Question

What message does Java give when you put an abstract method inside a class which is not, in it- self, declared abstract?

Short answer

Java throws an error: "MyClass cannot be instantiated; it contains abstract methods".

Step-by-step solution

Understanding the Problem

When we use an abstract method in Java, it does not contain a body and is meant to be overridden in a subclass. If such a method is placed within a class, that class is expected to be abstract as it has incomplete definitions.

Identify Condition for Error

A class with an abstract method must also be declared abstract. If a class contains an abstract method but isn't declared abstract itself, it lacks full implementations, which Java does not permit in a non-abstract class.

Message from Java Compiler

The Java compiler will not allow a non-abstract class to contain abstract methods. Upon compilation, it provides an error indicating this issue, ensuring proper implementation practices.

Key concepts

abstract class

In Java programming, an **abstract class** is a special kind of class that cannot be instantiated, which means you cannot create objects of an abstract class directly. This type of class is primarily used as a blueprint for other classes. Abstract classes can include abstract methods, which are methods without a body.
Unlike interfaces, abstract classes in Java are declared using the `abstract` keyword. An abstract class:

• Can have both abstract methods (methods without a body) and non-abstract methods (methods with a body).
• Can have member variables, constructors, and concrete methods like a regular class.
• Can provide a common base from which other classes can be derived, helping to promote code reuse and organization.

To ensure a meaningful use of abstract classes, subclasses are responsible for providing implementations for the abstract methods defined in their parent class. This mechanism allows developers to formulate a framework where subclasses must adhere to certain rules or methods, yet are free to implement them in their own way.

abstract method

An **abstract method** is a method declared within an abstract class, but it does not contain any implementation (i.e., it lacks a body). It's a placeholder for methods that must be implemented in subclasses. Key points about abstract methods include:

• Declared using the `abstract` keyword, followed by a method signature ending with a semicolon.
• They are designed to be overridden in the derived class, mandates subclasses to provide the method body.
• Cannot be static, synchronized, or declare constructors as abstract.

Abstract methods define a contract or policy that implementing subclasses should follow. This approach ensures that every subclass has a uniform method structure, promoting consistency across different implementations.

Java compiler error message

When a **Java compiler error message** appears due to an abstract method in a non-abstract class, it's essential to understand what the compiler expects. The error typically states that a class cannot contain abstract methods unless the class itself is declared abstract. The primary reason behind this is that non-abstract classes are expected to provide implementations for all of their methods, while abstract methods are inherently incomplete.
In practice:

• The compiler checks every method in a class during compilation.
• If it finds an abstract method in a non-abstract class, it raises an error like: "The class must be declared abstract or provide method implementations."
• This message ensures that any abstract method must have corresponding concrete implementations or the class marked as abstract.

By providing this error message, Java enforces a design pattern ensuring that all classes are capable of functioning properly with all required method definitions complete.