Variables & Data Types

Variables are how your program remembers things. Learn how Java stores numbers, text, and true/false values — and why types matter.

Learn Variables & Data Types in our free Java course — a beginner-friendly interactive lesson with worked examples, a practice exercise and a quick reference.

Part of the free Java course at LearnCodingFast — hands-on lessons with examples you run in your browser, plus practice exercises and a quick quiz.

A variable is a named container that holds a value . Your program uses variables to remember information — like a person's name, their age, or whether they're a student.

💡 Analogy: Think of variables as labeled jars in a kitchen. Each jar has a label (the variable name) and a specific shape (the type). A jar shaped for cookies can't hold soup — similarly, an int variable can't hold text.

Variables make your code readable and reusable . Instead of typing 25 everywhere, you type age — and if the age changes, you only update it in one place.

In Java, creating a variable has a strict format. You must tell Java three things:

In JavaScript or Python, you can put anything in any variable. In Java, you must declare the type upfront. This seems annoying at first, but it prevents bugs . If you accidentally try to store text in a number variable, the compiler catches it before your program runs.

Java has many types, but you really only need to know four to get started:

Start with just these four types. You can build entire programs with int , double , String , and boolean . Learn the other types later when you actually need them.

Java has exactly 8 primitive types . These store raw values directly in memory. You've already learned the main ones — here's the complete list:

Use int for whole numbers. Use double for decimals. Use boolean for yes/no. Use String for text. That covers 95% of cases.

String is the most commonly used reference type. It represents text — any sequence of characters wrapped in double quotes.

String Concatenation (joining text together):

Sometimes you have values that should never change — like the value of pi, tax rates, or maximum scores. Use the final keyword:

Constants use ALL_CAPS with underscores : MAX_USERS , TAX_RATE , DATABASE_URL . This makes them instantly recognizable in your code.

Sometimes you need to convert data from one type to another. Java has two kinds of casting:

💡 Analogy: Widening is like pouring water from a small cup into a big bucket — nothing is lost. Narrowing is like pouring from a bucket into a small cup — the overflow is gone forever.

Rules (Java will refuse to compile if you break these):

Conventions (not enforced, but everyone follows them):

Excellent work! You now understand Java's type system — the foundation of everything you'll build. You know the 4 essential types (int, double, String, boolean), how to declare constants, cast between types, and name variables like a professional.

Next up: Operators — learn arithmetic, comparison, and logical operations to make your programs calculate, compare, and decide.

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