Variables and Arrays

Variables in Java

1. Types

Primitive Types : Saves data literal. (boolean, byte, short, int, long, float, double, char) Reference Types : Saves address of the data. (String, etc…)

Ranges of various variable types

	byte	short	int	long
size	1 byte	2 bytes	4 bytes	8 bytes
range	-2^7 ~ 2^7-1	-2^15 ~ 2^15-1	-2^31~2^31	-2^63~2^63-1

	boolean	char	float	double
size	1 byte	2 bytes	4 bytes	8 bytes
range	true/false	0~2^16-1(\u0000~\uffff)	-1.4E-45~3.4E38	-4.9E-324~1.8E308

All java operation is done among ‘primitive’ types.
float and double have a concept of ‘accuracy’ denoting upto what digit the expression is hold accurate. (float is 7, double is 15). Though the size of float(double) is same as that of int(long), float(double) can express broader range of numbers by sacrificing its accuracy.

2. default values

	byte	short	int	long
default	0	0	0	0L

	boolean	char	float	double
default	false	\u0000	0.0f	0.0d

Arrays

1. Initialization

    int[] score = new int[5];

int[] score : defines a reference variable for the array. Array not yet made.
new int[5] : creates an array with five integers whose address is then assigned to ‘score’

2. Copying Arrays

System.arraycopy(arr, index_of_arr_to_start, newArr, index_of_newArr_to_start, arr.length);

public static void main(String[] args){
  char[] alphabet = {'A', 'B'};
  char[] number = {'1','2','3'};

  char[] sum = new char[alphabet.length + number.length];
  System.arraycopy(alphabet, 0, sum, 0, alphabet.length);
  System.arraycopy(number, 0, sum, alphabet.length, number.length);
}

3. N dimensional Arrays

String [4][1][5][9] arr : String array of 4 dimension.
int [3][4] numArr : int array of 2 dimension.

    String[][][][] arr = new String[][][][]; //Error! Array initializer expected
    String[][][][] arr = new String[4][][][]; //O.K

    int[][] numArr = new int[3][4];
    for(int[] i : numArr){
        for(int j : i){
            System.out.println("number=>"+j);
        }
    }

Note that when looping a multi-dimensional array, the reference variable holds the address of the outermost array as a reference. Hence for(int[] i : numArr).
Diagram explanation below.