How to access the outer class's field from a nested class?

Access to a non-static nested class (inner class) field of the outer class is done directly by the field name.

Access to a static nested class (static nested class) field of the outer class is only possible through a reference to an instance of the outer class, since the static nested class does not have an implicit connection to an instance of the outer class.

// Outer class
class Outer {
    private int outerField = 10;

    // Non-static nested class
    class Inner {
        void accessOuterField() {
            // Direct access to the outer class field
            System.out.println("From Inner class: " + outerField);
        }
    }

    // Static nested class
    static class StaticNested {
        // Does not have direct access to outerField
        void accessOuterField(Outer outerInstance) {
            // Access through a reference to an outer class object
            System.out.println("From StaticNested class: " + outerInstance.outerField);
        }
    }

    public static void main(String[] args) {
        Outer outer = new Outer();
        Outer.Inner inner = outer.new Inner();
        inner.accessOuterField(); // Output: From Inner class: 10

        Outer.StaticNested staticNested = new Outer.StaticNested();
        staticNested.accessOuterField(outer); // Output: From StaticNested class: 10
    }
}

To access an explicit outer class field when, for example, a nested class has a field with the same name, you can use the syntax Outer.this.fieldName.

class Outer {
    private int field = 10;

    class Inner {
        private int field = 20;

        void accessFields() {
            // Access to the nested class's field
            System.out.println("Inner field: " + field);
            // Access to the outer class's field
            System.out.println("Outer field: " + Outer.this.field);
        }
    }

    public static void main(String[] args) {
        Outer outer = new Outer();
        Outer.Inner inner = outer.new Inner();
        inner.accessFields();
        // Output:
        // Inner field: 20
        // Outer field: 10
    }
}