Month: April 2025

Static Initializer Block

• class 로딩 시 호출
• instance variable이나 method에 접근 못함
• static variable 초기화에 사용

public class StaticIntializerBlockTest {
    private static int id = 5;
    static {
        if (id <10) id = 20;
        else id = 100;
    }

    public static int getId() {
        return id;
    }

    public static void print() {
        System.out.println("StaticIntializerBlockTest.id=" + getId());
    }
}

public class StaticInstanceInitializerBlockTest {

    public static void main(String[] args) {
        StaticIntializerBlockTest.print();
    }
}

StaticIntializerBlockTest.id=20 // static block 이 호출되면서 20으로 셋팅

Instance Initializer Block

• 객체 생성시 호출
• super 생성자 이후에 실행하고, 생성자보다 먼저 실행
• instance variable이나 method에 접근 가능
• 모든 생성자의 공통 부분을 instance initializer block에 넣어줌

class InstanceInitializerBlockSuper {
    public InstanceInitializerBlockSuper() {
        System.out.println("InstanceInitializerBlockSuper");
    }
}

public class InstanceInitializerBlockTest extends InstanceInitializerBlockSuper {
    private int id = 5;
    {
        if (id <10) id = 20;
        else id = 100;
    }

    public InstanceInitializerBlockTest() {
        System.out.println("InstanceInitializerBlockTest.id=" + this.id);
    }

    public InstanceInitializerBlockTest(int id) {
        System.out.println(“InstanceInitializerBlockTest.id=" + this.id);
        this.id = id;
    }

    public int getId() {
        return id;
    }

    public void print() {
        System.out.println("StaticIntializerBlockTest.id=" + getId());
    }
}

public class StaticInstanceInitializerBlockTest {
    public static void main(String[] args) {
        InstanceInitializerBlockTest i = new InstanceInitializerBlockTest();
        i.print();
        i = new InstanceInitializerBlockTest(30);
        i.print();
    }
}

InstanceInitializerBlockSuper // super 생성자 이후에 실행
InstanceInitializerBlockTest.id=20 // instance block 호출되면서 20으로 셋팅
id=20
InstanceInitializerBlockSuper // super 생성자 이후에 실행
InstanceInitializerBlockTest.id=20 // instance block 호출되면서 20으로 셋팅된후 this.id = id를 통해서 30으로 셋팅
id=30

https://stackoverflow.com/questions/12550135/static-block-vs-initializer-block-in-java

BankAccount-InstanceStatic – instance vs static member field & method
BankAccount-InstanceStatic

import java.util.*;
public class BankAccount {
    private int balance;
    private static int interestRate; // (%)

    public BankAccount() { 
        balance = 0; 
        interestRate = 5; 
    }

    public void deposit(int amount) { 
        balance += amount;
    }
   
    public void withdrawl(int amount) { 
        balance -= amount; 
    }

    public void print() { 
        System.out.println("final balance=" + 
	        (balance + balance * interestRate * 0.01) +
		" balance=" +
		balance +
		" interest=" +
		(balance * interestRate * 0.01));
		printInterestRate(); 
    }

    public static void setInterestRate(int interestRate) { 
        BankAccount.interestRate = interestRate; 
    }

    public static void printInterestRate() { 
        System.out.println("interestRate=" +
        interestRate); 
    }

    public int getUserInputInt() {
        Scanner input = new Scanner(System.in);
        System.out.print("Please enter Integer value: ");
        int value = input.nextInt();
        return value;
    }

    public static void main(String[] args) {
        BankAccount b = new BankAccount();
        b.deposit(2000);
        b.withdrawl(1000);
        b.print();
        BankAccount.printInterestRate();

        int amount = b.getUserInputInt();
        b.deposit(amount);
        b.print();
    }
}

ValueClass-InstanceStatic

Instance member field: int value

Static member field: static int count

public class ValueClass {
    private int value = 0; // instance field
    private static int count = 0; // static field
    static Scanner input = new Scanner(System.in); // static field

    public ValueClass() { 
	this(0); 
    }

    public ValueClass(int value) { 
        this.value = value;
        count++;
    }

    public void setValue(int value) { // instance method
        this.value = value; 
    }

    public int getValue() { // instance method
        return this.value; 
    }

    public void print() { // instance method
        System.out.println("value=" + this.value + " # of value=" + count);
    }

    public static int getCount() { // static method
        return count;
    }

    public static void printString(String str) { // static method
        System.out.print(str);
    }

    public static int getUserInputInt() { // static method
        int value = input.nextInt();
        return value;
    }

    public static void main(String[] args) {
        ValueClass v = new ValueClass();
        v.print();
        v.setValue(2000);
        int value = v.getValue();
        v.print();

        ValueClass.printString("Please enter the integer value: ");		
        int val = ValueClass.getUserInputInt();
        v.setValue(val);
        v.print();

        ValueClass v2 = new ValueClass(300);
        v2.print();

        ValueClass[] vArray = new ValueClass[3];
        for (int i=0; i<3; i++) {
            ValueClass v3 = new ValueClass();
            ValueClass.printString("Please enter the integer value: ");
            v3.setValue(ValueClass.getUserInputInt());
            vArray[i] = v3;
        }
    }
}

PersonTest-InstanceStatic

PersonTest-InstanceStatic – instance member field & method vs static member field & method

public class Person {
    String name; // instance member field
    int age; // instance member field
    
    public Person() { // default constructor
        this("JAVA", 20); // call Person(String n, int a);
    }

    public Person(String name, int age) { // constructor
        this.name = name;
        this.age = age;
    }

    public void setName(String name) { // instance method
        this.name = name;
    }

    public void setAge(int age) { // instance method
        this.age = age;
    }

    public String getName() { // instance method
        return name;
    }

    public int getAge() { // instance method
        return age;
    }

    public void print() { // instance method
        System.out.println("Person Name=" + name + " Age=" + age);
    }

    @Override
    public String toString() { // instance method
        return "Person Name=" + name + " Age=" + age;
    }
}

public class Person2 {
    static String name; // static member field
    static int age; // static member field

    public Person2() { // default constructor
        this("JAVA", 20); // call Person2(String n, int a);
    }

    public Person2(String n, int a) { // constructor
        name = n;
        age = a;
    }

    public static void setName(String n) { // static method
        name = n;
    }

    public static void setAge(int a) { // static method
        age = a;
    }

    public static String getName() { // static method
        return name;
    }

    public static int getAge() { // static method
        return age;
    }

    public static void print() { // static method에서는 this 사용 불가
        System.out.println("Person Name=" + name + " Age=" + age);
    }

    @Override
    public String toString() {
        return "Person Name=" + name + " Age=" + age; // instance method에서는 static member variable 접근 가능
    }
}

public class Person3 {
    static String name; // static member field
    static int age; // static member field

    private Person3() { // private default constructor
    }

    public static void setName(String n) { // static method
        name = n;
    }

    public static void setAge(int a) { // static method
        age = a;
    }

    public static String getName() { // static method
        return name;
    }

    public static int getAge() { // static method
        return age;
    }

    public static void print() { // static method에서는 this 사용 불가
        System.out.println("Person Name=" + name + " Age=" + age);
    }
}

public class PersonTest {
    static Scanner input = new Scanner(System.in);

    public static void main(String[] args) {
        Person[] pArray = new Person[3]; 

        // 만약 Person 객체를 하나만 생성한 후 for문에서 공유해 사용할 경우  
        // 마지막으로 입력된 데이터로 모든 데이터값이 치환됨 
        Person p = new Person(); 
        for (int i = 0; i < 3; i++) {
            System.out.print("\n\nEnter Person name and age : ");
            p.setName(input.next()); // 입력정보
            p.setAge(input.nextInt()); // 입력정보
            p.Print(); 
            pArray[i] = p; //배열에 들어간 모든 원소는 동일한 p
        }
        System.out.println("pArray : " + Arrays.toString(pArray)); 

        Person[] pArray1 = new Person[3]; 
        // 아래와 같이 for문 안에 Person p = new Person()와같이 
        // 새로운 객체를 생성해야 각자 다르게 입력된 정보가 들어가게 됨 
        for (int i = 0; i < 3; i++) {
            Person p1 = new Person(); 
            System.out.print("\n\nEnter Person name and age : ");
            p1.setName(input.next()); // 입력정보
            p1.setAge(input.nextInt()); // 입력정보
            p1.Print();
            pArray1[i] = p1; // p1는 새로운 Person객체이므로 배열에서 각각 다른 원소가 들어간다  
        } 
        System.out.println("pArray1 : " + Arrays.toString(pArray1)); 

        // 만약 Person2 객체의 name과 age는 static이라서,  
        // static은 공유하는 것이므로, 마지막으로 입력된 데이터로 모든 데이터값이 치환됨 
        Person2[] pArray2 = new Person2[3]; 
        for (int i = 0; i < 3; i++) {  
            Person2 p2 = new Person2(); 
            System.out.print("\n\nEnter Person name and age : ");
            p2.setName(input.next()); // Person2.setName(input.next())와 동일
            p2.setAge(input.nextInt()); // Person2.setAge(input.nextInt())와 동일
            pArray2[i] = p2; // p2 객체의 name과 age는 static이므로 모두 공유하기때문에, 배열의 모든 원소는 마지막에 들어간 정보로 셋팅
        }
        System.out.println("pArray2 : " + Arrays.toString(pArray2));
 
        Person3[] pArray3 = new Person3[3]; // Person3 타입의 배열생성 OK 
        for (int i = 0; i < 3; i++) {  
            //Person3 p3 = new Person3(); // error Person3() has private access 
            System.out.print("\n\nEnter Person name and age : ");
            Person3.setName(input.next()); // p3.setName(input.next()) 사용을 못하게 함
            Person3.setAge(input.nextInt()); // p3.setAge(input.nextInt()) 사용을 못하게 함
            //pArray3[i] = p3; // error p3 객체를 생성할 수 없으므로 사용 불가능  
        } 
        Person3.print();

default (package private – access in other classes within the same package)

public (visible everywhere)

protected (only access in its own class, its subclass and other classes within the same package)

private (only access in its own class)

http://docs.oracle.com/javase/tutorial/java/javaOO/accesscontrol.html

Access Levels

Modifier	Class	Package	Subclass	World
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no modifier	Y	Y	N	N
private	Y	N	N	N

 

http://www.programmerinterview.com/index.php/java-questions/method-overriding-vs-overloading/

Method Overloading: 동일한 함수명에 매개변수가 다른 함수를 둘 이상 정의하는 것으로, 동일한 함수 기능을 수행하지만 다른 매개변수의 경우를 처리할 때 사용

//compiler error – can’t overload based on the
//type returned (one method returns int, the other returns a float):
//int changeDate(int Year) ;
//float changeDate (int Year);

//compiler error – can’t overload by changing just
//the name of the parameter (from Year to Month):
//int changeDate(int Year);
//int changeDate(int Month) ;

//valid case of overloading,
//since the methods have different number of parameters:
int changeDate(int Year, int Month) ;
int changeDate(int Year);

//also a valid case of overloading,
//since the parameters are of different types:
int changeDate(float Year) ;
int changeDate(int Year);

Method Overriding: 상속받은 파생 클래스에서 동일한 함수명에 동일한 매개변수로 정의하여 함수를 재정의하는 것으로 상속되어진 함수의 기능을 변경해서 재사용하고 싶을 때 사용

public class Parent {
    public int someMethod() {
        return 3;
    }
}

public class Child extends Parent{

    // this is method overriding:
    public int someMethod() {
        return 4;
    }
}

Person Struct (in C) PersonStructC

Person Class (in C++) PersonClassCPlusPlus

Person Class (in C#) PersonClassCSharp

Person Class (in Java) PersonClassJava

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