java8 使用方法引用（::）的四种方式

在java8中提供了方法引用的方式，简化了lambda表达式写法，方法引用本质上是lambda表达式的语法糖 并没有提供新的功能和特性，写法为两个冒号 "::"

import java.util.Arrays;
import java.util.Collections;
import java.util.List;
import java.util.function.BiFunction;

public class MethodReferenceTest {

	public static void main(String[] args) {
		MethodReferenceTest test = new MethodReferenceTest();
		
		Student s1 = new Student("zhangsna",90);
		Student s2 = new Student("wang",50);
		Student s3 = new Student("zhaosl",79);
		
		List<Student> list = Arrays.asList(s1,s2,s3);
		
		//集合排序lambda表达式 写法
		list.sort((o1,o2) -> {
			return Student.compareStudentByScore(o1, o2);
		});
		list.sort((o1,o2) -> Student.compareStudentByScore(o1, o2));
		
		//方法引用本质上是lambda表达式的语法糖 并没有提供新的功能和特性 
		
		//方法引用第一种模式  类名::静态方法  
		//相当于(o1,o2) -> Student.compareStudentByScore(o1, o2),参数按顺序传入方法 再方法引用时省略了声明参数的过程
		list.sort(Student::compareStudentByScore);
		
		//二、 引用名（对象名）::实例方法名
		//以下等价 参数按顺序传入方法 再方法引用时省略了声明参数的过程
		StudentComparator sc = new StudentComparator();
		list.sort((o1,o2) -> sc.compareScope(o1, o2));
		list.sort(sc::compareScope);
		
		//三  类名::实例方法名
		//以下等价 第一个元素调用方法第二个元素为方法的入参
		list.sort((o1,o2) -> o1.compareByScore(o2));
		list.sort(Student::compareByScore);
		
		//eg
		List<String> cities = Arrays.asList("qingdao","dalain","chongqing","beijing");
		
		Collections.sort(cities, (o1,o2) -> o1.compareToIgnoreCase(o2));
		
		Collections.sort(cities, String::compareToIgnoreCase);
		
		cities.forEach(System.out::println);
		
		//四 构造方法引用 类名::new
		//以下等价 调用构造函数参数依次传入
		Student student1 = test.initStudent("lishid", 50, (a1 ,a2) -> new Student(a1 ,a2));
		Student student2 = test.initStudent("lishid", 50, Student::new);
		System.out.println(student2.getName() + " " +student2.getScore());
		
		list.forEach(v -> System.out.println(v.getScore()));
		list.forEach(System.out::println);
	}
	
	public Student initStudent(String name,Integer score,BiFunction<String,Integer,Student> function) {
		return function.apply(name, score);
	}

}

public class Student {
	
	private String name;
	
	private int score;
	
	public Student(String name,int score) {
		this.name = name;
		this.score = score;
	}

	public String getName() {
		return name;
	}

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

	public int getScore() {
		return score;
	}

	public void setScore(int score) {
		this.score = score;
	}
	
	public static int compareStudentByScore(Student s1, Student s2) {
		return s1.getScore() - s2.getScore();
	}
	
	public static int compareStudentByName(Student s1, Student s2) {
		return s1.getName().compareToIgnoreCase(s2.getName());
	}
	
	public int compareByScore(Student student) {
		return this.score - student.getScore();
	}
	
	public int compareByName(Student student) {
		return this.name.compareToIgnoreCase(student.getName());
	}
	
	public String toString() {
		return name + " "+ score;
	}
}

public class StudentComparator {

	public int compareScope(Student s1, Student s2) {
		return s1.getScore() - s2.getScore();
	}
	
	public int compareByName(Student s1, Student s2) {
		return s1.getName().compareToIgnoreCase(s2.getName());
	}
}