[CS 기초] Java

4-1. 실행-Hello World

• main 안에 코드 작성.그대로 동작이 됨

• 저장을 누르면, HelloWorldApp.class & HelloWorldApp.java 2개가 생성됨

• RUN을 누를 것.

  HELLO WORLD가 출력될 것

public class HelloWorldApp{
	public static void main(String[] args) {
		System.out.println("Hello World!");
	}
}

HelloWorldApp.java

• 인간이 이해할 수 있는 텍스트

HelloWorldApp.class

• 이상하게 깨진 문자.
• 컴퓨터가 읽도록 생성된 텍스트이므로!

4-2. JAVA의 동작원리

• IN : source=code=language

• f(x) : JAVA

• OUT : application / program

Java Source Code ( .java )

• 사람만이 이해할수 있음.

Java Application ( .class )

• 기계가 이해하기 위해서 전환(=compile)해줘야 함
• RUN을 누르면, JVM에서 컴퓨터에서 작동시킴

JVM (Java Virtual Machine)

Computer

5. Hello Java World

5-1. Desktop Application 만들기

import javax.swing.*;   
import java.awt.Dimension;
import java.awt.Toolkit;
public class HelloWorldGUIApp{
    public static void main(String[] args){
        javax.swing.SwingUtilities.invokeLater(new Runnable() {
            public void run() {
                JFrame frame = new JFrame("HelloWorld GUI");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.setPreferredSize(new Dimension(800, 300));
                JLabel label = new JLabel("Hello World!!", SwingConstants.RIGHT);
                frame.getContentPane().add(label);
                Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();
                frame.setLocation(dim.width/2-400/2, dim.height/2-300/2);

                frame.pack();
                frame.setVisible(true);
            }
        });
    }
}

6-1. 데이터와 연산

다양한 data type들

• Number
  • +, - , / , x
• String
  • length, substring, search
• etc…

6-2. 데이터 타입

public class Datatype{
	public static void main(String[] args) {
		System.out.println(6); // Number 
		System.out.println("six"); // String ( 큰따옴표 주의! )
		System.out.println("6"); // String
		
		System.out.println(6+6); // Number+Number
		System.out.println("6"+"6"); // String+String
		
		System.out.println("abcd".length()); // length of string
	}
}

6-3. 숫자와 연산

public class Number {
	public static void main(String[] args) {
		// Operator 
		System.out.println(6+2); // 8
		System.out.println(6-2); // 4
		System.out.println(6*2); // 12
		System.out.println(6/2); // 3
		
		System.out.println(Math.PI);  // 3.1415926353...
		System.out.println(Math.floor(Math.PI)); // 3.0 
		System.out.println(Math.ceil(Math.PI)); //  4.0
	}
}

6-4. 문자열의 표현

public class StringApp {
	public static void main(String[] args) {
		System.out.println("Hello World"); // String (n글자) - 큰 따옴표
		System.out.println('H'); // character (1글자) - 작은 따옴표
		System.out.println("Hello "
				+ "World");
		System.out.println("Hello \nWorld"); // n = new line
		System.out.println("Hello \"World\""); // escape
	}
}

6-5. 문자열 다루기

public class String_operation {
	public static void main(String[] args) {
		System.out.println("Hello World".length());
		System.out.println("Hello World".replace("Hello","Bye"));
	}
}

8-1. 변수의 정의

public class variable {
	public static void main(String[] args) {
		int a = 1; // datatype 지정해야
		System.out.println(a);
		
		double b = 1.1; // double = real number
		System.out.println(b);
		
		String c = "Hello World!";
		System.out.println(c);
	}
}

8-2. 변수의 효용

public class Letter {
	public static void main(String[] args) {
		String name="egoing";
		System.out.println("Hello,"+name+" ... "+name);
		
		double VAT = 10.0;
		System.out.println(VAT);
	}
}

8-3. Casting

converting datatype

public class Casting {
	public static void main(String[] args) {
		double a = 1.1;
		double b = 1;
		double b2 = (double) 1;
		int c = (int) 1.1;
		String d = Integer.toString(1);
		
		System.out.println(a);
		System.out.println(b);
		System.out.println(b2);
		System.out.println(c);
		System.out.println(d);
		System.out.println(d.getClass());
	}
}

9. IOT 프로그램 만들기

import org.opentutorials.iot.Elevator;
import org.opentutorials.iot.Security;
import org.opentutorials.iot.Lighting;

public class OKJavaGoingHome {
	public static void main(String[] args) {
		String id = "Java APT 507";
		
		// Elevator call
		Elevator myElevator = new Elevator(id);
		myElevator.callForUp(1);
		
		// Security off
		Security mySecurity = new Security(id);
		mySecurity.off();
		
		// Light on
		Lighting hallLamp = new Lighting(id+" / Hall Lamp");
		hallLamp.on();
		
		Lighting floorLamp = new Lighting(id+" / Floor Lamp");		
		floorLamp.on();
	}
}

11. 입력 & 출력

Input -> Program -> Output

Input

• Argument / File / Network / Audio / Program

Output

• Monitor / File / Audio / Program

import javax.swing.JOptionPane;

import org.opentutorials.iot.DimmingLights;
import org.opentutorials.iot.Elevator;
import org.opentutorials.iot.Security;
import org.opentutorials.iot.Lighting;

public class OKJavaGoingHomeInput {
	public static void main(String[] args) {
		String id = JOptionPane.showInputDialog("Enter a ID");
		String bright = JOptionPane.showInputDialog("Enter brightness");
		
		// Elevator call
		Elevator myElevator = new Elevator(id);
		myElevator.callForUp(1);
		
		// Security off
		Security mySecurity = new Security(id);
		mySecurity.off();
		
		// Light on
		Lighting hallLamp = new Lighting(id+" / Hall Lamp");
		hallLamp.on();
		
		Lighting floorLamp = new Lighting(id+" / Floor Lamp");		
		floorLamp.on();
		
		DimmingLights moodLamp = new DimmingLights(id+" moodLamp");
		moodLamp.setBright(Double.parseDouble(bright));
		moodLamp.on();	
	}
}

import javax.swing.JOptionPane;

import org.opentutorials.iot.DimmingLights;
import org.opentutorials.iot.Elevator;
import org.opentutorials.iot.Security;
import org.opentutorials.iot.Lighting;

public class OKJavaGoingHomeInput {
	public static void main(String[] args) {
		String id = args[0];
		String bright = args[1];
		
		// Elevator call
		Elevator myElevator = new Elevator(id);
		myElevator.callForUp(1);
		
		// Security off
		Security mySecurity = new Security(id);
		mySecurity.off();
		
		// Light on
		Lighting hallLamp = new Lighting(id+" / Hall Lamp");
		hallLamp.on();
		
		Lighting floorLamp = new Lighting(id+" / Floor Lamp");		
		floorLamp.on();
		
		DimmingLights moodLamp = new DimmingLights(id+" moodLamp");
		moodLamp.setBright(Double.parseDouble(bright));
		moodLamp.on();	
	}
}

12. Compile

javac -cp ".:lib" OkJavaGoingHome.java

• class 파일의 path :
  • 1) “.” : 현재 directory
  • 2) “lib” : lib 폴더
• ( 윈도우의 경우 :대신 ;로)

13. Input ( args )

command line에 명령어 실행할때, 2개의 input을 넣어줘야!

• args[0] & args[1]

• ex) java OKJavaGoingHomeInput "Java APT507" 15.0

14. Packages & Class

[ 기본 구조 ]

Package # 1

• Class # 1-1
  • variable & method로 구성
• Class # 1-2
  • variable & method로 구성

Package # 2

• Class # 2-1
  • variable & method로 구성

[ Class ]

• ex) Math 클래스

public class ClassApp {
	public static void main(String[] args) {
		System.out.println(Math.PI);
		System.out.println(Math.floor(1.6));
		System.out.println(Math.ceil(1.6));
	}
}

[ Instance ]

• ex) PrintWriter 는 constructor가 있다 -> instance를 만들 수 있다

• p1 : instance

import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.io.IOException;

public class InstanceApp {
	public static void main(String[] args) throws IOException {	
        
		PrintWriter p1 = new PrintWriter("result1.txt");
		p1.write("Hello 1");
		p1.close();
		
		PrintWriter p2 = new PrintWriter("result2.txt");
		p2.write("Hello 2");
		p2.close();
	}
}

15. 상속

기존에 있는 class를 물려 받으면서, 새로운거 추가!

java.long.Object ………….(a)

• java.io.Writer ………..(b)
  • java.io.PrintWriter ………(c)

(c)는 (b)를 상속받아서, (b)는 (a)를 상속받아서 만들어졌다.

16. 나만의 App 만들기

public class AccountingApp {
	public static void main(String[] args) {
		double valueOfSupply = Double.parseDouble(args[0]);
		double vatRate = 0.1;
		double expenseRate = 0.3;
		
		double vat = valueOfSupply*vatRate;
		double total = valueOfSupply+ vat;
		double expense = valueOfSupply*expenseRate;
		double income = valueOfSupply - expense;
		double dividend1 =income*0.5;
		double dividend2 =income*0.3;
		double dividend3 =income*0.2;
		
		System.out.println("Value of Supply : "+valueOfSupply);
		System.out.println("VAT : "+vat);
		System.out.println("Total : "+total);
		System.out.println("Expense : "+expense);
		System.out.println("Income : "+income);
		System.out.println("Dividend 1 : "+dividend1);
		System.out.println("Dividend 2 : "+dividend2);
		System.out.println("Dividend 3 : "+dividend3);
	}
}

eclipse말고 terminal에서 실행하기

• cd ~ ( java파일이 있는 경로로 이동 )
• javac AccountingApp.java
• java AccountingApp 33333.0

17. If문

public class AccountingIfApp {
	public static void main(String[] args) {
		double valueOfSupply = Double.parseDouble(args[0]);
		double vatRate = 0.1;
		double expenseRate = 0.3;
		
		double vat = valueOfSupply*vatRate;
		double total = valueOfSupply+ vat;
		double expense = valueOfSupply*expenseRate;
		double income = valueOfSupply - expense;
		
		double dividend1;
		double dividend2;
		double dividend3;
		
		if(income>10000.0) {
			dividend1 =income*0.5;
			dividend2 =income*0.3;
			dividend3 =income*0.2;
		}else {
			dividend1 =income*1.0;
			dividend2 =income*0.0;
			dividend3 =income*0.0;
		}
		
		System.out.println("Value of Supply : "+valueOfSupply);
		System.out.println("VAT : "+vat);
		System.out.println("Total : "+total);
		System.out.println("Expense : "+expense);
		System.out.println("Income : "+income);
		System.out.println("Dividend 1 : "+dividend1);
		System.out.println("Dividend 2 : "+dividend2);
		System.out.println("Dividend 3 : "+dividend3);
	}
}

18. Method

public class AccountingMethodApp {
	public static void double valueOfSupply;
	public static void double vatRate;
	private static double expenseRate;
	public static void main(String[] args) {
		valueOfSupply = 10000.0;
		vatRate = 0.1;
		expenseRate = 0.3;
		print();
	}

	private static void print() {
		System.out.println("Value of Supply : "+valueOfSupply);
		System.out.println("VAT : "+getVAT());
		System.out.println("Total : "+getTotal());
		System.out.println("Expense : "+getExpense());
		System.out.println("Income : "+getIncome());
		System.out.println("Dividend 1 : "+getDividend1());
		System.out.println("Dividend 2 : "+getDividend2());
		System.out.println("Dividend 3 : "+getDividend3());
	}

	private static double getDividend1() {
		return getIncome() *0.5;
	}
	private static double getDividend2() {
		return getIncome() *0.3;
	}
	private static double getDividend3() {
		return getIncome() *0.2;
	}

	private static void getIncome(){
		return valueOfSupply - getExpense();
	}

	private static void getExpense(){
		return valueOfSupply*expenseRate;
	}

	private static void getTotal(){
		return valueOfSupply+ getVAT();
	}

	private static double getVAT() {
		return valueOfSupply*vatRate;
	}
}

19. Class

class Accounting{
	public static void double valueOfSupply;
	public static void double vatRate;
	private static double expenseRate;
	
	private static void print() {
		System.out.println("Value of Supply : "+valueOfSupply);
		System.out.println("VAT : "+getVAT());
		System.out.println("Total : "+getTotal());
		System.out.println("Expense : "+getExpense());
		System.out.println("Income : "+getIncome());
		System.out.println("Dividend 1 : "+getDividend1());
		System.out.println("Dividend 2 : "+getDividend2());
		System.out.println("Dividend 3 : "+getDividend3());
	}

	private static double getDividend1() {
		return getIncome() *0.5;
	}
	private static double getDividend2() {
		return getIncome() *0.3;
	}
	private static double getDividend3() {
		return getIncome() *0.2;
	}

	private static void getIncome(){
		return valueOfSupply - getExpense();
	}

	private static void getExpense(){
		return valueOfSupply*expenseRate;
	}

	private static void getTotal(){
		return valueOfSupply+ getVAT();
	}

	private static double getVAT() {
		return valueOfSupply*vatRate;
	}
}


public class AccountingMethodApp {
	public static void main(String[] args) {
		Accounting.valueOfSupply = 10000.0;
		Accounting.vatRate = 0.1;
		Accounting.expenseRate = 0.3;
		Accounting.print();
	}
}

12. Instance

class Accounting{
	public static void double valueOfSupply;
	public static void double vatRate;
	private static double expenseRate;
	
	private static void print() {
		System.out.println("Value of Supply : "+valueOfSupply);
		System.out.println("VAT : "+getVAT());
		System.out.println("Total : "+getTotal());
		System.out.println("Expense : "+getExpense());
		System.out.println("Income : "+getIncome());
		System.out.println("Dividend 1 : "+getDividend1());
		System.out.println("Dividend 2 : "+getDividend2());
		System.out.println("Dividend 3 : "+getDividend3());
	}

	private double getDividend1() {
		return getIncome() *0.5;
	}
	private double getDividend2() {
		return getIncome() *0.3;
	}
	private double getDividend3() {
		return getIncome() *0.2;
	}

	private void getIncome(){
		return valueOfSupply - getExpense();
	}

	private void getExpense(){
		return valueOfSupply*expenseRate;
	}

	private void getTotal(){
		return valueOfSupply+ getVAT();
	}

	private double getVAT() {
		return valueOfSupply*vatRate;
	}
}

public class AccountingMethodApp {
	public static void main(String[] args) {
		// Instance a1 & a2
		Accounting a1 = new Accounting();
		Accounting a2 = new Accounting();
		
		a1.valueOfSupply = 10000.0;
		a1.vatRate = 0.1;
		a1.expenseRate = 0.3;
		a1.print();
		
		a2.valueOfSupply = 20000.0;
		a2.vatRate = 0.05;
		a2.expenseRate = 0.2;
		a2.print();
	}
}