Java Data Types

Primitive Data Types

Java has 8 built-in primitive data types. They are not objects and hold their values directly in memory.

Type	Size	Default	Range
`byte`	1 byte	0	-128 to 127
`short`	2 bytes	0	-32,768 to 32,767
`int`	4 bytes	0	-2,147,483,648 to 2,147,483,647
`long`	8 bytes	0L	-9.2Ã—10Â¹â¸ to 9.2Ã—10Â¹â¸
`float`	4 bytes	0.0f	~±3.4Ã—10³â¸ (7 decimal digits)
`double`	8 bytes	0.0d	~±1.7Ã—10³â°â¸ (15 decimal digits)
`char`	2 bytes	'\u0000'	0 to 65,535 (Unicode)
`boolean`	1 bit	false	true or false

Primitive Types Example

public class PrimitiveTypes {
    public static void main(String[] args) {
        byte   b  = 100;
        short  s  = 30000;
        int    i  = 2_000_000;   // underscores improve readability
        long   l  = 9_000_000_000L;
        float  f  = 3.14f;
        double d  = 3.141592653589793;
        char   c  = 'A';
        boolean flag = true;

        System.out.println("byte:    " + b);
        System.out.println("short:   " + s);
        System.out.println("int:     " + i);
        System.out.println("long:    " + l);
        System.out.println("float:   " + f);
        System.out.println("double:  " + d);
        System.out.println("char:    " + c);
        System.out.println("boolean: " + flag);
    }
}

Wrapper Classes

Every primitive type has a corresponding wrapper class in java.lang. Wrapper classes allow primitives to be used as objects (e.g., in collections).

Primitive	Wrapper Class	Useful Methods
byte	Byte	Byte.parseByte(), Byte.MAX_VALUE
short	Short	Short.parseShort()
int	Integer	Integer.parseInt(), Integer.MAX_VALUE, Integer.toBinaryString()
long	Long	Long.parseLong()
float	Float	Float.parseFloat()
double	Double	Double.parseDouble()
char	Character	Character.isLetter(), Character.toUpperCase()
boolean	Boolean	Boolean.parseBoolean()

Autoboxing and Unboxing

Autoboxing is the automatic conversion of a primitive to its wrapper class. Unboxing is the reverse — wrapper to primitive. Java does this automatically.

Autoboxing & Unboxing

import java.util.ArrayList;

public class Autoboxing {
    public static void main(String[] args) {
        // Autoboxing: int -> Integer
        int primitive = 42;
        Integer wrapped = primitive;  // compiler inserts Integer.valueOf(42)

        // Unboxing: Integer -> int
        int back = wrapped;           // compiler inserts wrapped.intValue()

        // Autoboxing in collections (collections only accept objects)
        ArrayList<Integer> list = new ArrayList<>();
        list.add(10);   // autoboxed to Integer.valueOf(10)
        int val = list.get(0);  // unboxed automatically

        System.out.println("Wrapped: " + wrapped);
        System.out.println("Back:    " + back);
        System.out.println("From list: " + val);
    }
}

Type Casting

Widening casting (implicit) converts a smaller type to a larger type automatically. Narrowing casting (explicit) requires a cast operator and may lose data.

Type Casting

public class TypeCasting {
    public static void main(String[] args) {
        // Widening: byte -> short -> int -> long -> float -> double
        int i = 100;
        long l = i;       // implicit widening
        double d = l;     // implicit widening
        System.out.println("Widening: " + d);  // 100.0

        // Narrowing: double -> int (explicit cast required)
        double pi = 3.99;
        int truncated = (int) pi;  // fractional part is dropped, NOT rounded
        System.out.println("Narrowing: " + truncated);  // 3

        // char <-> int
        char ch = 'A';
        int ascii = ch;           // widening: char to int
        char back = (char) 66;    // narrowing: int to char
        System.out.println("char to int: " + ascii);  // 65
        System.out.println("int to char: " + back);   // B
    }
}

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Java Data Types

Primitive Data Types

Wrapper Classes

Autoboxing and Unboxing

Type Casting

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