Java详细分析Lambda表达式与Stream流的使用方法

ArrayList<String> list = new ArrayList<>();
list.add("a");
list.add("b");
list.add("c");
list.add("d");
list.sort(new Comparator<String>() {
    @Override
    public int compare(String o1, String o2) {
        return o2.compareTo(o1);
    }
});
System.out.println(list);

// Lambda表达式
list.sort((o1,o2) -> {return o2.compareTo(o1);});

Integer[] arr = new Integer[]{1,4,3,2,5,5};
Arrays.stream(arr)    //拿到流
    .filter((a) -> {return a>3;})  //中间操作,过滤
    .forEach((a) -> {   //终端操作,遍历
        System.out.println(a);
    });
Integer[] arr = new Integer[]{1,4,3,2,5,5};
Object[] objects = Arrays.stream(arr)
    .sorted().distinct() //排序并去重
    .toArray();  //转数组
System.out.println(Arrays.toString(objects));
Integer[] arr = new Integer[]{1,4,3,2,5,5};
Integer max = Arrays.stream(arr).distinct()
    .max(((o1, o2) -> {    //去重返回最大值
        return o1 - o2;
    })).get();  //拿到这个值
//此处max为终端操作,返回的已经不是流,而是一个OPtion的对象
//它里面有一个get()方法可以返回这个值
System.out.println(max);
Integer[] arr = new Integer[]{1,4,3,2,5,5};
long count = Arrays.stream(arr).distinct()
    .count();   //返回总个数
System.out.println(count);
Integer[] arr = new Integer[]{1,4,3,2,5,5};
Integer i = Arrays.stream(arr).distinct()
    .reduce((o1, o2) -> {  //所有元素求和
        return o1 + o2;
    })
    .get();
System.out.println(i);

public class Apple {
    private Integer num;
    private String name;
    private String color;
}

List<Apple> list = new ArrayList<>();
list.add(new Apple(100,"苹果1","红色"));
list.add(new Apple(105,"苹果5","红色"));
list.add(new Apple(104,"苹果4","红色"));
list.add(new Apple(103,"苹果3","红色"));
list.add(new Apple(102,"苹果2","红色"));

List<String> collect = list.stream()
    //将属性的一列通过get方法映射成流
    .map(Apple::getName)
    //转为一个list集合
    .collect(Collectors.toList());
System.out.println(collect);
List<Apple> collect = list.stream().sorted(((o1, o2) -> {
    //通过num属性自定义排序
    return o1.getNum() - o2.getNum();
    }))
    //转为一个list集合
    .collect(Collectors.toList());
System.out.println(collect);
Map<Integer, String> map = list.stream().sorted(((o1, o2) -> {
    return o1.getNum() - o2.getNum();
    }))
    //转map, 第一个参数为键,第二个参数为值
    .collect(Collectors.toMap(Apple::getNum, Apple::getName));
System.out.println(map);