方法引用（Java）

把已经有的方法拿过来用，当做函数式接口中抽象方法的方法体

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.被引用的方法形参的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需求

package function;import java.util.Arrays;public class FunctionDemo {public static void main(String[] args) {Integer[] arr = {2, 6, 7, 1, 5, 9, 8};Arrays.sort(arr,FunctionDemo::subtraction);System.out.println(Arrays.toString(arr));}public static int subtraction(int num1, int num2) {return num2 - num1;}
}

方法引用的分类

1.引用静态方法

package function;import java.util.ArrayList;
import java.util.Collections;public class FunctionDemo2 {public static void main(String[] args) {ArrayList<String> list = new ArrayList<>();Collections.addAll(list,"1","2","3","4","5");list.stream().map(Integer::parseInt).forEach(s-> System.out.print(s+" "));}
}

2.引用成员方法

①引用其他类的chengyuanfangfa

package function;import java.util.ArrayList;
import java.util.Collections;public class FunctionDemo3 {public static void main(String[] args) {ArrayList<String> list = new ArrayList<>();Collections.addAll(list,"张三丰","张无忌","张三","张强","韦小宝");list.stream().filter(new StringOperation()::stringJudge).forEach(s-> System.out.println(s));}
}

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

②引用本类的成员方法

静态方法中是没有this的所以只能调用本类的方法

package function;import java.util.ArrayList;
import java.util.Collections;public class FunctionDemo3 {public static void main(String[] args) {ArrayList<String> list = new ArrayList<>();Collections.addAll(list,"张三丰","张无忌","张三","张强","韦小宝");
//        list.stream().filter(new StringOperation()::stringJudge).forEach(s-> System.out.println(s));list.stream().filter(new FunctionDemo3()::stringJudge).forEach(s-> System.out.println(s));}public boolean stringJudge(String s) {return s.startsWith("张") && s.length() == 3;}
}

③引用父类的成员方法

3引用构造方法

package function;import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.stream.Collectors;public class FunctionDemo4 {public static void main(String[] args) {ArrayList<String> list = new ArrayList<>();Collections.addAll(list, "张三丰,24", "张无忌,26", "张三,18", "张强,31", "韦小宝,27");
//        list.stream().filter(new StringOperation()::stringJudge).forEach(s-> System.out.println(s));List<Student> collect = list.stream().map(Student::new).collect(Collectors.toList());System.out.println(collect);}
}

package function;public class Student {private String name;private int age;public Student() {}public Student(String str) {String[] split = str.split(",");this.name = split[0];this.age = Integer.parseInt(split[1]);}public Student(String name, int age) {this.name = name;this.age = age;}/*** 获取** @return name*/public String getName() {return name;}/*** 设置** @param name*/public void setName(String name) {this.name = name;}/*** 获取** @return age*/public int getAge() {return age;}/*** 设置** @param age*/public void setAge(int age) {this.age = age;}public String toString() {return "Student{name = " + name + ", age = " + age + "}";}
}