Why Are Constructors Not Part Of The Interface?

CAN interface have concrete methods?

Concrete methods in interfaces The simplest form of this feature is the ability to declare a concrete method in an interface, which is a method with a body.

A class that implements this interface need not implement its concrete method..

CAN interface have main method?

Answer : Yes, we can provide different implementation of main() declared in an interface and classes implementing that interface by overriding method and can overload static main method if defined in an interface. … Prior to Java 8, it was not possible to DEFINE methods inside interface.

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.

Are all methods in an interface public?

All abstract, default, and static methods in an interface are implicitly public , so you can omit the public modifier. In addition, an interface can contain constant declarations. All constant values defined in an interface are implicitly public , static , and final . Once again, you can omit these modifiers.

Can we write 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.

Can abstract class have constructors?

Yes! Abstract classes can have constructors! Yes, when we define a class to be an Abstract Class it cannot be instantiated but that does not mean an Abstract class cannot have a constructor. Each abstract class must have a concrete subclass which will implement the abstract methods of that abstract class.

Can a constructor be abstract?

You can’t have an abstract constructor, as abstract means you need to provide the implementation for that at some point of time in your subclass. But you cannot override constructor. There will be no point in having an abstract constructor : Since the constructor needs to be of the same name as of class.

What happens if you give method body in interface?

It is a compile-time error if a method declared in an interface is declared static, because static methods cannot be abstract. … A static method also has a block body, which provides the implementation of the method. So it explicitly states that in Java 8, they can exist.

What happens when a constructor is defined for an interface?

What happens when a constructor is defined for an interface? Explanation: Constructor is not provided by interface as objects cannot be instantiated. 9. What happens when we access the same variable defined in two interfaces implemented by the same class?

Why Java interfaces Cannot have constructor but abstract classes can have?

Since we cannot call the methods in the interface, there is no need for creating the object for interface and there is no need of having a constructor in it. … The constructor inside the abstract class can only be called during constructor chaining i.e. when we create an instance of sub-classes.

Can abstract class have body?

Abstract methods are declaration only and it will not have implementation. It will not have a method body. A Java class containing an abstract class must be declared as abstract class. An abstract method can only set a visibility modifier, one of public or protected.

Can we have this and super in the same constructor?

Because if you use this() and super() together in a constructor it will give compile time error. Because this() and super() must be the first executable statement. If you write this() first than super() will become the second statement and vice-versa. That’s why we can’t use this() and super() together.

Is it compulsory to implement all methods of interface?

Yes, it is mandatory to implement all the methods in a class that implements an interface until and unless that class is declared as an abstract class. … Declare the class as an abstract class, as a result, forces you to subclass the class (and implement the missing methods) before you can create any objects.

Can we declare an interface as final?

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 an interface be instantiated?

Interfaces cannot be instantiated, but rather are implemented. A class that implements an interface must implement all of the non-default methods described in the interface, or be an abstract class.