What is the difference between an abstract method and a concrete method in Java?

In Java, an abstract method is a method that is declared, but not defined, in an abstract class or interface. The method definition is left to the subclasses or implementing classes. The purpose of an abstract method is to provide a common interface for its subclasses or implementers. Abstract methods are declared using the 'abstract' keyword.

A concrete method, on the other hand, is a method that is fully defined in a class. Concrete methods are also known as non-abstract methods or implemented methods. Concrete methods have a method body that provides the actual implementation of the method.

One of the key differences between abstract and concrete methods is that abstract methods cannot be instantiated or called directly, while concrete methods can be.