Month: April 2020

//pass value type by value (값형식을 값에 의한 전달) – copy of value 전달

static void square1(int x) {
    x *= x;
    System.out.printf("The value inside square1: %d\n", x);
}

public static void main(String[] args) {
    int i = 5; 
    System.out.println("Before i=" + i);
    square1(i);
    System.out.println("After i=" + i + "\n\n");
}

//Before i=5
//The value inside square1: 25
//After i=5

//pass reference type by value (참조형식을 값에 의한 전달) – copy of reference 전달

static void square2(IntValue value) {
    value.x *= value.x;
    System.out.printf("The value inside square2: %d\n", value.x);
}

public static void main(String[] args) {
    int i = 5;
    IntValue value = new IntValue(i);
    System.out.println("Before value.x=" + value.x);
    square2(value);
    System.out.println("After value.x=" + value.x + "\n\n");
}

//Before value.x=5
//The value inside square2: 25
//After value.x=25

static void changeArray1(int[] arr) {
    arr[0]=888; // arr -> myArray이므로 원본 배열의 첫번째 값은 888로 변경
    System.out.println("changeArray1 arr=" + Arrays.stream(arr).mapToObj(String::valueOf).collect(Collectors.joining(",")));
    arr = new int[] {-3, -1, -2, -3, -4}; // local 변수로 새롭게 할당하여 지정 그러나 원본 배열 변경 안됨
    System.out.println("changeArray1 arr=" + Arrays.stream(arr).mapToObj(String::valueOf).collect(Collectors.joining(",")));
}

public static void main(String[] args) {
    int[] myArray = {1, 4, 5};
    System.out.println("Before myArray=" + Arrays.stream(myArray).mapToObj(String::valueOf).collect(Collectors.joining(",")));
    changeArray1(myArray);
    System.out.println("After changeArray1 myArray=" + Arrays.stream(myArray).mapToObj(String::valueOf).collect(Collectors.joining(",")));
}

//Before myArray=1, 4, 5
//changeArray1 arr=888, 4, 5
//changeArray1 arr=-3, -1, -2, -3, -4
//After myArray=888, 4, 5

static void changeArray2(IntValue[] arr) {
    arr[0].x=888; // 원본 배열의 첫번째 값은 888로 변경
    System.out.println("changeArray2 arr=" + Arrays.stream(arr).map(Object::toString).collect(Collectors.joining(",")));
    arr = new IntValue[] { new IntValue(-3), new IntValue(-1), new IntValue(-2), new IntValue(-3), new IntValue(-4)};  // local 변수로 새롭게 할당하여 지정 그러나 원본 배열 변경 안됨
    System.out.println("changeArray2 arr=" + Arrays.stream(arr).map(Object::toString).collect(Collectors.joining(",")));
}

public static void main(String[] args) {
    IntValue[] myArray2 = { new IntValue(1), new IntValue(4), new IntValue(5)};
    System.out.println("Before myArray2=" + Arrays.stream(myArray2).map(Object::toString).collect(Collectors.joining(",")));
    changeArray2(myArray2);
    System.out.println("After myArray2=" + Arrays.stream(myArray2).map(Object::toString).collect(Collectors.joining(",")));
}

// Before myArray2=1, 4, 5
// changeArray2 arr=888, 4, 5
// changeArray2 arr=-3, -1, -2, -3, -4
// After myArray2=888,4,5

TripleCalculator – TripleCalculator calculates MIN/MAX/MEDIAN/MEAN/SUM of 3 numbers.

PersonTest-Gender – Person has name, age, gender

EnumTest – Weekday

https://www.baeldung.com/java-permgen-metaspace

Permgen (JDK7 이전)

Metaspace (JDK8 이후)

클래스의 정보가 로드되는 영역, 정적 멤버 (static member) 저장, 바이트코드나 JIT 정보 저장

https://www.baeldung.com/java-stack-heap

Object − objects have states and behaviors.
Class – defines the grouping of data and code, the “type” of an object
Instance – a specific allocation of a class
Message – sent to objects to make them act
Method – a “function” that an object knows how to perform
Local Variables − variables defined inside methods, constructors or blocks
Instance Variables – variables within a class but outside any method (a specific piece of data belonging to an object)
Class Variables − variables declared within a class, outside any method, with the static keyword
Encapsulation – keep implementation private and seperate from interface
Polymorphism – different objects, same interface
Inheritance – hierarchical organization, share code, customize or extend behaviors