Author: kpark

/* command line arguments in C */
void main(int argc, char** argv)
{
    char name[128]; 
    int age = 0; 
    if (argc >= 2) { 
        strcpy(name, argv[1]); 
        age = atoi(argv[2]); 
        printf("%s, name = %s, age = %d\n", argv[0], name, age); 
    } 
}

~>person.exe Park 20

argv[0] = “person.exe”

argv[1] = “Park”

argv[2] = “20”

// command line arguments in Java
public class Person {
    public static void main(String[] args) {
        String name = ""; 
        int age = 0; 
        if (args.length >= 2) { 
            name = args[0]; 
            age = Integer.parseInt(args[1]); 
            System.out.printf("name = %s, age = %d\n", name, age); 
        } 
    }
}

~>java Person Park 30

args[0] = “Park”

args[1] = “30”

//http://javadevnotes.com/java-integer-to-string-examples
// int => String type conversion
int i = -10; // -10
String s1 = Integer.toString(i); // “-10”
String s2 = String.valueOf(i); // “-10”
String s3 = new Integer(i).toString(); // “-10”
String s4 = String.format(“%d”, i); // “-10”
System.out.println(“s1 = ” + s1);
System.out.println(“s2 = ” + s2);
System.out.println(“s3 = ” + s3);
System.out.println(“s4 = ” + s4);

//http://javadevnotes.com/java-string-to-integer-examples
// String => int type conversion
String x = “-123”; // “-123”
int y = Integer.parseInt(x); // -123 (integer로 변환이 가능한 경우만 변환 가능 그 외엔 run-time exception error)
int z = Integer.valueOf(x); // -123 (integer로 변환이 가능한 경우만 변환 가능 그 외엔 run-time exception error)
int w = new Integer(x).intValue(); // 123 (integer로 변환이 가능한 경우만 변환 가능 그 외엔 run-time exception error)
System.out.println(“x = ” + x);
System.out.println(“y = ” + y);
System.out.println(“z = ” + z);
System.out.println(“w = ” + w);

byte bVal = 127;
int iVal = 100;
System.out.println(bVal+iVal); // 127 + 100 = 227
System.out.println(10/4); // int/ int 2.5 => 2
System.out.println(10.0/4); // double / int => double divided by double => 2.5
System.out.println((char)0x12340041);
System.out.println((byte)(bVal+iVal)); // 127 + 100 => (byte)227 => 1110 0011 => 1 + 2 + 32 + 64 – 128 => -29
System.out.println((int)2.9 + 1.8); // 2 + 1.8 => 2.0 + 1.8 => 3.8
System.out.println((int)(2.9 + 1.8)); // (int)4.1 => 4
System.out.println((int)2.9 + (int)1.8); // 2 + 1 => 3

Implicit type conversion vs Explicit type conversion

int intVal = 32555;
byte byteVal = 25;
long longVal = intVal; // implicit type conversion
System.out.println(longVal); // 32555
intVal = (int) longVal; // explicit type conversion
System.out.println(intVal); // 32555
longVal = byteVal; // implicit type conversion
System.out.println(longVal); // 25
byteVal = (byte) longVal; // explicit type conversion
System.out.println(byteVal); // 25

Java’s byte is a signed 8-bit numeric type whose range is -128 to 127 (JLS 4.2.1). 233 is outside of this range; the same bit pattern represents -23 instead.

11101001 = 1 + 8 + 32 + 64 + 128 = 233 (int)
           1 + 8 + 32 + 64 - 128 = -23 (byte)

A byte plus a byte is an int in Java.

byte a = 1;
byte b = 2; 
byte c = a + b; // compile error (byte + byte = int)
byte c = (byte)(a + b); // corrected

Primitive Type vs Reference Type

8 Primitive Data Type
https://en.wikibooks.org/wiki/Java_Programming/Primitive_Types

Object Types

Reference Types

instance variable vs static variable vs local variable

public class Hello {

private String name; // instance variable
private static int value = 10; // static variable

public int sum(int n, int m) {

int result = n + m; // result : local variable
return result;

}

public static void main(String args[]) {

Hello h = new Hello();
h.name = “Hello”;
System.out.println(h.name);
System.out.println(Hello.value);

}

}

http://www.oracle.com/technetwork/java/codeconventions-135099.html

int             i;
char            c;
float           myWidth;

eclipse
eclipse-inst-win64