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Today I Learned Method Ref
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‎methodref/MethodRef3.java‎

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package methodref;
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import java.util.function.Function;
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public class MethodRef3 {
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public static void main(String[] args) {
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// 임의 객체의 인스턴스 메서드 참조(특정 타입의)
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Person person1 = new Person("Kim");
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Person person2 = new Person("Park");
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Person person3 = new Person("Lee");
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// 람다
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Function<Person, String> fun1 = (Person person) -> person.introduce();
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System.out.println("person1 = " + fun1.apply(person1));
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System.out.println("person2 = " + fun1.apply(person2));
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System.out.println("person3 = " + fun1.apply(person3));
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System.out.println();
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// 메서드 참조, 타입이 첫 번째 매개변수가 됨
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// 그리고 첫 번쨰 매개변수의 메서드를 호출, 나머지는 순서대로 매개변수에 전달
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Function<Person, String> fun2 = Person::introduce; // 타입::인스턴스 메서드 (타입 이름) -> 이름.메서드
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System.out.println("person1 = " + fun2.apply(person1));
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System.out.println("person2 = " + fun2.apply(person2));
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System.out.println("person3 = " + fun2.apply(person3));
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}
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}

‎methodref/MethodRef4.java‎

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package methodref;
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import java.util.ArrayList;
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import java.util.List;
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import java.util.function.Function;
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public class MethodRef4 {
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public static void main(String[] args) {
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List<Person> personList = List.of(new Person("Kim"),
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new Person("Park"),
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new Person("Lee")
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);
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List<String> result1 = mapPersonToString(personList, (Person p) -> p.introduce());
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List<String> result2 = mapPersonToString(personList, Person::introduce);
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System.out.println("result1 = " + result1);
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System.out.println("result2 = " + result2);
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List<String> stringResult1 = mapStringToString(result1, (String s)-> s.toUpperCase());
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List<String> stringResult2 = mapStringToString(result1, String::toUpperCase);
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System.out.println("stringResult1 = " + stringResult1);
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}
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public static List<String> mapPersonToString(List<Person> personList, Function<Person, String> fun){
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List<String> result = new ArrayList<>();
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for (Person p : personList) {
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String apply = fun.apply(p);
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result.add(apply);
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}
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return result;
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}
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public static List<String> mapStringToString(List<String> stringList, Function<String, String> fun){
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List<String> result = new ArrayList<>();
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for (String p : stringList) {
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String apply = fun.apply(p);
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result.add(apply);
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}
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return result;
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}
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}

‎methodref/MethodRef5.java‎

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package methodref;
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import lambda.lambda5.mystream.MyStreamV3;
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import java.util.List;
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public class MethodRef5 {
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public static void main(String[] args) {
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List<Person> personList = List.of(
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new Person("Kim"),
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new Person("Park"),
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new Person("Lee")
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);
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List<String> result1 = MyStreamV3.of(personList)
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.map(person -> person.introduce())
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.map(str -> str.toUpperCase())
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.toList();
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System.out.println("result1 = " + result1);
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List<String> result2 = MyStreamV3.of(personList)
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.map(Person::introduce)
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.map(String::toUpperCase)
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.toList();
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System.out.println("result2 = " + result2);
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}
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}

‎methodref/MethodeRefEx1.java‎

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package methodref;
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import java.util.function.Supplier;
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public class MethodeRefEx1 {
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public static void main(String[] args) {
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// 1. 정적 메서드 참조
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Supplier<String> staticMethod1 = () -> Person.greeting();
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Supplier<String> staticMethod2 = Person::greeting; // 클래스::정적 메서드
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System.out.println("staticMethod1 = " + staticMethod1.get());
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System.out.println("staticMethod2 = " + staticMethod2.get());
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// 2. 특정 객체의 인스턴스 참조
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Person person = new Person("Choi");
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Supplier<String> instanceMethod1 = () -> person.introduce();
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Supplier<String> instanceMethod2 = person::introduce; // 객체::인스턴스메서드
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System.out.println("instanceMethod1 = " + instanceMethod1.get());
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System.out.println("instanceMethod2 = " + instanceMethod2.get());
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// 3. 생성자 참조
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Supplier<Person> newPerson1 = () -> new Person();
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Supplier<Person> newPerson2 = Person::new; // 클래스::new
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System.out.println("newPerson1 = " + newPerson1.get());
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System.out.println("newPerson2 = " + newPerson2.get());
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}
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}

‎methodref/MethodeRefEx2.java‎

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package methodref;
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import java.util.function.Function;
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import java.util.function.Supplier;
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public class MethodeRefEx2 {
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public static void main(String[] args) {
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// 1. 정적 메서드 참조
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Function<String, String> staticMethod1 = (name) -> Person.greetingWithName(name);
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Function<String, String> staticMethod2 = Person::greetingWithName;
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System.out.println("staticMethod1 = " + staticMethod1.apply("Kim"));
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System.out.println("staticMethod2 = " + staticMethod2.apply("Kim"));
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// 2. 특정 객체의 인스턴스 참조
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Person person = new Person("Choi");
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Function<Integer, String> instanceMethod1 = (n) -> person.introduceWithNumber(n);
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Function<Integer, String> instanceMethod2 = person::introduceWithNumber; // 객체::인스턴스메서드
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System.out.println("instanceMethod1 = " + instanceMethod1.apply(1));
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System.out.println("instanceMethod2 = " + instanceMethod2.apply(2));
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// 3. 생성자 참조
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Function<String, Person> newPerson1 = (name) -> new Person(name);
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Function<String, Person> newPerson2 = Person::new; // 클래스::new
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System.out.println("newPerson1 = " + newPerson1.apply("Choi"));
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System.out.println("newPerson2 = " + newPerson2.apply("Choi"));
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}
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}

‎methodref/MethodeRefEx6.java‎

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package methodref;
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import java.util.function.BiFunction;
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// 매개 변수 추가
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public class MethodeRefEx6 {
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public static void main(String[] args) {
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// 4. 임의 객체의 인스턴스 메서드 참조(특정 타입의)
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Person person = new Person("Kim");
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// 람다
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BiFunction<Person, Integer, String> fun1 =
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(Person p, Integer n) -> p.introduceWithNumber(n);
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System.out.println("person.introduceWithNumber = " + fun1.apply(person, 5));
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// 메서드 참조, 타입이 첫 번쨰 매개변수가 됨, 그리고 첫 번째 매개변수의 메서드를 호출
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// 나머지는 순서대로 매개변수에 전달
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BiFunction<Person, Integer, String> fun2 = Person::introduceWithNumber; // 타입::메서드 명
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System.out.println("person.introduceWithNumber = " + fun2.apply(person, 10));
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}
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}

‎methodref/Person.java‎

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package methodref;
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public class Person {
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String name;
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public Person() {
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this("unkown");
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}
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public Person(String name) {
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this.name = name;
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}
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// 정적 매서드
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public static String greeting() {
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return "hello";
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}
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// 정적 매서드 + 매개변수
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public static String greetingWithName(String name) {
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return "hello " + name;
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}
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public String getName() {
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return name;
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}
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// 인스턴스 메서드
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public String introduce(){
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return "I am " + name;
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}
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// 인스턴스 메서드, 매개변수
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public String introduceWithNumber(int number){
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return "I am " + name + ", my number is " + number;
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}
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}

‎methodref/start/MethodRefStartV1.java‎

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package methodref.start;
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import java.util.function.BinaryOperator;
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public class MethodRefStartV1 {
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public static void main(String[] args) {
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BinaryOperator<Integer> add1 = (x,y) -> x+y;
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BinaryOperator<Integer> add2 = (x,y) -> x+y;
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Integer result1 = add1.apply(1,2);
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System.out.println("result1 = " + result1);
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Integer result2 = add2.apply(3,4);
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System.out.println("result2 = " + result2);
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}
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}

‎methodref/start/MethodRefStartV2.java‎

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package methodref.start;
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import java.util.function.BinaryOperator;
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public class MethodRefStartV2 {
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public static void main(String[] args) {
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BinaryOperator<Integer> add1 = (x,y) -> add(x,y);
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BinaryOperator<Integer> add2 = (x,y) -> add(x,y);
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Integer result1 = add1.apply(1,2);
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System.out.println("result1 = " + result1);
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Integer result2 = add2.apply(3,4);
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System.out.println("result2 = " + result2);
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}
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public static int add(int x, int y){
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return x + y;
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}
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}

‎methodref/start/MethodRefStartV3.java‎

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package methodref.start;
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import java.util.function.BinaryOperator;
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public class MethodRefStartV3 {
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public static void main(String[] args) {
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BinaryOperator<Integer> add1 = MethodRefStartV3::add;
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BinaryOperator<Integer> add2 = MethodRefStartV3::add;
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Integer result1 = add1.apply(1,2);
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System.out.println("result1 = " + result1);
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Integer result2 = add2.apply(3,4);
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System.out.println("result2 = " + result2);
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}
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public static int add(int x, int y){
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return x + y;
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}
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}

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