Java8メソッド参照

今回みなさんと一緒にJava８のメソッド参照を勉強しましょう。

まずは、Java８以前の方法で年齢ソードのメソットを作りましょうか。

lombokを使う

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.*;

public class App {
    public static void main( String[] args ) {
        List<Person> personList = new ArrayList<Person>();
        personList.add(new Person("Jack",25));
        personList.add(new Person("Mary",23));
        personList.add(new Person("Jill",24));
        personList.add(new Person("Dock",34));
        personList.add(new Person("Lily",27));


        // 匿名クラス
        Collections.sort(personList, new Comparator<Person>() {
            @Override
            public int compare(Person o1, Person o2) {
                return o1.getAge() - o2.getAge();
            }
        });

        System.out.println(personList);
    }
}

@Data
@AllArgsConstructor
@NoArgsConstructor
class Person {
    private String name;
    private int age;
}

実装結果：[Person(name=Mary, age=23), Person(name=Jill, age=24), Person(name=Jack, age=25), Person(name=Lily, age=27), Person(name=Dock, age=34)]

Lambda式

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.*;

public class App {
    public static void main( String[] args ) {
        List<Person> personList = new ArrayList<Person>();
        personList.add(new Person("Jack",25));
        personList.add(new Person("Mary",23));
        personList.add(new Person("Jill",24));
        personList.add(new Person("Dock",34));
        personList.add(new Person("Lily",27));

        // Lambda式
        Collections.sort(personList,(p1,p2) -> p1.getAge() - p2.getAge());
        System.out.println(personList);
    }
}

@Data
@AllArgsConstructor
@NoArgsConstructor
class Person {
    private String name;
    private int age;
    
}

実装結果：[Person(name=Mary, age=23), Person(name=Jill, age=24), Person(name=Jack, age=25), Person(name=Lily, age=27), Person(name=Dock, age=34)]

static メソッド参照

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

import java.util.*;

public class App {
    public static void main( String[] args ) {
        List<Person> personList = new ArrayList<Person>();
        personList.add(new Person("Jack",25));
        personList.add(new Person("Mary",23));
        personList.add(new Person("Jill",24));
        personList.add(new Person("Dock",34));
        personList.add(new Person("Lily",27));

        // static メソッド参照
        Collections.sort(personList, Person::CompareAge);
        System.out.println(personList);

    }
}

@Data
@AllArgsConstructor
@NoArgsConstructor
class Person {
    private String name;
    private int age;

    public static int CompareAge(Person o1, Person o2) {
        return o1.getAge() - o2.getAge();
    }
}