Java Nested Classes

Java Inner & Static Nested Classes: A Concise Guide

Java offers two types of nested classes: inner classes and static nested classes. The key difference lies in their relationship with the enclosing class.

Inner Classes: These have implicit access to the enclosing class's members (instance variables and methods), even private ones. They are instances tied to a specific instance of the outer class.

class Outer {
    private int x = 10;

    class Inner {
        void printX() { System.out.println(x); }
    }
}

Outer outer = new Outer();
Outer.Inner inner = outer.new Inner(); //Inner instance requires Outer instance
inner.printX(); // Accesses Outer's private x

Static Nested Classes: These behave like regular top-level classes, lacking implicit access to the enclosing class's instance members. They are accessed using the enclosing class name.

class Outer {
    private int x = 10;

    static class Nested {
        //Cannot directly access Outer's x
        void printSomething(){ System.out.println("Hello");}
    }
}

Outer.Nested nested = new Outer.Nested();
nested.printSomething();

Spring Boot Context: In Spring Boot, static nested classes are often preferred for utility classes or configurations that don't need access to the enclosing class's state. Inner classes might be used for helper classes tightly coupled to the enclosing class's instance. Choose based on the class's dependencies and lifecycle.

In short: Use inner classes when needing access to the outer class's instance members; use static nested classes for independent utility classes. Avoid unnecessary nesting for better code clarity and maintainability.