How to Fix Class Is Not Abstract and Does Not Override Error in Java

1. Understanding the Error Class is not abstract and does not override abstract method in Java
2. Solution 1: Override the canSpeak() Method
3. Solution 2: Declare the Class as Abstract
4. Solution 3: Make Human a Class and Extend It in the Baby Class
5. Conclusion

When working with abstract classes and interfaces in Java, it’s not uncommon to encounter the error message: "Class is not abstract and does not override abstract method". This error occurs when a class claims to implement an interface or extend an abstract class but fails to provide implementations for all the abstract methods defined in the interface or abstract class.

This article will find solutions to the error Class is not abstract and does not override abstract method that occurs when we use the concept of abstraction in Java, providing detailed explanations and example codes for each approach.

Understanding the Error Class is not abstract and does not override abstract method in Java

To understand this error, let’s consider the following scenario:

interface MyInterface {
  void myMethod();
}

class MyClass implements MyInterface {
  // This class does not provide an implementation for myMethod()
}

In this example, MyClass claims to implement the MyInterface interface, which declares the abstract method myMethod(). However, MyClass does not provide an implementation for myMethod(), leading to the error message.

Why Does the Error Class is not abstract and does not override abstract method Occur in Java?

In the code below, we have two classes and an interface. The class JavaExample has a main() method without any body part.

We create a Human interface with an abstract method canSpeak() with boolean as a return type. We do not specify a body for canSpeak() because an abstract method doesn’t have a body.

In the second class, Baby, we inherit the Human interface using the implements keyword. If we use an IDE, there will be an error, and when we run the code, the error will occur, as shown in the output.

public class JavaExample {
  public static void main(String[] args) {}
}

class Baby implements Human {}

interface Human {
  abstract boolean canSpeak();
}

Output:

JavaExample.java:5: error: Baby is not abstract and does not override abstract method canSpeak() in Human
class Baby implements Human {}
^
1 error
error: compilation failed

If we look at the error Baby is not abstract and does not override abstract method canSpeak() in Human, we can understand why it occurred. It says that the class Baby is not abstract, and it doesn’t override the method canSpeak() of the Human interface.

This error appears because we have to override the abstract methods to define the body when implementing any class interface with abstract methods.

Solution 1: Override the canSpeak() Method

To fix the Baby is not abstract and does not override abstract method speak() in Human error, the first solution is to override the abstract method canSpeak() in the Baby class that implements the Human interface.

The canSpeak() function returns false and in the main() method we create an object of the Baby class and call the overridden function canSpeak(). In the output, we can see that there’s no error and the expected value shows.

public class JavaExample {
  public static void main(String[] args) {
    Baby baby = new Baby();
    System.out.println("Can Speak? " + baby.canSpeak());
  }
}

class Baby implements Human {
  @Override
  public boolean canSpeak() {
    return false;
  }
}

interface Human {
  abstract boolean canSpeak();
}

Output:

Can Speak? false

This code defines a program to determine if a baby can speak. It consists of three components: the JavaExample class, the Baby class, and the Human interface.

In the JavaExample class, the main method is the entry point. Inside it, an instance of the Baby class is created, and the canSpeak method of the Baby instance is called.

The result is printed to the console using System.out.println. The Baby class implements the Human interface, which requires it to provide an implementation for the canSpeak method.

Here, the Baby class overrides canSpeak and returns false, indicating that a baby cannot speak. The Human interface declares a single abstract method, canSpeak, specifying that any class implementing it must provide an implementation for this method.

This code demonstrates the use of interfaces to define common behaviors, and in this specific case, it asserts that a Baby cannot speak according to the canSpeak method’s implementation in the Baby class.

Solution 2: Declare the Class as Abstract

If you cannot provide an implementation for all the abstract methods in an interface or abstract class within the current class, you can declare the class as abstract.

This solution involves making the class Baby an abstract.

We have the same code here, but Baby is an abstract class. It cannot be initialized; we only create an instance of the Baby class in the main() method.

public class JavaExample {
  public static void main(String[] args) {
    Baby baby;
  }
}

abstract class Baby implements Human {}

interface Human {
  abstract boolean canSpeak();
}

By declaring Baby as abstract, it is not mandatory to provide implementations for all the abstract methods immediately. Subclasses that extend Baby will be responsible for implementing the required methods.

Solution 3: Make Human a Class and Extend It in the Baby Class

The last solution is a different one. Instead of implementing an interface, we can change the interface Human to a class and extend that class in the Baby class using the keyword extends.

We specify the body of canSpeak() in the Human class itself, which removes the error.

public class JavaExample {
  public static void main(String[] args) {
    Baby baby = new Baby();
    System.out.println("Can Speak? " + baby.canSpeak());
  }
}

class Baby extends Human {}

class Human {
  boolean canSpeak() {
    return false;
  }
}

Output:

Can Speak? false

This code defines a simple program that checks if a Baby can speak. It consists of three components: the JavaExample class, the Baby class, and the Human class.

In the JavaExample class, the main method serves as the entry point of the program. Within it, an instance of the Baby class is created using the new keyword.

Subsequently, the canSpeak method of the Baby instance is called. The result is then printed to the console using System.out.println.

The Baby class extends the Human class, which means it inherits the canSpeak method from the Human class. In this case, the canSpeak method in the Human class returns false, indicating that a human, including a baby, cannot speak in this context.

Overall, this code demonstrates class inheritance in Java, where the Baby class inherits behavior from the Human class and checks if a baby can speak based on the canSpeak method’s implementation in the Human class.

Conclusion

This article provides comprehensive solutions for resolving the error "Class is not abstract and does not override abstract method" in Java. This error occurs when a class claims to implement an interface or extend an abstract class but lacks implementations for all the abstract methods defined in the interface or abstract class.

1. The first solution involves overriding the abstract method in the implementing class to provide the required implementation.

2. The second solution suggests declaring the class as abstract, allowing subclasses to be responsible for implementing the required methods.

3. The third solution transforms the interface into a class and extends it in the implementing class, enabling a default implementation of the method in the base class.

Each solution is accompanied by an example code and detailed explanations to illustrate their application. By applying these solutions, developers can effectively address the specified error, ensuring their Java programs function correctly.