Java primitive data types and variable declaration

Primitive Data Types

Java has eight primitive types — values stored directly on the stack, not as objects. Choosing the right type avoids silent overflow and wasted memory.

The Four You Will Use Most

int age = 30;           // 32-bit, range ±2.1 billion
long population = 8_000_000_000L; // 64-bit, suffix L required
double price = 19.99;  // 64-bit IEEE 754 decimal
boolean active = true; // true or false only

The Other Four

byte b = 127;           // 8-bit, range -128 to 127
short s = 32_000;       // 16-bit, range ±32,767
float f = 3.14f;        // 32-bit decimal, suffix f required
char c = 'A';           // 16-bit Unicode character

Key declaration rules:

• Declare type before name: int count = 0;

• Java is statically typed — you cannot assign a String to an int variable after declaration.

• Integer literals default to int; floating-point literals default to double.

• Use underscores in large numbers (1_000_000) for readability — the compiler ignores them.

Uninitialized local variables cause a compile error in Java. Class-level fields get zero-equivalent defaults (0, false, \u0000), but relying on defaults is considered poor practice because it obscures intent.

Widening vs narrowing: Java automatically widens smaller types to larger ones (e.g., int to long), but narrowing requires an explicit cast: (int) myLong. Narrowing truncates — data loss is possible and the compiler makes you acknowledge it.