Java之方法引用笔记

方法引用实际上是Lambda表达式的一种语法糖

 

方法引用分为4类:

1.类名::静态方法名

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

    public Student(String name, int mark) {
        this.name = name;
        this.mark = mark;
    }

    public String getName() {
        return name;
    }

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

    public int getMark() {
        return mark;
    }

    public void setMark(int mark) {
        this.mark = mark;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", mark=" + mark +
                '}';
    }

    //静态方法名
    public static int computerByName(Student student1, Student student2){
        return student1.getName().compareToIgnoreCase(student2.getName());
    }

    public static int computerByMark(Student student1,Student student2){
        return student2.getMark() - student1.getMark();
    }


}

 

public static void main(String[] args) {
        Student student1 = new Student("明明",100);
        Student student2 = new Student("小李",88);
        Student student3 = new Student("静静",95);

        List<Student> studentList = Arrays.asList(student1,student2,student3);

        studentList.sort((theStudent1,theStudent2)-> Student.computerByName(theStudent1,theStudent2));
        studentList.forEach(student -> System.out.println(student));
        //等价于:
        studentList.sort(Student::computerByName);
        studentList.forEach(student -> System.out.println(student));

        System.out.println("--------------------------------");

        studentList.sort((theStudent1,theStudent2)-> Student.computerByMark(theStudent1,theStudent2));
        studentList.forEach(student -> System.out.println(student));
        //等价于
        studentList.sort(Student::computerByMark);
        studentList.forEach(student -> System.out.println(student));

    }

2.引用名::实例方法名

public class StudentComparator {
    //实例方法名
    public  int computerByName(Student student1, Student student2){
        return student1.getName().compareToIgnoreCase(student2.getName());
    }

    public  int computerByMark(Student student1,Student student2){
        return student2.getMark() - student1.getMark();
    }
}

public static void main(String[] args) {
        Student student1 = new Student("明明",100);
        Student student2 = new Student("小李",88);
        Student student3 = new Student("静静",95);

        List<Student> studentList = Arrays.asList(student1,student2,student3);

        StudentComparator studentComparator = new StudentComparator();
        studentList.sort((theStudent1, theStudent2) -> studentComparator.computerByMark(theStudent1,theStudent2));
        studentList.forEach(student -> System.out.println(student));
        //等价于：
        studentList.sort(studentComparator::computerByMark);
        studentList.forEach(student -> System.out.println(student));

        System.out.println("--------------------------------");

        studentList.sort((theStudent1, theStudent2) -> studentComparator.computerByName(theStudent1,theStudent2));
        studentList.forEach(student -> System.out.println(student));
        //等价于：
        studentList.sort(studentComparator::computerByName);
        studentList.forEach(student -> System.out.println(student));
    }

 3.类名::实例方法名

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

    public Student(String name, int mark) {
        this.name = name;
        this.mark = mark;
    }

    public String getName() {
        return name;
    }

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

    public int getMark() {
        return mark;
    }

    public void setMark(int mark) {
        this.mark = mark;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", mark=" + mark +
                '}';
    }

    //实例方法名
    public  int computerByName2(Student student){
        return  getName().compareToIgnoreCase(student.getName());
    }

    public  int computerByMark2(Student student){
        return getMark() - student.getMark();
    }

}

    public static void main(String[] args) {

        Student student1 = new Student("明明",100);
        Student student2 = new Student("小李",88);
        Student student3 = new Student("静静",95);

        List<Student> studentList = Arrays.asList(student1,student2,student3);

        //实例方法 为什么方法只有一个参数 是Lambda中第一个参数调用了这个方法，剩下的参数作为入参传入这个实例方法
        studentList.sort(Student::computerByName2);
        studentList.forEach(student -> System.out.println(student));
        //等价于：
        studentList.sort((theStudent1,theStudent2) -> theStudent1.computerByName2(student2));
        studentList.forEach(student -> System.out.println(student));

        System.out.println("--------------------------------");

        studentList.sort(Student::computerByMark2);
        studentList.forEach(student -> System.out.println(student));
        //等价于：
        studentList.sort((theStudent1,theStudent2) -> theStudent1.computerByMark2(student2));
        studentList.forEach(student -> System.out.println(student));


        //比如这个方法调用
        List<String> citys = Arrays.asList("beijing","shanghai","guangzhou","changsha");
        citys.sort((city1,city2) -> {
            return city1.compareToIgnoreCase(city2);
        });
        citys.forEach(city -> System.out.println(city));

        System.out.println("--------------------------------");

        //等价于：
        citys.sort(String::compareToIgnoreCase);
        citys.forEach(city -> System.out.println(city));
    }

 

4.构造方法引用 类名::new

public class MethodReferenceTest {
    public static void main(String[] args) {
        MethodReferenceTest methodReferenceTest = new MethodReferenceTest();
        System.out.println(methodReferenceTest.getString(() -> new String()));
        //等价于：
        System.out.println(methodReferenceTest.getString(String::new));

    }

    public String  getString(Supplier<String>supplier){
        return supplier.get() + "test";
    }
}

@FunctionalInterface
public interface Supplier<T> {

    /**
     * Gets a result.
     *
     * @return a result
     */
    T get();
}

Supplier<T>不接收参数，但是返回一个类型，在上面测试代码中吗就是返回一个String类型。用方法引用就可以简写为：String::new