In Java, there are several differences between abstract classes and interfaces. Here are the main differences:
1. Definition: An abstract class is a class that cannot be instantiated, meaning it is extended by other classes. It can have abstract and non-abstract methods. On the other hand, an interface is a contract that defines a set of methods that a class implementing the interface must provide. It cannot have any method implementation.
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Inheritance: A class can only extend one abstract class, but it can implement multiple interfaces. This provides greater flexibility in terms of code reuse and class relationships.
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Method Implementation: An abstract class can have both abstract and non-abstract methods. Abstract methods are declared without implementation and must be overridden by subclasses. Non-abstract methods in abstract classes can have implementations which can be inherited by subclasses. In contrast, all methods in an interface are implicitly abstract without any implementation. The implementing class must provide implementations for all methods defined in the interface.
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Variables: An abstract class can have instance variables, constructors, and non-abstract methods. It can also have state and behavior. In contrast, interfaces cannot have instance variables or constructors. It can only have static final variables (constants) and abstract methods.
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Accessibility: Abstract class members can have different access modifiers such as private, protected or public. Interface members are implicitly public and cannot be private or protected.
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Usage: Abstract classes are used when you want to provide a common base implementation for subclasses and when you need to define a class hierarchy. Interfaces are used when you want to define a contract that multiple unrelated classes can implement, thus achieving polymorphism.
In summary, abstract classes are used to provide a common base implementation and define class hierarchies, while interfaces are used to define contracts that classes can implement, enabling polymorphism and inheritance-like multiple behaviors.