Both interfaces and abstract classes with all abstract methods can be used to define a contract for implementing classes. However, there are some differences:

1. An interface only defines method signatures and constants, while an abstract class can also have instance variables and concrete methods.

2. A class can implement multiple interfaces, but can only extend one abstract class.

Here’s an example of an interface:

public interface Vehicle {
 void start();
 void stop();
 int getSpeed();
}

And here’s an example of an abstract class with all abstract methods:

public abstract class Animal {
 abstract void eat();
 abstract void sleep();
 abstract void makeSound();
}

Note that in the second example, the Animal class do not have any instance variables or concrete methods. If it did, those would have to be implemented by any non-abstract subclass.

An interface and an abstract class with all abstract methods both provide a blueprint for implementing classes, but there are some key differences between the two:

Inheritance: An interface can be implemented by multiple classes, whereas an abstract class can only be inherited by a single class.

Accessibility: Members of an interface are public by default, while members of an abstract class can have any accessibility level.

Implementation: In an interface, the implementing class must provide implementations for all methods, while in an abstract class, some or all methods can be left abstract and implemented by subclasses.

Properties and fields: Interfaces can only have method signatures and events, while abstract classes can have fields, properties, and instance variables.

Constructors: An interface cannot have a constructor, while an abstract class can have a constructor.

In general, you should use an interface if you want to specify a contract that can be implemented by multiple classes, and use an abstract class if you want to create a base class that provides some default behavior.