자바 스트림 API - 최종처리(4)_collect2

collect(partitioningBy(Predicate))

=> true 값과 false로 나누어서 반환

 

collect(Collectors.groupingBy(Function))

=> Function에서 리턴값으로 그룹핑해서 반환

 

 

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8. 성별 분류

//익명클래스
	    Predicate<Stu> predicate = new Predicate<Stu>() {
	    	@Override
	    	public boolean test(Stu t) {
	    		return t.isMale();
	    	}
	    };
	    Map<Boolean, List<Stu>> xxx = Stream.of(stuArr).collect(Collectors.partitioningBy(predicate));
	    List<Stu> male = xxx.get(true);
	    List<Stu> female = xxx.get(false);
	    
	    System.out.println("male: " + male);
	    System.out.println("female: " + female);
	    
//male: [Stu [username=홍길동, grade=3, isMale=true, age=25], Stu [username=이순신, grade=1, isMale=true, age=55], Stu [username=안중근, grade=3, isMale=true, age=31], Stu [username=홍범도, grade=1, isMale=true, age=58], Stu [username=강감찬, grade=3, isMale=true, age=44]]
//female: [Stu [username=유관순, grade=2, isMale=false, age=18]]

 //람다표현식
	    Map<Boolean, List<Stu>> xxx2 = 
	    		Stream.of(stuArr)
//	    			  .collect(Collectors.partitioningBy(t->t.isMale()));
	    			  .collect(Collectors.partitioningBy(Stu::isMale));    //method reference
	    List<Stu> male2 = xxx2.get(true);
	    List<Stu> female2 = xxx2.get(false);
	    
	    System.out.println("male2: " + male2);
	    System.out.println("female2: " + female2);

//male2: [Stu [username=홍길동, grade=3, isMale=true, age=25], Stu [username=이순신, grade=1, isMale=true, age=55], Stu [username=안중근, grade=3, isMale=true, age=31], Stu [username=홍범도, grade=1, isMale=true, age=58], Stu [username=강감찬, grade=3, isMale=true, age=44]]
//female2: [Stu [username=유관순, grade=2, isMale=false, age=18]]

 

9. grade 그룹핑

//익명클래스
	    Function<Stu, Integer> classifier = new Function<Stu, Integer>(){
	    	@Override
	    	public Integer apply(Stu t) {
	    		return t.getGrade();
	    	}
	    };
	    
	    Map<Integer, List<Stu>> yyy = Stream.of(stuArr)
	    									.collect(Collectors.groupingBy(classifier));
	    Set<Integer> keys3 = yyy.keySet();
	    for (Integer key: keys3) {
	    	System.out.println(key+"=" + yyy.get(key));
	    }

//1=[Stu [username=이순신, grade=1, isMale=true, age=55], Stu [username=홍범도, grade=1, isMale=true, age=58]]
//2=[Stu [username=유관순, grade=2, isMale=false, age=18]]
//3=[Stu [username=홍길동, grade=3, isMale=true, age=25], Stu [username=안중근, grade=3, isMale=true, age=31], Stu [username=강감찬, grade=3, isMale=true, age=44]]

 

grade 그룹핑 갯수 구하기

Map<Integer, Long> yyy2 = Stream.of(stuArr)
				.collect(Collectors.groupingBy(classifier, Collectors.counting()));
		Set<Integer> keys4 = yyy2.keySet();
		for (Integer key: keys4) {
			System.out.println(key+"=" + yyy2.get(key));
		}

//1=2
//2=1
//3=3