What is a nested class and its types

💡 Concept: Nested Classes

Nested classes are classes defined within another class in Java, used to logically group classes and improve encapsulation.

📘 Quick Intro

Java supports static nested classes, inner classes, local classes, and anonymous classes as types of nested classes.

🧠 Analogy

Nested classes are like a toolbox within a toolbox: smaller tools (classes) grouped inside a larger tool (class) for better organization.

🔧 Technical Explanation

Static nested classes are like static members; do not require an instance of outer class.
Inner classes are associated with an instance of the outer class.
Local classes are defined within a method.
Anonymous classes have no name and are used for quick implementations.
Nested classes increase encapsulation and readability.

🎯 Use Cases

✅ Group helper classes within main classes.
✅ Implement event listeners or callbacks with anonymous classes.
✅ Improve modularity and encapsulation.

💻 Example: Nested Classes in Java


public class Outer {
    private int outerValue = 10;

    // Static nested class
    static class StaticNested {
        void display() {
            System.out.println("Inside static nested class");
        }
    }

    // Inner class
    class Inner {
        void display() {
            System.out.println("Outer value is " + outerValue);
        }
    }
}

public class TestNested {
    public static void main(String[] args) {
        Outer.StaticNested staticNested = new Outer.StaticNested();
        staticNested.display();

        Outer outer = new Outer();
        Outer.Inner inner = outer.new Inner();
        inner.display();
    }
}

❓ Interview Q&A

Q1: What is a nested class?
A: A class inside another class.

Q2: What types of nested classes are there?
A: Static nested, inner, local, anonymous.

Q3: How is static nested different from inner?
A: Static nested does not require outer instance.

Q4: Can nested classes access outer class members?
A: Inner classes can; static nested cannot.

Q5: What is an anonymous class?
A: A class without a name used for quick implementation.

Q6: Are nested classes recommended?
A: Yes, for encapsulation.

Q7: Can nested classes be static?
A: Yes.

Q8: Can nested classes implement interfaces?
A: Yes.

Q9: Are nested classes type-safe?
A: Yes.

Q10: How to instantiate inner classes?
A: Using outer.new Inner().

📝 MCQs

Q1. What is a nested class?

Class inside another class
Interface
Method
Variable

Q2. Types of nested classes?

Static only
Inner only
Static nested, inner, local, anonymous
None

Q3. Static nested vs inner?

Same
Static nested no outer instance
Inner no outer instance
Both need outer instance

Q4. Can nested classes access outer members?

Yes all
Inner yes, static no
No
Sometimes

Q5. What is anonymous class?

Class with name
Class without name
Interface
Enum

Q6. Are nested classes recommended?

No
Yes
Sometimes
Rarely

Q7. Can nested classes be static?

No
Yes
Only inner
Depends

Q8. Can nested classes implement interfaces?

No
Yes
Sometimes
Never

Q9. Are nested classes type-safe?

No
Yes
Maybe
Depends

Q10. How to instantiate inner classes?

Using new Inner()
Using outer.new Inner()
Using Outer.Inner()
Using Inner()

💡 Bonus Insight

Nested classes help organize code logically and improve encapsulation in complex applications.

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