Java Data Types

Java has primitive types (raw values) and reference types (objects).

The eight primitives

Note the suffixes: L for long, f for float. Without them, 15000000000 is an int (overflow) and 5.75 is a double (type error).

Defaults: use int for whole numbers and double for decimals unless you have a reason not to.

Reference types

Everything else — String, arrays, and every class — is a reference type. Its default value is null.

String name = "Sita";
int[] marks = {88, 74, 91};
Scanner sc = new Scanner(System.in);

Type casting

Widening (small → large) happens automatically:

int i = 100;
long l = i;      // fine
double d = l;    // fine

Narrowing (large → small) must be explicit and can lose data:

double d = 9.78;
int i = (int) d;   // 9 — the decimal part is discarded, not rounded

Integer division

A classic bug: dividing two ints gives an int.

System.out.println(5 / 2);        // 2   ← not 2.5
System.out.println(5 / 2.0);      // 2.5
System.out.println((double) 5 / 2); // 2.5

Wrapper classes

Each primitive has an object counterpart, needed when a class requires an object (collections, generics):

int → Integer, double → Double, char → Character, boolean → Boolean, and so on.

Integer boxed = 42;         // autoboxing
int unboxed = boxed;         // auto-unboxing

int parsed = Integer.parseInt("123");
double d = Double.parseDouble("3.14");
String s = String.valueOf(42);

Beware: an Integer can be null, and unboxing a null throws a NullPointerException.