02 — Variables & Types

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▶️ java pkg1core/core2Variables.java · java pkg1core/core3DataTypes.java

Variables store data

A variable is a named container for a value.

1int age = 30;2double price = 19.99;3boolean active = true;4String name = "Ravi";

The 8 primitive types

Primitives store raw values directly (fast, no object overhead).

💡 Use int for integers and double for decimals unless you have a specific reason not to.

Reference types

Everything that is not a primitive is an object (stored on the heap; variable holds a reference).

1String greeting = "Hello";     // String object2int[] numbers = {1, 2, 3};     // array object3Scanner sc = new Scanner(System.in);

Wrappers & autoboxing

Each primitive has a wrapper class for use in generics/collections:

1Integer boxed = 42;    // autoboxing: int → Integer2int unboxed = boxed;   // unboxing: Integer → int

⚠️ Gotcha: Integer cache for -128..127 — Integer.valueOf(127) == Integer.valueOf(127) is true, but 128 is false.

Variable kinds

`var` — type inference (Java 10+)

1var message = "Hello";        // inferred as String2var list = new ArrayList<String>();

Rules: local variables only, must have an initializer, still statically typed.

Casting & overflow

1long big = 9_000_000_000L;2int small = (int) big;           // narrowing cast — may lose data3 4int max = Integer.MAX_VALUE;5System.out.println(max + 1);     // overflows to Integer.MIN_VALUE

Practice

1. Run core2Variables and core3DataTypes.
2. Declare one variable of each primitive type and print them.
3. Try var for a List<String>.

Interview drill → 01 Core Java (questions 1–7)

Next → 03 Operators & Casting