How to write constructor for anonymous class in Java?

How to write constructor for anonymous class in Java?

Writing a constructor for an anonymous class in Java can be done by using the new keyword followed by the name of the interface or superclass that the anonymous class implements or extends. However, it's important to note that there are some nuances and considerations when writing constructors for anonymous classes.

Here is an example of how you might write a constructor for an anonymous class in Java:

Runnable r = new Runnable() {
public void run() {
System.out.println("Hello World");
}
};

In this example, we're creating an anonymous class that implements the Runnable interface. The constructor is implicit because we didn't specify a custom one. This means that Java will automatically generate a no-arg constructor for our anonymous class.

However, what if you want to write a custom constructor for your anonymous class? Well, it's possible but it requires using an initializer block. Here's how you would do it:

Runnable r = new Runnable() {
private int x;
public Runnable(int x) {
this.x = x;
}
public void run() {
System.out.println("Hello World: " + x);
}
};

In this example, we're creating an anonymous class that implements the Runnable interface with a constructor that takes one parameter. This means that whenever you create an instance of our anonymous class, you'll need to specify a value for that parameter.

One thing to note is that you can't have a constructor with parameters and also have another constructor without parameters in Java. If you try to do this, you'll get a compiler error because the default no-arg constructor will be automatically generated by Java if it doesn't find one with parameters. So, you might want to reconsider your design if you need multiple constructors.

Another thing to consider is that when working with anonymous classes, they can't have their own static variables or methods. This means that any static variables or methods would actually belong to the class where you declare them, not the anonymous class itself. However, this shouldn't be a problem unless you're trying to do something very complex.

Finally, keep in mind that using anonymous classes extensively can make your code harder to read and maintain because they often have limited scope and functionality. This means it's generally better to use them sparingly for simple tasks like creating a one-off Runnable or Comparator.

In conclusion, writing a constructor for an anonymous class in Java is possible but requires using an initializer block if you want to create a custom constructor. You should consider the implications of using anonymous classes and try to keep your code as readable and maintainable as possible.

Anonymous class in Java w3schools

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W3Schools is a popular online platform that provides tutorials and reference materials for web development, including programming languages like Java. According to W3Schools, an anonymous class in Java is a class that can be instantiated without knowing its name at compile time.

In other words, an anonymous class is created at runtime using the new keyword followed by a lambda expression or a method handle. This type of class is often used as a one-time utility class to perform a specific task, and then it's discarded.

Here's a simple example of how you can create an anonymous class in Java:

import java.lang.reflect.Method;
public class Main {
public static void main(String[] args) throws Exception {
// Create an anonymous class with a method that prints "Hello World"
Method myMethod = new Object() {
public void sayHello() {
System.out.println("Hello World");
}
}.getClass().getMethod("sayHello");
// Call the method
myMethod.invoke(new Object());
}
}

In this example, we create an anonymous class with a single method sayHello() that prints "Hello World". We then retrieve the method using reflection and invoke it.

Anonymous classes can also be used as lambda expressions. Lambda expressions are concise ways to represent small, one-time-use functions or methods. Here's an example of how you can use an anonymous class as a lambda expression:

public class Main {
public static void main(String[] args) {
// Create an anonymous class with a method that prints "Hello World"
Runnable myRunnable = new Runnable() {
public void run() {
System.out.println("Hello World");
}
};
// Start the thread and call the method
Thread myThread = new Thread(myRunnable);
myThread.start();
}
}

In this example, we create an anonymous class that implements the Runnable interface. We then start a new thread with our anonymous class as its runable.

Overall, anonymous classes in Java provide a flexible way to create one-time-use utility classes or methods without having to declare a separate class or interface.