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Java Anonymous Class: Java Explained

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Java anonymous classes are classes without a name. They are used when a custom class is required in an immutable block of code, such as when implementing a new interface on the fly or overriding a method of an existing class. Anonymous classes enable developers to create custom classes without the need to write a separate class definition. In this article, we will explore how to create, debug and use anonymous classes in Java.

Advantages of Using an Anonymous Class

There are several advantages to using anonymous classes in your Java programming. First, these classes provide succinct and clean code which is easy to read. Second, an anonymous class can be used to add new functionality to existing classes without actually modifying the existing class. Finally, these classes allow developers to create custom classes quickly and easily without having to write the full code for the class.

Anonymous classes also provide a great way to create objects with specific properties without having to create a new class. This can be useful when you need to create a one-off object with specific properties that don’t fit into an existing class. Additionally, anonymous classes can be used to create objects with a specific scope, which can be useful for creating objects that are only accessible within a certain context.

Creating an Anonymous Class

Creating an anonymous class in Java is a straightforward process. To start, declare a reference to an interface or class and assign it an anonymous class instance that is declared as part of the assignment. The anonymous class must contain implementations for any methods that the interface or base class defines. The syntax for using an anonymous class should look like this:

Reference r = new Interface/Class() {    @Override     method() {        // Implementation here    }    // Any additional methods     method2(){        // Implementation here    }};

It is important to note that an anonymous class can contain only one constructor declaration, no private members and no static members. As a result, all fields must be declared inside the instance initializer.

Anonymous classes are useful when you need to create a one-time implementation of an interface or abstract class. They are also useful for creating a class with a small number of methods that are used only once. Anonymous classes can also be used to create a class with a single method, such as a Runnable or Comparator.

When to Use Anonymous Classes

Anonymous classes are commonly used in two situations. The first is when creating a custom class on-the-fly, such as when implementing a new interface or overriding a method of an existing class. This allows developers to quickly and easily create custom classes without having to write a separate class definition. The second situation is when creating a background thread or task – i.e., a thread with its own run() method – where a named inner class is not required or desired.

Anonymous classes can also be used to create a single instance of a class, such as when creating a singleton object. This is useful when a class needs to be instantiated only once, and the code for the class is not complex enough to warrant a separate class definition. Additionally, anonymous classes can be used to create a local class, which is a class defined within a method and can only be used within that method.

Sample Code for Anonymous Classes

The following example shows a simple anonymous class in use. This anonymous class implements the java.lang.Runnable interface and prints out a message in its run() method:

Runnable r = new Runnable() {    @Override     public void run() {        System.out.println("I am running!");    }};

Anonymous classes can also be used to override methods from existing classes. In the following example, an anonymous class is used to override the toString() method of java.lang.String:

String s = "Hello World"; String t = new String(s) {     @Override     public String toString() {         return "Goodbye World";     } }; System.out.println(t);  // Prints "Goodbye World" 

Common Pitfalls of Anonymous Classes

When writing anonymous classes in Java, there are several important things to remember. First, as previously mentioned, anonymous classes cannot have private members or static fields, as these are not declared within the scope of the anonymous class. Second, lambda expressions cannot be used within anonymous classes, even though both are used for anonymous instances of interfaces. Third, it is important to remember that an anonymous class does not have a constructor – only the instance initializer is executed – so any initialization logic must take place within the instance initializer.

Working with Interfaces and Abstract Classes

Anonymous classes can also be used in combination with either interfaces or abstract classes. When using interfaces, both the abstract and non-abstract methods of the interface must be implemented; whereas, when using abstract classes, all of the abstract methods must be implemented plus any non-abstract methods may also be overridden.

Understanding the Syntax of an Anonymous Class

An anonymous class may appear intimidating at first glance, but its syntax is remarkably simple and straightforward. The syntax consists of two parts: first, the reference variable and second, the anonymous inner class. The reference variable is less important as its job is simply to reference the anonymous inner class, which contains all of the code for implementing the interface/class that was declared. Here is a simplified version of an anonymous inner class:

Reference r = new Interface/Class() {     // Method implementations here }; 

When working with more complex anonymous inner classes, understanding their syntax becomes increasingly important.

What Is the Difference between a Named and Anonymous Class?

Anonymous and named (or inner) classes differ in two key ways. First, an anonymous class has no name whereas a named class does. Second, a named inner class has access to all of the members declared within its enclosing scope – including both instance and static variables – whereas an anonymous class has no access to these members. Consequently, it is only suitable for implementations that do not need this access.

Debugging an Anonymous Class

Debugging an anonymous class can sometimes be difficult due to the lack of an explicit name for the class, making it difficult to identify where bugs are located within code. Fortunately, there are a few techniques for debugging anonymous classes which can help to identify problems more quickly. First, it is possible to use code breakpoints to debug an anonymous class by setting a breakpoint on a line within the code body of the anonymous inner class. Second, whenever possible, it is best to use named inner classes instead of anonymous classes – since these have explicit names – as these are easier to debug.

In conclusion, an understanding of Java’s anonymous classes can help developers create concise and clean code that is easy to read and understand. When used properly, these classes can provide immense value by enabling developers to quickly and easily create custom implementations of interfaces or abstract classes without having to write additional code for each individual implementation.

Anand Das

Anand Das

Anand is Co-founder and CTO of Bito. He leads technical strategy and engineering, and is our biggest user! Formerly, Anand was CTO of Eyeota, a data company acquired by Dun & Bradstreet. He is co-founder of PubMatic, where he led the building of an ad exchange system that handles over 1 Trillion bids per day.

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