jdk8中list,map的stream常見操作

本文的知識點：

map 與flatmap的差別

從list中取出第一個元素，需要需要list為null,empty的情況

package com.alioo.stream;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.Optional;

import static java.util.stream.Collectors.toList;

public class ListDemo {

    public static void main(String[] args) {

        List<Integer> numbers = Arrays.asList(3, 2, 2, 3, 7, 3, 5);
        // map 簡單應用擷取對應的平方數
        List<Integer> squaresList = numbers.stream().map(i -> i * i).distinct().collect(toList());
        System.out.println("squaresList=" + squaresList);
        //結果：
        // squaresList=[9, 4, 49, 25]

        // map 與flatmap的差別
        String[] words = new String[] {"Hello", "World"};
        List<String[]> mapResult = Arrays.stream(words)
            .map(word -> word.split(""))
            .distinct()
            .collect(toList());
        mapResult.forEach(obj -> {
            System.out.println(obj + "=" + Arrays.toString(obj));

        });
        //結果：
        //[Ljava.lang.String;@42110406=[H, e, l, l, o]
        //[Ljava.lang.String;@531d72ca=[W, o, r, l, d]

        List<String> flatMap = Arrays.stream(words)
            .flatMap(word -> Arrays.stream(word.split("")))
            .distinct()
            .collect(toList());
        System.out.println("flatMap=" + flatMap);
        //結果：
        //flatMap=[H, e, l, o, W, r, d]

        //forEach裡的return隻相當于continue,沒有break文法
        //forEach裡的return隻相當于continue,沒有break文法，在這裡我總結了3種解決方案供你選擇
        List<String> strs = Arrays.asList("a", "b", "c", "d", "e");
        strs.forEach(o -> {
            if (o.equals("d")) {
                System.out.println("hits char is:" + o);
                return;
            }
            System.out.println("forEach return o=" + o);
        });
        //forEach return o=a
        //forEach return o=b
        //forEach return o=c
        //hits char is:d
        //forEach return o=e

        try {
            strs.forEach(o -> {
                if (o.equals("d")) {
                    System.out.println("hits char is:" + o);
                    throw new RuntimeException();
                }
                System.out.println("forEach exception o=" + o);

            });
        } catch (Exception e) {
            //
        }
        //forEach exception o=a
        //forEach exception o=b
        //forEach exception o=c
        //hits char is:d

        strs.stream()
            .filter(o -> (!o.equals("d") && !o.equals("e")))
            .forEach(o -> {
                System.out.println("forEach filter=" + o);
            });
        //forEach filter=a
        //forEach filter=b
        //forEach filter=c

        strs.stream().anyMatch(o -> {
            boolean flag = o.equals("d");
            if (flag) {
                return true;
            }
            System.out.println("forEach anyMatch=" + o);
            return false;
        });
        //forEach anyMatch=a
        //forEach anyMatch=b
        //forEach anyMatch=c


        //List<Integer> numbers2 = Arrays.asList(3, 2, 1, 3, 7, 8, 5);
        //List<Integer> numbers2 = null;
        List<Integer> numbers2 = new ArrayList<>();

        //從list中取出第一個元素，需要需要list為null,empty的情況
        Integer firstNumber = Optional.ofNullable(numbers2).orElse(Collections.emptyList()).stream()
            .findFirst()
            //.map(c -> c * c)
            .orElse(0);
        System.out.println("firstNumber=" + firstNumber);

        firstNumber =Optional.ofNullable(numbers2)
            .map(list->list.stream().findFirst().orElse(0) )
            .orElse(0);
        System.out.println("firstNumber=" + firstNumber);


        List<Integer> result2 = new ArrayList<>();
        //parallelStream的一個坑小記：最終各個item的執行順序不是固定的
        //numbers2.parallelStream().forEach(o->{
        //    System.out.println(Thread.currentThread()+" o="+o);
        //    result2.add(o*o);
        //});

        System.out.println(Thread.currentThread() + " result2=" + result2);



    }
}