Fundamentals

Java Fundamentals

Variables & Data Types

A variable is a named memory location used to store data.

java

1int age = 25;
2String name = "Akash";

Concept	Example	Meaning
Declaration	`int age;`	Creates a variable
Initialization	`age = 25;`	Assigns a value
Declaration + Initialization	`int age = 25;`	Both together
Reassignment	`age = 26;`	Changes the value

Java has 8 primitive data types.

Type	Size	Example	Default Value*
`byte`	8-bit	`byte x = 10;`	`0`
`short`	16-bit	`short x = 100;`	`0`
`int`	32-bit	`int x = 1000;`	`0`
`long`	64-bit	`long x = 100000L;`	`0L`
`float`	32-bit	`float x = 10.5f;`	`0.0f`
`double`	64-bit	`double x = 10.5;`	`0.0d`
`char`	16-bit	`char c = 'A';`	`'\u0000'`
`boolean`	JVM-dependent	`boolean flag = true;`	`false`

Note: Default values apply to instance/static variables, not local variables.

Reference variables store a reference to an object.

Type	Example
`String`	`String name = "Akash";`
Array	`int[] nums = {1, 2, 3};`
Class	`User user = new User();`
Interface	`List<Integer> list = new ArrayList<>();`
Enum	`Day day = Day.MONDAY;`

Primitive	Reference
Stores actual value	Stores reference to an object
8 built-in types	Classes, arrays, interfaces, enums, etc.
Cannot be `null`	Can be `null`
Generally more lightweight	Objects may require heap allocation
`int x = 10`	`String s = "Hello"`

Operators are symbols used to perform operations on variables and values.

Important: Integer division removes the decimal part.

java

1int x = 5 / 2;       // 2
2double y = 5 / 2;    // 2.0
3double z = 5 / 2.0;  // 2.5

Key interview point: && and || use short-circuit evaluation.

java

1int a = 5;
2int b = a++; // b = 5, a = 6

Short form of if-else.

java

1int max = a > b ? a : b;