Can we have concrete methods in interface?

Interfaces cannot have any concrete methods. If you need the ability to have abstract method definitions and concrete methods then you should use an abstract class.

Can we have concrete methods in Java interface?

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 an interface have methods?

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. It is the blueprint of the class.

What happens if we provide concrete implementation of method in interface?

What will happen if we provide concrete implementation of method in interface? Explanation: The methods of interfaces are always abstract. They provide only method definition. 8.

CAN interfaces have 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).

Java Interview 09 - Concrete Methods In Interface

Can we have constructor in 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 have static methods?

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 final methods?

If you make an interface final, you cannot implement its methods which defies the very purpose of the interfaces. Therefore, you cannot make an interface final in Java. Still if you try to do so, a compile time exception is generated saying “illegal combination of modifiers − interface and final”.

CAN interface have non abstract methods?

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

CAN interface have private methods?

An interface can have private methods since Java 9 version. These methods are visible only inside the class/interface, so it's recommended to use private methods for confidential code. That's the reason behind the addition of private methods in interfaces.

Can we create non static variables in an interface?

No you cannot have non-static variables in an interface. By default, All the members (methods and fields) of an interface are public. All the methods in an interface are public and abstract (except static and default).

CAN interface have default methods?

Interfaces can have default methods with implementation in Java 8 on later. Interfaces can have static methods as well, similar to static methods in classes. Default methods were introduced to provide backward compatibility for old interfaces so that they can have new methods without affecting existing code.

Can we override static method of interface?

You cannot override the static method of the interface; you can just access them using the name of the interface. If you try to override a static method of an interface by defining a similar method in the implementing interface, it will be considered as another (static) method of the class.

CAN interface have final variable?

The interface contains the static final variables. The variables defined in an interface can not be modified by the class that implements the interface, but it may use as it defined in the interface. ? The variable in an interface is public, static, and final by default.

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.

Can we override methods in interface?

No. Interface methods can be implemented. To overload/override a method, that method must be defined first. Interface do not contain method definition.

Why interface Cannot have static methods?

All methods in an interface are explicitly abstract and hence you cannot define them as static because static methods cannot be abstract.

Can we use static and final in interface?

Interface variables are static because java interfaces cannot be instantiated on their own. The value of the variable must be assigned in a static context in which no instance exists. The final modifier ensures the value assigned to the interface variable is a true constant that cannot be re-assigned.

Can interface contain nested types?

Yes, if we define a class inside the interface, the Java compiler creates a static nested class. Let's see how can we define a class within the interface: interface M{

Can we have static abstract method in interface?

A static method belongs to class not to object instance thus it cannot be overridden or implemented in a child class. So there is no use of making a static method as abstract.

Can we override default method in interface?

If you have default method in an interface, it is not mandatory to override (provide body) it in the classes that are already implementing this interface. In short, you can access the default methods of an interface using the objects of the implementing classes.

Can abstract class have concrete methods?

Abstract class can have both an abstract as well as concrete methods. A concrete class can only have concrete methods. Even a single abstract method makes the class abstract.

CAN interface have objects?

Like abstract classes, interfaces cannot be used to create objects (in the example above, it is not possible to create an "Animal" object in the MyMainClass) Interface methods do not have a body - the body is provided by the "implement" class. On implementation of an interface, you must override all of its methods.

Can an interface implement multiple classes?

Yes an interface can be implemented by multiple classes.

Can we overload final method?

Can We Override a Final Method? No, the Methods that are declared as final cannot be Overridden or hidden.