Introduction

In the dynamic realm of Java programming, two powerful constructs, Anonymous Inner Classes, and Lambda Expressions, stand out for their ability to enhance code flexibility. Both serve the purpose of implementing functional interfaces, but they differ significantly in their syntax, use cases, and behavior. Let’s delve into a detailed comparison to uncover the distinctions between these two essential features.

Anonymous Inner Class: A Closer Look

Definition and Purpose: An Anonymous Inner Class is essentially an inner class without a name, instantiated at the point of creation. It shines when you need a one-time-use class, particularly when overloading methods of a class or interface is necessary without the need to create a dedicated subclass.

Use Cases:

• Creating implementation classes for listener interfaces, often employed in graphics programming.
• Instantiating an object with added functionalities, especially when subclassing seems impractical.

Syntax:

Test t = new Test() {
   // Data members and methods
   public void test_method() {
      // Implementation
   }   
};

Example:

interface Age {
    int x = 21;
    void getAge();
}

class AnonymousDemo {
    public static void main(String[] args) {
        Age obj = new Age() {
            public void getAge() {
                System.out.print("Age is " + x);
            }
        };
        obj.getAge();
    }
}

Output:

Age is 21

Characteristics:

1. Can extend abstract and concrete classes.
2. Allows the implementation of interfaces with any number of abstract methods.
3. Can declare instance variables.

Memory and Compilation:

• Generates a separate .class file at compile time.
• Memory allocation occurs on-demand when creating an object.

Lambda Expressions: Embracing Conciseness

Definition and Purpose: Lambda Expressions, introduced in Java 8, are concise and expressive representations of instances of functional interfaces. They facilitate treating functionality as a method argument and bring a functional programming paradigm to Java.

Use Cases:

• Implement a functional interface using a single abstract method.
• Simplifying code, especially in scenarios involving event listeners.

Syntax:

FuncInterface fobj = (int x) -> System.out.println(2 * x);

Example:

interface FuncInterface {
    void abstractFun(int x);

    default void normalFun() {
        System.out.println("Hello");
    }
}

class Test {
    public static void main(String args[]) {
        FuncInterface fobj = (int x) -> System.out.println(2 * x);
        fobj.abstractFun(5);
    }
}

Output:

10

Characteristics:

1. Cannot extend abstract or concrete classes.
2. Limited to implementing interfaces with a single abstract method.
3. Does not allow the declaration of instance variables; variables act as local variables.

Memory and Compilation:

• No separate .class file generated at compile time.
• Resides in the permanent memory of the JVM.

Table of Difference:

Feature	Anonymous Inner Class	Lambda Expression
Nature	Class without a name	Method without a name (anonymous function)
Inheritance	Can extend abstract and concrete classes	Cannot extend abstract or concrete classes
Interface Implementation	Can implement an interface with any methods	Can implement an interface with a single method
Variable Declaration	Allows declaration of instance variables	No instance variable declaration allowed
Instantiation	Can be instantiated	Cannot be instantiated
“this” Reference	Refers to the current anonymous class object	Refers to the current outer class object
Handling Methods	Suitable for handling multiple methods	Best choice for handling a single interface method
Compilation Output	Generates a separate .class file	No separate .class file generated
Memory Allocation	On-demand when creating an object	Resides in permanent memory of the JVM

Conclusion

In the vast landscape of Java programming, choosing between Anonymous Inner Classes and Lambda Expressions depends on the specific requirements of your code. While Anonymous Inner Classes offer flexibility and versatility, Lambda Expressions bring concise and expressive code to the forefront. Understanding the nuances between these constructs empowers developers to make informed decisions, ultimately enhancing the quality and readability of their code.