Understand the idea

A non-static inner class is associated with an enclosing object and can access its members. A static nested class is different: it does not require an enclosing instance. Use nesting when it clarifies the relationship, not just to shorten a file.

Explore the details

Java Inner Classes

In Java, it is also possible to nest classes (a class within a class). The purpose of nested classes is to group classes that belong together, which makes your code more readable and maintainable.

To access the inner class, create an object of the outer class, and then create an object of the inner class:

Private Inner Class

Unlike a "regular" class, an inner class can be private or protected. If you don't want outside objects to access the inner class, declare the class as private:

Static Inner Class

An inner class can also be static, which means that you can access it without creating an object of the outer class:

Access Outer Class From Inner Class

class OuterClass {
    int x = 10;

    class InnerClass {
        void show() {
            // Inner class accesses outer class field
            System.out.println("Outer x = " + x);
        }
    }

    static class StaticInner {
        void greet() {
            System.out.println("Hello from static inner class!");
        }
    }
}

// Regular inner class (needs outer instance)
OuterClass outer = new OuterClass();
OuterClass.InnerClass inner = outer.new InnerClass();
inner.show();   // Outer x = 10

// Static inner class (no outer instance needed)
OuterClass.StaticInner si = new OuterClass.StaticInner();
si.greet();     // Hello from static inner class!

One advantage of inner classes, is that they can access attributes and methods of the outer class:

A complete runnable example

This complete Main.java program focuses on the lesson idea. Reference fragments above may demonstrate individual statements; this example includes the imports, entry point, and supporting types it needs.

public class Main {
    public static void main(String[] args) throws Exception {
        Outer outer = new Outer();
        Outer.Inner inner = outer.new Inner();
        inner.show();
    }
}
class Outer {
    int value = 10;
    class Inner { void show() { System.out.println("Outer value: " + value); } }
}

Outer value: 10

Change it. Run it. Understand it.

Load this lesson example into the editor. Change one value or statement, predict the result, then press RUN to execute Java.

Make it your own

Your learning checkpoint

• Explain the example without reading the output first.
• Make the change requested in the practice task.
• Describe why the result changed—or why compilation failed.

Ready to move on? Follow the next lesson in course order, or return to the curriculum to review a prerequisite.