方法引用的学习

把已经有的方法拿过来用，当做函数式接口中的抽象方法的方法体。

1.引用处必须是函数式接口

2.被引用的方法必须已经存在

3.被引用方法的形参和返回值需要跟抽象方法保持一致

4.被引用方法的功能要满足当前的需求

方法引用的分类

1.引用静态方法

格式：类名::静态方法

package myFunction;

import java.util.ArrayList;
import java.util.Collections;
import java.util.function.Function;

public class Functiondemo01 {
    public static void main(String[] args) {
        ArrayList<String>  list=new ArrayList<>();
        Collections.addAll(list,"1","2","3","4","5");
       /* list.stream().map(new Function<String,Integer>() {
            @Override
            public Integer apply(String s) {
                int i = Integer.parseInt(s);
                return i;
            }
        }).forEach(s-> System.out.println(s));*/
        list.stream().map(Integer::parseInt).forEach(s-> System.out.println(s));
    }
}

2.引用成员方法

格式：对象：：成员方法

其他类：其他类对象：：方法名

本类：this：：方法名

父类：super::方法名

package myFunction;

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

public class Functiondemo02 {
    public static void main(String[] args) {
     ArrayList<String> list=new ArrayList<>();
        Collections.addAll(list,"张无忌","周芷若","赵敏","张强","张三丰");
      //  list.stream().filter(s->s.startsWith("张")).filter(s->s.length()==3).forEach(s-> System.out.println(s));
list.stream().filter(new StringOperation()::stringJudge).forEach(s-> System.out.println(s));
    }
    
}
package myFunction;

public class StringOperation {
    public boolean stringJudge(String s){
     return s.startsWith("张")&&s.length()==3;
    }
}

package myFunction;

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

public class Functiondemo02 {
    public static void main(String[] args) {
     ArrayList<String> list=new ArrayList<>();
        Collections.addAll(list,"张无忌","周芷若","赵敏","张强","张三丰");
      //  list.stream().filter(s->s.startsWith("张")).filter(s->s.length()==3).forEach(s-> System.out.println(s));
list.stream().filter(new Functiondemo02()::stringJudge).forEach(s-> System.out.println(s));
    }
        public boolean stringJudge(String s){
            return s.startsWith("张")&&s.length()==3;
        }
    }

3.引用构造方法

格式：类名：：new

package myFunction;

public class Student {
    private String name;
    private int age;

    public Student() {
    }
    public Student(String str) {
        String []arr=  str.split(",");
        this.name=arr[0];
        this.age=Integer.parseInt(arr[1]);
    }
    public Student(String name, int age) {
        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 String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}
package myFunction;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.function.Function;
import java.util.stream.Collectors;

public class FunctionDemo03 {
    public static void main(String[] args) {
        ArrayList<String> list=new ArrayList<>();
        Collections.addAll(list,"张无忌,15","周芷若,14","赵敏,13");
  /*List<Student> newList=     list.stream().map(new Function<String,Student>() {
           @Override
           public Student apply(String s) {
            String []arr=  s.split(",");
            String name=arr[0];
           int age=Integer.parseInt(arr[1]);
               return new Student(name,age);
           }
       }).collect(Collectors.toList());
        System.out.println(newList);*/
   List<Student> newList2=list.stream().map(Student::new).collect(Collectors.toList());
        System.out.println(newList2);
    }
}

其他调用方式：

1.使用类名引用成员方法

格式：类名：：成员方法

package myFunction;

import java.util.ArrayList;
import java.util.Collections;
import java.util.Locale;
import java.util.function.Function;

public class Funtiondemo04 {
    public static void main(String[] args) {
        ArrayList<String> list=new ArrayList<>();
        Collections.addAll(list,"aaa","bbb","ccc","ddd");
       /* list.stream().map(new Function<String, String>() {
            @Override
            public String apply(String s) {
                return s.toUpperCase();
            }
        }).forEach(s-> System.out.println(s));*/
        list.stream().map(String::toUpperCase).forEach(s-> System.out.println(s));
    }
}

引用数组的构造方法

格式:数组类型[]::new

package myFunction;

import java.util.ArrayList;
import java.util.Collections;
import java.util.function.IntFunction;

public class FunctionDemo05 {
    public static void main(String[] args) {
        ArrayList<Integer> list=new ArrayList<>();
        Collections.addAll(list,1,2,3,4,5);
    /*  Integer[]ar=  list.stream().toArray(new IntFunction<Integer[]>() {
            @Override
            public Integer[] apply(int value) {
                return new Integer[value];
            }
        });
        System.out.println(ar);*/
      Integer[] ar2=  list.stream().toArray(Integer[]::new);
        System.out.println(ar2);
    }
}