Module 11 - Nested & Inner Classes

What You Will Learn

Kind	Keyword	Has outer `this`?	When to use
Static nested class	`static class Foo` inside a class	No	Logically grouped with outer; no outer-instance needed
Non-static inner class	`class Foo` inside a class	Yes	Tightly coupled to outer instance; rare in modern Java
Local class	`class Foo` inside a method	Yes (if non-static method)	One-off implementation local to a block
Anonymous class	`new Interface() { ... }`	Yes (if non-static context)	Single-use; largely replaced by lambdas in Java 8+

Memory Model

Static Nested Class                   Inner (Non-static) Class
───────────────────                   ────────────────────────
  OuterClass                            OuterClass instance
  ├─ static fields                      ├─ fields
  └─ NestedClass (static)               └─ InnerClass (non-static)
       no link back to outer                  hidden field: outer$this
                                              can access outer.privateField

The Four Kinds

class Outer {
    // 1. Static nested - no outer instance needed
    static class StaticNested { }

    // 2. Inner (non-static) - carries a reference to Outer.this
    class Inner { void hi() { System.out.println(outerField); } }

    // 3. Local class - defined inside a method body
    void method() {
        class Local implements Runnable {
            public void run() { System.out.println("local"); }
        }
        new Local().run();
    }

    // 4. Anonymous class - instantiated inline, no name
    Runnable r = new Runnable() {
        public void run() { System.out.println("anon"); }
    };
}

When Lambda Replaces Anonymous Class

Anonymous class is REQUIRED when:               Lambda is PREFERRED when:
  • Implementing an interface with              • Exactly one abstract method
    multiple abstract methods                  • No need to store state in fields
  • Need to hold multiple fields               • No need for super/this reference
  • Need to call super                         • Conciseness matters
  • Need to reference 'this' (the anon obj)

// Pre-Java 8 - anonymous class
Collections.sort(list, new Comparator<String>() {
    public int compare(String a, String b) { return a.compareTo(b); }
});

// Java 8+ - lambda (Comparator is @FunctionalInterface)
list.sort(String::compareTo);

Source Files

File	What it Demonstrates
`StaticNestedDemo.java`	Static nested classes: Builder, Node (linked list), Entry (map)
`InnerClassDemo.java`	Non-static inner class: Iterator, event listener, outer access
`LocalAndAnonymous.java`	Local classes, anonymous classes, and when lambdas replace them
`NestedClassPatterns.java`	Real patterns: private static nested for encapsulation, builder, composite

Running

cd module-11-nested-classes
mvn test