Author: kpark

DataTypeTest
Boolean
Char
String
Numeric Type: byte, short, int, long, float, double
Type Conversion
Enum

https://www.baeldung.com/java-permgen-metaspace

Permgen (JDK7 이전)

Metaspace (JDK8 이후)

클래스의 정보가 로드되는 영역, 정적 멤버 (static member) 저장, 바이트코드나 JIT 정보 저장

https://www.baeldung.com/java-stack-heap

//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

int i = 10;
System.out.println(“i++=” + (i++)); // prints 10 and then increments
System.out.println(“i=” + i); // prints 11

int j = 20;
System.out.println(“++j=” + (++j)); // increments and then prints 21
System.out.println(“j=” + j); // prints 21

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);

}

}

https://docs.oracle.com/javase/tutorial/java/nutsandbolts/operators.html

 

Operators	Precedence
postfix	expr++ expr--
unary	++expr --expr +expr -expr ~ !
multiplicative	* / %
additive	+ -
shift	<< >> >>>
relational	< > <= >= instanceof
equality	== !=
bitwise AND	&
bitwise exclusive OR	^
bitwise inclusive OR	|
logical AND	&&
logical OR	||
ternary	? :
assignment	= += -= *= /= %= &= ^= |= <<= >>= >>>=