Nested and Inner Class in Java

Nested and Inner class in Java is a class declared within the body of another class. Nested classes are a powerful feature of Java that can be used to improve the readability, maintainability, and performance of our code.

What is a nested class?

A nested class is a class that is declared within the body of another class. Nested classes are also known as inner classes. Nested classes can be static or non-static.

What are the benefits of using nested classes?

There are several benefits to using nested classes, including:

Improved readability and maintainability: Nested classes can help to improve the readability and maintainability of our code by logically grouping related classes together. This makes it easier to understand how the classes interact with each other.
Increased encapsulation: Nested classes can help to increase the encapsulation of our code by restricting access to the members of the outer class. This can help to protect our data from unauthorized access.
Improved performance: Nested classes can sometimes improve the performance of our code by allowing us to share data between the inner and outer classes.

What are the different types of nested classes in Java?

There are two main types of nested classes in Java:

Non-static nested classes (inner classes): Non-static nested classes are nested classes that are not declared static. This means that they cannot be accessed independently of the outer class.
Static nested classes: Static nested classes are nested classes that are declared static. This means that they can be accessed independently of the outer class.

Non-static nested classes (inner classes)

How to declare a non-static nested class?

To declare a non-static nested class, we use the following syntax:

class OuterClass {
    class InnerClass {
        // Inner class members
    }
}

How to access a non-static nested class?

To access a non-static nested class, we must first create an instance of the outer class. Then, we can access the inner class using the dot notation. For example:

OuterClass outerClass = new OuterClass();
OuterClass.InnerClass innerClass = outerClass.new InnerClass();

Benefits of using non-static nested classes

Non-static nested classes have several benefits, including:

They can access the members of the outer class, including private members.
They can be used to implement the strategy pattern.
They can be used to implement the observer pattern.

Static nested classes

How to declare a static nested class?

To declare a static nested class, we use the following syntax:

class OuterClass {
    static class InnerClass {
        // Inner class members
    }
}

How to access a static nested class?

To access a static nested class, we can use the dot notation without first creating an instance of the outer class. For example:

OuterClass.InnerClass innerClass = OuterClass.InnerClass.new();

Benefits of using static nested classes

Static nested classes have several benefits, including:

They can be accessed independently of the outer class.
They can be used to implement utility classes.
They can be used to implement singleton classes.

Other types of nested classes

In addition to non-static nested classes and static nested classes, there are two other types of nested classes in Java:

Method local inner classes: Method local inner classes are nested classes that are declared within the body of a method.
Anonymous inner classes: Anonymous inner classes are nested classes that are created without a name.

Conclusion

Nested classes are a powerful feature of Java that can be used to improve the readability, maintainability, and performance of our code. There are several different types of nested classes, each with its own benefits. By understanding the different types of nested classes and how to use them, we can write more effective and efficient Java code.

Summary of the key points

Nested classes are classes that are declared within the body of another class.
There are two main types of nested classes in Java: non-static nested classes (inner classes) and static nested classes.
Nested classes can be used to improve the readability, maintainability, and performance of our code.
There are several other types of nested classes in Java, including method local

FAQs

What is a nested class in Java?

A nested class is a class that is declared inside another class. Nested classes can be either static or non-static.

What is an inner class in Java?

An inner class is a non-static nested class. Inner classes have access to the private members of the outer class.

What are some common mistakes to avoid when using nested and inner classes in Java?

Following are some common mistakes to avoid when using nested and inner classes in Java:

Not understanding the difference between nested and inner classes: Nested classes can be either static or non-static, while inner classes are always non-static. Inner classes have access to the private members of the outer class, while nested classes do not.

Using nested and inner classes unnecessarily: Nested and inner classes should only be used when they offer a clear benefit. Using nested and inner classes unnecessarily can make your code more difficult to read and maintain.

Not using nested and inner classes correctly: There are some specific rules that must be followed when using nested and inner classes. For example, you cannot create an instance of an inner class without first creating an instance of the outer class.

How to instantiate inner class in Java?

To instantiate an inner class, we must first instantiate the outer class. Then, create the inner object within the outer object with this syntax:

OuterClass outerObject = new OuterClass(); OuterClass.InnerClass innerObject = outerObject.new InnerClass();

Reference: https://docs.oracle.com/javase/tutorial/java/javaOO/nested.html