[java] Abstract methods in Java

I want to write an abstract method but the compiler persistently gives this error:

abstract methods cannot have a body

I have a method like this:

public abstract boolean isChanged() {
    return smt else...
}

What is wrong here?

The error message tells the exact reason: "abstract methods cannot have a body".

They can only be defined in abstract classes and interfaces (interface methods are implicitly abstract!) and the idea is, that the subclass implements the method.

Example:

 public abstract class AbstractGreeter {
   public abstract String getHelloMessage();

   public void sayHello() {
     System.out.println(getHelloMessage());
   }
 }

 public class FrenchGreeter extends AbstractGreeter{

   // we must implement the abstract method
   @Override
   public String getHelloMessage() {
     return "bonjour";
   }
 }

If you use the java keyword abstract you cannot provide an implementation.

Sometimes this idea comes from having a background in C++ and mistaking the virtual keyword in C++ as being "almost the same" as the abstract keyword in Java.

In C++ virtual indicates that a method can be overridden and polymorphism will follow, but abstract in Java is not the same thing. In Java abstract is more like a pure virtual method, or one where the implementation must be provided by a subclass. Since Java supports polymorphism without the need to declare it, all methods are virtual from a C++ point of view. So if you want to provide a method that might be overridden, just write it as a "normal" method.

Now to protect a method from being overridden, Java uses the keyword final in coordination with the method declaration to indicate that subclasses cannot override the method.