2.3 Data Types, Primitives & Wrapper Classes

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Java is a statically typed language. Every variable must have a declared type before compilation. Java's type system divides into 8 Primitive Types and Reference Types.

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1. The 8 Primitive Types in Java

Type	Size (Bits / Bytes)	Min Value	Max Value	Default Value	Wrapper Class
byte	8 bits (1 byte)	-128	127	0	Byte
short	16 bits (2 bytes)	-32,768	32,767	0	Short
int	32 bits (4 bytes)	-2³¹	2³¹ - 1 (~2.14B)	0	Integer
long	64 bits (8 bytes)	-2⁶³	2⁶³ - 1	0L	Long
float	32 bits (4 bytes)	IEEE-754 single	IEEE-754 single	0.0f	Float
double	64 bits (8 bytes)	IEEE-754 double	IEEE-754 double	0.0d	Double
char	16 bits (2 bytes)	\u0000 (0)	\uffff (65,535)	\u0000	Character
boolean	JVM dependent (1-8b)	false	true	false	Boolean

Financial Calculations: Avoid float/double

Never use float or double for currency or exact financial balances due to floating-point rounding inaccuracies. Always use java.math.BigDecimal.

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2. Autoboxing & The Integer Cache Trap

Java compiler automatically converts between primitives and their wrapper classes:

Integer a = 100; // Autoboxing: Integer.valueOf(100)
int b = a;       // Unboxing: a.intValue()

The Integer Cache Gotcha (-128 to 127)

Java caches Integer objects between -128 and 127 (per JLS §5.1.7):

Integer x = 127;
Integer y = 127;
System.out.println(x == y); // true (Both refer to same cached instance)

Integer p = 128;
Integer q = 128;
System.out.println(p == q); // false (Two separate Heap objects!)
System.out.println(p.equals(q)); // true (Compares underlying int value)

Best Practice

Always compare object wrappers using .equals(), never ==.