memory.md

Memory

Basics

Bit - 0 or 1

Byte - 8 bits

Mega Byte - 1 million or $$2^{20}$$ bytes

Giga Byte - 1 Billion or $$2^{30}$$ bytes

{% hint style="success" %} Old Machines, we used to assume 32 bits with 4 byte pointers

Modern Machines, we used to assume 64 bits with 8 byte pointers

• Can address more memory
• Pointers uses more memory {% endhint %}

Typical memory usage of primitive types and arrays

Type	Bytes
boolean	1
byte	1
char	2
int	4
float	4
long	8
double	8

For one Dimensional arrays ( N )

Note: Over head of 24 bytes

Type	Bytes
char[]	2N + 24
int[]	4N + 24
double[]	8N + 24

For two Dimensional arrays ( M * N )

Type	Bytes
char[][]	2MN + 24
int[][]	4MN + 24
double[][]	8MN + 24

Java Objects

Object overhead - 16 bytes

Object Reference - 8 bytes

Padding - Each object uses a multiple of 8 bytes

{% tabs %} {% tab title="Date" %}

/**
* Memory Calculation
* Object Overhead + day + month + year + padding 
* 16 + 4 + 4 + 4 + 4 = 32 bytes
*/
public class date {
  private int day;      // 4 bytes
  private int month;    // 4 bytes
  private int year;     // 4 bytes 
} 

{% endtab %}

{% tab title="String" %}

/**
* Memory Calculation
* Object Overhead + value + offset + count + hash + padding 
* 16 + (8 + 2N + 24) + ( 4 + 4 + 4 ) + 4 = 2N + 64 bytes
*/
public class string {

    private char[] value; // Reference to array : 8 + ( 2N + 24 )
    private int offset;   // 4 bytes
    private int count;    // 4 bytes
    private int hash;     // 4 bytes
    
}

{% endtab %} {% endtabs %}