list.add(5);  
    list.add(2);  
    list.add(2);  
      
    //过滤  
    list.stream().filter(a->a>3).forEach(System.out::println);  
    //截断  
    System.out.println("==========");  
    list.stream().limit(3).forEach(System.out::println);  
    //跳过  
    System.out.println("==========");  
    list.stream().skip(3).forEach(System.out::println);  
    //去重  
    System.out.println("==========");  
    list.stream().distinct().forEach(System.out::println);  
    //映射  
    System.out.println("==========");  
    list.stream().map(a->a+1).forEach(System.out::println);  
    //排序  
    System.out.println("==========");  
    list.stream().sorted().forEach(System.out::println);  
    System.out.println("==========");  
    list.stream().sorted((a,b)->b-a).forEach(System.out::println);

### 终止操作 ###

//所有都满足  
    System.out.println("==========");  
    System.out.println(list.stream().allMatch(a -> a > 3));  
    //至少有一个满足  
    System.out.println("==========");  
    System.out.println(list.stream().anyMatch(a -> a > 3));  
    //没有元素满足  
    System.out.println("==========");  
    System.out.println(list.stream().noneMatch(a -> a > 3));  
    //返回第一个元素  
    System.out.println("==========");  
    System.out.println(list.stream().findFirst());  
    //返回任意一个元素  
    System.out.println("==========");  
    System.out.println(list.stream().findAny());  
    //返回任意一个元素  
    System.out.println("==========");  
    System.out.println(list.stream().count());  
    //返回最值  
    System.out.println("==========");  
    System.out.println(list.stream().max((a,b)->a-b));  
    System.out.println(list.stream().max((Integer::compare)));  
    System.out.println(list.stream().max((a,b)->b-a));  
    //内部迭代  
    System.out.println("==========");  
    list.stream().forEach(System.out::println);  
    //求和  
    System.out.println("==========");  
    System.out.println(list.stream().reduce(0,Integer::sum));  
    //收集  
    System.out.println("==========");  
    list.stream().filter(a->a>3).collect(Collectors.toList()).stream().forEach(System.out::println);

## optional ##

![在这里插入图片描述][0fc86fc1c6fd40b9b85411eaf8032463.png]

![在这里插入图片描述][a79bbf8198b04498a5e1d3f66fa437b8.png]

import java.util.Optional;
    
    public class OP {
        
        public static void main(String[] args) {
        
            //存一个空
            Optional<Object> op1 = Optional.empty();
            System.out.println(op1);//Optional.empty
            //op1.get();//报错，Exception in thread "main" java.util.NoSuchElementException: No value present
            if(!op1.isPresent()){
        
                System.out.println("数据不存在");
            }
            //存一个值，非空
            //get()和of()常搭配使用
            Optional<String> op2 = Optional.of("aaa");
            System.out.println(op2);//Optional[aaa]
    
                System.out.println("数据存在");
                System.out.println(op2.get());//aaa
                System.out.println(op2.get().getClass());//class java.lang.String
    
            //存一个值，可以为空
            Optional<Object> op3 = Optional.ofNullable(null);
            System.out.println(op3);
            if(op2.isPresent()){
        
                System.out.println("数据存在");
                //System.out.println(op3.get());//报错，Exception in thread "main" java.util.NoSuchElementException: No value present
                System.out.println(op3.orElse("bbb"));//bbb
            }
        }
    }

[1107c10097f5432bb415b32065101abc.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/26/8f6a9469ef6c494baca2e1019a6ac905.png
[0fc86fc1c6fd40b9b85411eaf8032463.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/26/2e3a5da265d34be4b15014edeee54393.png
[a79bbf8198b04498a5e1d3f66fa437b8.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/26/49b6e4cc91d447919d24ab7ff2a2c87d.png