Lecture 02: Java Programming Basics (1)

<1.1 A "Hello, World!" program>

- In Java, everything is an object. An object is an instance of a class.

 

ex) A "Hello, World!" program

package cse3040;
 
public class HelloWorld {
 
    public static void main(String[] args) {
        System.out.println("Hello, World!");
        // TODO Auto-generated method stub
 
    }
 
}

• A class may contain methods. The main method is the entry point(starting point) of a program.
• It is declared as static, which means this method does not require an instance. entry point인 main method는 항상 static이다. (static 메소드 VS instance 메소드)
• There are four access modifier: public, protected, private, and nothing
• I can put multiple related classes in a package and can use hierarchical packages. ex)package sogang.cse.cse3040
• 한 줄 주석은 //, 여러 줄 주석은 /* ~~ */으로
• System.out is an instance of class PrintStream.
• Println is a method defined in the class PrintStream. This method is called "instance method", because it can be called using the instance of a class.

- We create an instance of a class using the keyword new.

ex) Random generator = new Random(); 으로 generator가 Random클래스의 instance가 된다.

<1.2 Primitive Types(원시 자료형)>

- In Java, most variables are objects(=instance of a class). still, Java has primitive types(원시 자료형).

- default값으로 정수는 int형, 실수는 double형이다.

- In Java, String is used much more frequently than char.

- In Java, you cannot convert boolean to integers.(1은 ture, 0은 false 아니다)

<1.3 Variables(변수)>

- The basic syntax of Java is very similar to C/C++.

ex) int total = 0, count; (multiple definitions in one line),

     Random generator = new Random(); (definition + class instantiation)

- Variable name must start with a letter. (A number is not allowed)

- Variable name is case-sensitive. (대소문자 구분한다)

- According to naming convention, a constant variable should be named using upper_case letters.(상수는 대문자로)

ex) final int DAYS_PER_WEEK = 7; (keyword final를 통해 상수선언)

 

cf) import할거 있을 때 커서 올리고 Ctrl+Shift+O하면 import 자동으로 해준다.

<EX 소스코드>

github.com/HoYoungChun/Java_Language_Lecture/tree/master/EX02

HoYoungChun/Java_Language_Lecture