Classes and primitives

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Goal

Read and run a tiny Java program that uses both a primitive type and an object type. After this page you will know the minimum vocabulary needed to read the code on later pages.

Prerequisites

• JDK 21 installed

The pieces

A class is Java's basic container for code. Every program lives inside at least one class. A class with a main method is something you can run.

Java has two kinds of values:

• Primitives — int, long, double, boolean, char. They hold the value directly and have no methods.
• Objects — instances of a class. String is the one you will use most. Objects have methods you call with a dot: name.length().

var is a Java keyword that lets the compiler figure out the type for a local variable. var name = "Mei"; is the same as String name = "Mei";. Use it when the type is obvious from the right-hand side.

For the full language spec, see the Java 21 tutorial. We only need the four lines below right now.

Code

Create a file called Hello.java:

public class Hello {
    public static void main(String[] args) {
        int year = 2026;
        var greeting = "hello from Java " + year;
        System.out.println(greeting);
    }
}

Run it (JDK 21 lets you run a single .java file with no compile step):

java Hello.java

Output:

hello from Java 2026

What each line does:

• int year = 2026; — a primitive integer.
• var greeting = ... — the compiler infers String because the right-hand side is a string concatenation.
• System.out.println(...) — call the println method on the out field of the System class.

For frontend developers

var here is the opposite of JavaScript's var: in Java it is a compile-time inference, not a runtime mutable binding. Think TypeScript's local type inference (let x = 1 infers number), not JavaScript's pre-let var.

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