1. Program and OS
- 1.1. Program
- 1.2. Operating System
2. Data representation
3. JVM(Java Virual Machine)
4. Hello SSAFY
5. Java IDE Intro
6. Variable and Data type
7. Operator
8. Control flow - condition
- 8.1. if
- 8.2. switch
9. Control flow - loop
10. Array
- 10.1. Declaration
- 10.2. Usage
11. Multidimensional Array
- 11.1. Declaration
- 11.2. Search

1. Program and OS

1.1. Program

the collection of commands to perfrom sepcific task

1.2. Operating System

Software that manage computer hardware and provide service and platform for application SW

2. Data representation

Binary data
1Byte as numerical value(==the first one is sign bit==)
- -128 ~ 127

3. JVM(Java Virual Machine)

Subjects executing Java Byte code
Java Byte Code
- the middle file(.class) between source code and binary code
Makes Java Byte code be able to excuted on every OS

4. Hello SSAFY

public class Hello {
    public static void main(String[] args){
        System.out.println("Hello SSAFY")
    }
}

javac Hello.java
- Hello.java -> Hello.class
java Hello
- excuting Hello.class
eclipse
- IDE(Integrated Developmend Environment)
- Support programming including upper procedures

5. Java IDE Intro

<package> // folder
    <library>
    <src> // source code folder
        <.java> //make class in the package.src
    <bin> // java byte code folder
        <.clss>

5-1. main method

where the code starts!!
fixed form public static void main(String[] args){}

5-2. comment

line comment: //
range comment: /* */
documentation comment: /** */
- explanation for the function

5-3. Print

System.out.

print: print
println: print and change the line

printf

public static void main(String[] args) {
    System.out.printf("%d \n", 10);     // 10
    System.out.printf("%o \n", 10);     // 12
    System.out.printf("%x \n", 10);     // a
    System.out.printf("%X \n", 10);     // A

    System.out.printf("%4d \n", 10);    //  10
    System.out.printf("%-4d \n", 10);   //10
    System.out.printf("%04d \n", 10);   //0010

    System.out.printf("%f \n", 10.1);   //10.100000
    System.out.printf("%.2f \n", 10.1); //10.10

    System.out.printf("%s \n", "Kim");  //Kim
    System.out.printf("%c \n", "O");    //O
}

6. Variable and Data type

6.1. Variable

name of the memory that contains the data
Rules
- Letters and numbers '$' and '_'
- can't start with numbers
- camelCase
  - for variable and functions
  - PascalCase: for class

6.2. Data Type (Primitive) <-> (Reference)

Primitive Type
- pre-fixed memory size
- variable itself has the data
- types
  - Bool
  - char(2B)
  - byte(1B), short(2B), ==int(4B)==, long(8B)
  - float(4B), ==double(8B)==
- declaration -> assgining
```
int age;
age = 30;

int age = 30; // initilazation = declaration + assigning
```

6.3. Type Casting

byte < short < int < long < float < double
- ==float is bigger than long==
Implicit Casting
- put smaller data type into bigger data type
Explicit Casting
- put bigger data type into smaller data type
- ```
int i = 100;
byte b = (byte)i;
```

7. Operator

Equality comparison operator
- ==: compare the value in the variable
  - ==not suitable for reference data type==
- equals(): compare the value in the object
  - ==suitable for reference data type==

8. Control flow - condition

8.1. if

public class example {
    public static void main(String[] args) {
        if(conditional statement) {
            statement to run;
        }else if(conditional statement2) {
            statement to run;
        }else {
            statement to run;
        }

        if(conditional statement)
            statement to run; // when there's only one statement {}can be skipped
            statement to run2; // it isn't included regardless of the index. Not Python!!!
    }
}

8.2. switch

public class example {
    public static void main(String[] args) {
        switch(variable) {
            case value1:
                statement;
                break   // break is not mandotry
                        // but from here all the statements under will be excuted until meet another break
            case value2:
                statement;
                break
            default:
                statement;
        }
    }
}

Example.

public class example {
    public static void main(String[] args) {

        int month = 12

        switch(month) {
            case 1:
            case 3:
            case 5:
            case 7:
            case 8:
            case 10:
            case 12:
                System.Out.println("31");
                break
            case 4:
            case 6:
            case 9:
            case 11:
                System.Out.println("30");
            case 2:
                System.Out.println("28");
                break
            default:
                System.Out.println("Wrong input");
        }
    }
}

9. Control flow - loop

9.1. if

public class example {
    public static void main(String[] args) {
        for (intialization; condition; Incre/Dcre){
            statement
        }
        // intialization, Incre/Decre can be expressed with more than 2 vairables!!

        for(int i = 0; i < 10; i++){
            System.outl.println(i); // 0 ~ 9
        }

        for(int i = 0, j = 10; i < 10; i += 2, j--){
            System.outl.println(i); // 0, 2, 4, 6, 8
            System.outl.println(j); // 10, 9, 8, 7, 6

        // multiplication table
        for (int i = 2; i <= 9; i++){
            for (int j = 2; j <= 9; j++)
                System.outl.printf("%d * %d = %d\n", i, j, i * j);
        }
    }
}

9.2. while

public class example {
    public static void main(String[] args) {
        while(conditional statement) {
            statement;
        } // while conditional statement is true

        do {
            statement;
        } while (conditional statement);
        // the statement is excuted at least once
    }
}

9.3. continue and break

can be used with ==label=

public class example {
    public static void main(String[] args) {
        label:
        for (int i = 2; i <= 9; i++){
            for (int j = 2; j <= 9; j++)
                System.outl.printf("%d * %d = %d\n", i, j, i * j);
                if (j==3){
                    continue label;
                }
        }
}

10. Array

Data structure for saving the same type of data
==The size is fixed==

It's an object == reference data

The data is saved in heap
The variable in stack has the address pointing the data

String c = "Hi";
String d = "Hi";
String e = new String("Hi");

System.out.println(c == d); //true. There's string pool in heap
System.out.println(c == e); //false
System.out.println(c.equals(d)); //true

can be referenced with index(int)

10.1. Declaration

Delacration

type[] variable

type variable[]

int[] iArr;
char[] cArr;
boolean[] bArr;
String[] strArr;
Date[] dateArr;

Creation

type[] variable = {v1, v2, v3, }
variable = new type[] {v1, v2, v3, }

variable = new type[len]

default value

public class array {
    public static void main(String[] args) {
        //1-dimensional array
        int[] score1;
        int score2[];

        score1 = {1, 2, 3, 4, 5}; // Impossible
        score1 = new int[] {1, 2, 3, 4, 5};

        int[] score2 = {1, 2, 3, 4, 5}

        int[] score3 = new int[5];
        score3[0] = 1;
        score3[1] = 2;
        score3[2] = 3;
        score3[3] = 4;
        score3[4] = 5;
     }
}

10.2. Usage

Arr[index]
Arr.length

for-each

access to the elements in the array

take the elements ==by copying it==

public class foreach{
    public static void main(String[] args) {
        int[] arr = {77, 50, 10, 12, 64, 15}

        for(int x : arr) {
            System.out.println(X); // 77, 50, 10, 12, 64, 15
        }
    }
}

Arrays.toString(arr)
- change the arr into '[v1, v2, v3, ...]'

System.arraycopy(Object src, int srcPos, Object dest, int destPos, int length)

   public class copy{
       public static void main(String[] args) {
           int[] arr = {77, 50, 10, 12, 64, 15}

           int[] tmp = new int[arr.length*2];

           System.arraycopy(arr, 0, tmp, 0, arr.length);
           //[77, 50, 10, 12, 64, 15, 0, 0, 0, 0, 0, 0]
       }
   }
   ```

11. Multidimensional Array

elements of the array have the address of another array

11.1. Declaration

Declaration
- int[][] iArr
- int iArr[][]
- int[] iARr[]