JAVA自學筆記（10）—Stream流、方法引用

開胃菜

——實現遍歷集合，開啟Stream流的便利化

import java.util.ArrayList;
import java.util.Collections;


public class Main{
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<String>();
        ArrayList<String> list1 = new ArrayList<String>();
        Collections.addAll(list,"ab","cea","fshwe","dea");
        list.stream().
                filter(str -> str.contains("a")).
                filter(str -> str.length() == 3).
                forEach(str-> System.out.println(str));

    }
}

列印結果：

cea
dea

走進Stream流的世界

1.0 獲取stream流的方法

import java.util.*;
import java.util.stream.Stream;

public class Main{
    public static void main(String[] args) {
        ArrayList<String> list = new ArrayList<>();
        list.stream();//第一種方式

        //特別的Map集合也可以間接的獲取
        Map<String,Integer> map=new HashMap<>();
        Set<String> keySet = map.keySet();
        keySet.stream();
        Collection<Integer> integers = map.values();
        integers.stream();

        Set<Map.Entry<String, Integer>> entries = map.entrySet();

        entries.stream();


        //第二種方式
        Stream<Integer> integerStream = Stream.of(1, 2, 3, 4, 5);//數組

    }
}

2.0 Stream流的方法

（1）forEach

import java.util.stream.Stream;

public class Main{
    public static void main(String[] args) {
        Stream<String> stream = Stream.of("a", "b", "c", "d", "e");
        stream.forEach((str)-> System.out.print(str+" "));
        //a b c d e 
    }
}

(2) filter()

import java.util.stream.Stream;

public class Main{
    public static void main(String[] args) {
        Stream<String> stream = Stream.of("baby", "boy", "home", "dream", "eage");
        Stream<String> a = stream.filter(str -> str.contains("a")).filter(str -> str.length() >=3);
        a.forEach((str)-> System.out.print(str+" "));
        //baby dream eage
    }
}

(3)Stream流的特點

（4）Map方法

import java.util.stream.Stream;

public class Main{
    public static void main(String[] args) {
        Stream<String> stream = Stream.of("12", "34", "56");
        stream.map((str)->Integer.parseInt(str)+1).
                forEach((Str)-> System.out.print(Str+" "));

        //13 35 57 
    }
}

（5）count方法

（6）limit方法

（7）skip方法

（8）concat方法

方法引用

—— 簡便你我他

1.0 通過對象名引用成員方法

public class Main{
    public static void PrintString(MessageBuilder mes)
    {
        mes.bulidString("hello");
    }
    public static void main(String[] args) {
        //Lambda
        PrintString((s)->{
            Printer pri=new Printer();
            pri.PrintUpper(s);//HELLO
        });

        //方法引用
        Printer printer=new Printer();
        PrintString(printer::PrintUpper);//HELLO
    }
}

2.0 通過類名引用靜態方法　　

public class Main{
    public static int calmathod(int num,MessageBuilder mes)
    {
        return mes.Cal(num);
    }
    public static void main(String[] args) {
        int num=-10;
        /*
        int calmathod = calmathod(num, (number) -> {
            return Math.abs(number);
        });*/
        int calmathod=calmathod(num,Math::abs);
        System.out.println(calmathod);//10
    }
}

3.0 通過super引用父類的方法

public class Main extends FU{
    @Override
    public void show(){
        System.out.println("Hello,My soulmate!");
    }
    public void method(MessageBuilder mes){
        mes.greet();
    }
    public void Print()//調用父類方法
    {
        /*
        method(()->{
            FU fu = new FU();
            fu.show();
        });
        */
       method(super::show);//Hello,My friend!
    }
    public static void main(String[] args) {
     new Main().Print();
    }
}

4.0 通過this調用本類中的成員方法　　

public class Main{
    public void method()
    {
        System.out.println("Hello world!");
    }
    public void showMethod(MessageBuilder mes){//函數式介面
        mes.show();
    }
    public void MethodPrint()
    {
        /*
        showMethod(()->{
            this.method();
        });
         */
        showMethod(this::method);
    }

    public static void main(String[] args) {
        new Main().MethodPrint();//Hello world!
    }
}

5.0 類的構造器（構造方法）　　

public class Person {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Person() {
    }

    public Person(String name) {
        this.name = name;
    }
}

@FunctionalInterface
public  interface MessageBuilder {
    public abstract Person show(String name);
}

public class Main{
    public static String Method(String name,MessageBuilder mes)
    {
        Person person = mes.show(name);
        return person.getName();
    }
    public static void main(String[] args) {
        String name="布丁";
        /*
        String method = Method(name,(str)->{
             return new Person(str);
        });
         */

        String method = Method(name, Person::new);

        System.out.println(method);

    }
}

6.0 數組的構造器引用

@FunctionalInterface
public  interface MessageBuilder {
    public abstract int[] Builder(int length);
}

public class Main{
    public static int[] Method(int length,MessageBuilder mes)
    {
        int[] builder = mes.Builder(length);
        return builder;
    }
    public static void main(String[] args) {
        /*
        int[] method = Method(3, (length) -> {
            return new int[length];
        });
         */
        int[] method = Method(3, int[]::new);

        System.out.println(method.length);
    }
}