Does a class need to be abstract to implement an interface?

It's not necessary for an abstract class to have abstract method. We can mark a class as abstract even if it doesn't declare any abstract methods. If abstract class doesn't have any method implementation, its better to use interface because java doesn't support multiple class inheritance.

Can an interface have non-abstract method?

Type of methods: Interface can have only abstract methods. An abstract class can have abstract and non-abstract methods.

Can we use interface instead of abstract class?

Abstract classes should be used primarily for objects that are closely related, whereas interfaces are best suited for providing common functionality to unrelated classes. If you are designing small, concise bits of functionality, use interfaces. If you are designing large functional units, use an abstract class.

Can a class implement multiple interfaces?

Your class can implement more than one interface, so the implements keyword is followed by a comma-separated list of the interfaces implemented by the class. By convention, the implements clause follows the extends clause, if there is one.

Abstract Classes vs Interfaces: Interview Question with a Twist!

Can we instantiate an interface if yes how?

No, you cannot instantiate an interface. Generally, it contains abstract methods (except default and static methods introduced in Java8), which are incomplete. Still if you try to instantiate an interface, a compile time error will be generated saying “MyInterface is abstract; cannot be instantiated”.

Can we extend non-abstract class in Java?

Yes, but if the class is marked as final, it can't be extended.

Does interface contain only abstract methods?

The interface body can contain abstract methods, default methods, and static methods. An abstract method within an interface is followed by a semicolon, but no braces (an abstract method does not contain an implementation).

Are interfaces always abstract?

Yes, Interfaces can only have abstract methods. In the Java programming language, an interface is a reference type, similar to a class, that can contain only constants, method signatures, default methods, static methods, and nested types. Method bodies exist only for default methods and static methods.

Can a class implement an interface?

A class can implement more than one interface at a time. A class can extend only one class, but implement many interfaces. An interface can extend another interface, in a similar way as a class can extend another class.

Can an abstract class have non abstract methods?

Yes, we can declare an abstract class with no abstract methods in Java. An abstract class means that hiding the implementation and showing the function definition to the user. An abstract class having both abstract methods and non-abstract methods.

Why interface has no method implementation?

It cannot have a method body. Java Interface also represents the IS-A relationship. Like a class, an interface can have methods and variables, but the methods declared in an interface are by default abstract (only method signature, no body). Interfaces specify what a class must do and not how.

Can functional interface have 2 abstract methods?

A functional interface is an interface that contains only one abstract method. They can have only one functionality to exhibit. From Java 8 onwards, lambda expressions can be used to represent the instance of a functional interface.

Can we inherit abstract class in Java?

If a class is declared abstract, it cannot be instantiated. To use an abstract class, you have to inherit it from another class, provide implementations to the abstract methods in it. If you inherit an abstract class, you have to provide implementations to all the abstract methods in it.

Why interface have only abstract methods?

Because by default all methods are abstract inside the interface. So this example states that we can not have final methods inside the interfaces. Hence, this example also shows that we can have abstract methods only inside the interface.

Can abstract class have final methods?

therefore, a final abstract combination is illegal for classes. Hence, a final class cannot contain abstract methods whereas an abstract class can contain a final method.

Can we have static method in interface?

Static methods in an interface since java8

Since Java8 you can have static methods in an interface (with body). You need to call them using the name of the interface, just like static methods of a class.

CAN interface have private methods?

As of Java 9, methods in an interface can be private. A private method can be static or an instance method, but it cannot be a default method since that can be overridden.

Can interface be instantiated?

An interface can't be instantiated directly. Its members are implemented by any class or struct that implements the interface. A class or struct can implement multiple interfaces.

Can abstract class inherit non abstract class?

An abstract class cannot be inherited by structures. It can contains constructors or destructors. It can implement functions with non-Abstract methods. It cannot support multiple inheritance.

Can constructor be inherited?

Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.

Can you inherit multiple interfaces?

No you cannot inherit multiple interfaces, because interfaces cannot be inherited. Interfaces are IMPLEMENTED, not inherited.

Can we create constructor of interface?

No, you cannot have a constructor within an interface in Java. You can have only public, static, final variables and, public, abstract, methods as of Java7. From Java8 onwards interfaces allow default methods and static methods. From Java9 onwards interfaces allow private and private static methods.

Can interface contain concrete methods?

All the methods in an interface must be abstract, you cannot have a concrete method (the one which has body) if you try to do so, it gives you a compile time error saying “interface abstract methods cannot have body”.

CAN interface have non abstract methods in Java?

Interface methods are by definition public and abstract, so you cannot have non-abstract methods in your interface.