Predicate의 결합

@FunctionalInterface
public interface Predicate<T> {

    boolean test(T t);

    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    default Predicate<T> negate() {
        return (t) -> !test(t);
    }

    default Predicate<T> or(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

    static <T> Predicate<T> isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }

    @SuppressWarnings("unchecked")
    static <T> Predicate<T> not(Predicate<? super T> target) {
        Objects.requireNonNull(target);
        return (Predicate<T>)target.negate();
    }
}

and(), or(), negate()

• and(), or(), negate()로 두 Predicate(두 조건식)를 하나로 결합

• default메소드

• and() : && / or() : || / negate() : !
  ⇒ obj1.and(obj2)/obj1.or(obj2)/obj.negate()

Predicate<Integer> p = i -> i < 100;
Predicate<Integer> q = i -> i < 200;
Predicate<Integer> r = i -> i%2 == 0;

-> 결합!!

Predicate<Integer> notP = p.negate();	//i >=100
Predicate<Integer> all = notP.and(q).or(r);	//100<= i && i <200 || i%2 == 0
Predicate<Integer> all2 = notP.and(q.or(r));	//100<= i && (i <200 || i%2 == 0)
		
System.out.println(all.test(2));	//f && t || t -> f || t -> t
System.out.println(all2.test(2));	//f && (t || t) -> f && t -> f

---

static isEqual()

• 등가비교를 위한 Predicate의 작성에는 isEqual() 사용
• ✨static 메소드
  ⇒ Predicate.isEqual(obj1)
  ⇒ Predicate.isEqual(obj1).test(obj2)

Predicate<String> p2 = Predicate.isEqual("abc");
boolean result = p2.test("abc22");
System.out.println(result);	//false

//***한줄로 줄이면!!
boolean result2 = Predicate.isEqual("abc").test("abc22");
System.out.println(result2);	//false

• 그냥 ("abc").equals("abc22") 와 같은 거!

---

Function의 결합

@FunctionalInterface 
public interface Function<T, R> { 
	R apply(T t); 

	default <V> Function<V, R> compose(Function<? super V, ? extends T> before) { 
	Objects.requireNonNull(before); 
	return (V v) -> apply(before.apply(v)); 
} 

default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) { 
	Objects.requireNonNull(after); 
	return (T t) -> after.apply(apply(t)); 
} 

static <T> Function<T, T> identity() { return t -> t; } 
}

• 두 함수를 결합할 경우 : ✨✨(앞 함수의 반환타입 == 뒤 함수의 매개타입)

---

ex14_03

import java.util.function.Function;
import java.util.function.Predicate;

public class Ex14_03 {

	public static void main(String[] args) {
		Function<String, Integer> f = (s) -> Integer.parseInt(s, 16);	//문자열 s를 16진수 Integer로
		Function<Integer, String> g = (i) -> Integer.toBinaryString(i);	//i를 2진수 문자열로
		
		Function<String, String> h = f.andThen(g);
		Function<Integer, Integer> h2 = f.compose(g);	//== g.andThen(f)
		
		System.out.println(h.apply("FF"));	//"FF" -> 255 -> "11111111"
		System.out.println(h2.apply(2));	//2 -> "10" -> 16
		
		Function<String, String> f2 = x -> x;	//항등 함수
		System.out.println(f2.apply("AAA"));	//"AAA"그대로 출력
		
		Predicate<Integer> p = i -> i < 100;
		Predicate<Integer> q = i -> i < 200;
		Predicate<Integer> r = i -> i%2 == 0;
		
		Predicate<Integer> notP = p.negate();	//i >=100
		Predicate<Integer> all = notP.and(q).or(r);	//100<= i && i <200 || i%2 == 0
		Predicate<Integer> all2 = notP.and(q.or(r));	//100<= i && (i <200 || i%2 == 0)
		
		System.out.println(all.test(2));	//f && t || t -> f || t -> t
		System.out.println(all2.test(2));	//f && (t || t) -> f && t -> f
		System.out.println(all.test(150));	//t && t || t -> t
		
		String str1 = "abc";
		String str2 = "abc";
		
		Predicate<String> p2 = Predicate.isEqual(str1);
		boolean result = p2.test(str2);
		System.out.println(result);
		//***한줄로 줄이면!!
		boolean result2 = Predicate.isEqual(str1).test(str2);
		//boolean result2 = str1.equals(str2); 랑 같다.
		System.out.println(result2);

	}

}

---

11111111
16
AAA
true
false
true
true
true

---

---

Ref

• 생활코딩 JAVA1
• 객체화 설명
• w3schools JAVA
• 나동빈
• 자바의정석
• 자바의정석 깃허브
• 클래스와 객체