Inner Classes

JavaMidjava

The Question

What is an inner class in Java?

What a Strong Answer Covers

• class defined within another class
• member inner class
• static nested class
• local inner class
• anonymous inner class
• access to outer class members

Senior-Level Answer

An inner class is a class defined within the body of another class or interface. Java has four distinct kinds, each with different scoping and access rules.

**Non-static inner class** (member inner class): defined at the class level without `static`. It has an implicit reference to the enclosing instance and can access all members of the outer class, including private ones. Because of the implicit reference, you need an outer instance to create one: `Outer.Inner i = outer.new Inner()`. The implicit outer reference also means these classes are a common source of memory leaks — they prevent GC of the outer instance.

**Static nested class**: declared with `static`. It does not hold a reference to the enclosing instance. It can only access static members of the outer class directly. It is instantiated as `new Outer.Nested()`. This is the preferred default when inner class access to instance state is not needed, precisely because there is no implicit reference and no leak risk.

**Local class**: defined inside a method body. It can access local variables of the enclosing method, but only if they are effectively final. Scope is limited to the method.

**Anonymous class**: a one-off, nameless class defined inline, typically as an argument or assignment. Common before Java 8 for event listeners and callbacks — largely superseded by lambda expressions for single-abstract-method interfaces.

The key design guidance: prefer static nested classes over non-static inner classes unless you genuinely need access to the enclosing instance's state. Non-static inner classes are the primary source of hard-to-diagnose memory leaks in Android and long-lived Java applications — the outer object cannot be GC'd as long as the inner object is alive.

All four types can access `private` members of the outer class through compiler-generated synthetic accessor methods.

What Separates a 2/3 from a 3/3

Common Mistakes

• Conflating static nested classes with non-static inner classes — the implicit outer reference is the critical difference.
• Not knowing about the memory leak risk from non-static inner classes holding a reference to the outer instance.
• Thinking anonymous classes were replaced entirely by lambdas — lambdas only work for functional interfaces; anonymous classes still support multi-method interfaces.

Follow-Up Questions

• How can a non-static inner class cause a memory leak? — The inner class holds an implicit reference to the outer class instance, preventing GC even after the outer object is logically done.
• Why must local classes only access effectively final local variables? — The local class instance may outlive the method stack frame; a copy is captured, and mutation would cause inconsistency.
• When would you still use an anonymous class in Java 8+ instead of a lambda? — When implementing an interface with more than one abstract method, or when you need to maintain state between method calls.

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