Type Casting & Conversion

In Plain English

Type casting is when you assign a value of one primitive data type to another type. Java supports Widening Casting (automatic) when moving to a larger type, and Narrowing Casting (manual) when moving to a smaller type where data loss could occur.

The Mental Model

"Widening casting is pouring a small glass of water into a giant bucket: nothing spills. Narrowing casting is pouring a giant bucket into a small cup: you must manually confirm and accept that excess water will overflow."

Deep Dive: How It Works

Widening Casting (Automatic): byte -> short -> char -> int -> long -> float -> double. Safe, no precision loss.

Narrowing Casting (Explicit): double -> float -> long -> int -> char -> short -> byte. Requires explicit syntax `(type) value`.

String conversion: `Integer.parseInt("123")`, `Double.parseDouble("45.67")`, `String.valueOf(100)`.

Syntax Blueprint

int myInt = 9;
double myDouble = myInt; // Widening (automatic)
int backToInt = (int) myDouble; // Narrowing (manual)

Place target type in parentheses for manual narrowing cast: (int) 3.99 becomes 3.

Core Rules to Remember

Truncation on Narrowing: (int) 9.99 truncates the decimal part to 9, not rounding.

String Parsing: Use Integer.parseInt("42") to convert numeric text into real ints.

Preventing Division Bugs: Cast operands to double: (double) a / b.

Common Beginner Traps & How to Fix Them

Mistake:Assuming (int) rounds up or down: (int) 7.89 yields 7

Why it happens: Narrowing cast simply chops off the decimal digits.

How to fix: Use Math.round(7.89) if you want standard rounding before casting.

Pro Tip: Use String.valueOf(number) when you need to safely convert any primitive into a String.

Live Interactive Example

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Widening and Narrowing In Action

Java 13

Output Console

Click "Run Code" to view the rendered output.

How it works: (int) 49.99 truncates to 49. Integer.parseInt turns "150" into 150 so it can be added to 50.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Convert string price to int cents

Parse the string "25.75" into a double named "price".
Calculate total cents: (int) (price * 100).
Print "Cents: " + cents.

Sandbox Output

Click "Run & Check" to test your solution.

Finished reading and practicing?

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Deep Dive: How It Works

Widening Casting (Automatic): byte -> short -> char -> int -> long -> float -> double. Safe, no precision loss.

Narrowing Casting (Explicit): double -> float -> long -> int -> char -> short -> byte. Requires explicit syntax `(type) value`.

String conversion: `Integer.parseInt("123")`, `Double.parseDouble("45.67")`, `String.valueOf(100)`.