Core java interview Questions
11. What is method overloading?
Method Overloading means to have two or more methods with same name in the same class with different arguments. The benefit of method overloading is that it allows you to implement methods that support the same semantic operation but differ by argument number or type.
Note:
- Overloaded methods MUST change the argument list
- Overloaded methods CAN change the return type
- Overloaded methods CAN change the access modifier
- Overloaded methods CAN declare new or broader checked exceptions
- A method can be overloaded in the same class or in a subclass
12. What is method overriding?
Method overriding occurs when sub class declares a method that has the same type arguments as a method declared by one of its super class. The key benefit of overriding is the ability to define behavior that’s specific to a particular subclass type.
Note:
- The overriding method cannot have a more restrictive access modifier than the method being overridden (Ex: You can’t override a method marked public and make it protected).
- You cannot override a method marked final
- You cannot override a method marked static
14. Can overloaded methods be override too?
Yes, derived classes still can override the overloaded methods. Polymorphism can still happen. Compiler will not binding the method calls since it is overloaded, because it might be overridden now or in the future.
15. Is it possible to override the main method?
NO, because main is a static method. A static method can't be overridden in Java.
16. How to invoke a super class version of an Overridden method?
To invoke a super class method that has been overridden in a subclass, you must either call the method directly through a super class instance, or use the super prefix in the subclass itself. From the point of the view of the subclass, the super prefix provides an explicit reference to the super class' implementation of the method.
// From subclass
super.overriddenMethod();
17. What is super?
super is a keyword which is used to access the method or member variables from the super class. If a method hides one of the member variables in its super class, the method can refer to the hidden variable through the use of the super keyword. In the same way, if a method overrides one of the methods in its super class, the method can invoke the overridden method through the use of the super keyword.
Note:
- You can only go back one level.
- In the constructor, if you use super(), it must be the very first code, and you cannot access any this.xxx variables or methods to compute its parameters.
18. How do you prevent a method from being overridden?
To prevent a specific method from being overridden in a subclass, use the final modifier on the method declaration, which means "this is the final implementation of this method", the end of its inheritance hierarchy.
public final void exampleMethod() {
// Method statements
}
19. What is an Interface?
An interface is a description of a set of methods that conforming implementing classes must have.
Note:
- You can’t mark an interface as final.
- Interface variables must be static.
- An Interface cannot extend anything but another interfaces.
20. Can we instantiate an interface?
You can’t instantiate an interface directly, but you can instantiate a class that implements an interface.
21. Can we create an object for an interface?
Yes, it is always necessary to create an object implementation for an interface. Interfaces cannot be instantiated in their own right, so you must write a class that implements the interface and fulfill all the methods defined in it.
22. Do interfaces have member variables?
Interfaces may have member variables, but these are implicitly public, static, and final- in other words, interfaces can declare only constants, not instance variables that are available to all implementations and may be used as key references for method arguments for example.
23. What modifiers are allowed for methods in an Interface?
Only public and abstract modifiers are allowed for methods in interfaces.
24. What is a marker interface?
Marker interfaces are those which do not declare any required methods, but signify their compatibility with certain operations. The java.io.Serializable interface and Cloneable are typical marker interfaces. These do not contain any methods, but classes must implement this interface in order to be serialized and de-serialized.
25. What is an abstract class?
Abstract classes are classes that contain one or more abstract methods. An abstract method is a method that is declared, but contains no implementation.
Note:
- If even a single method is abstract, the whole class must be declared abstract.
- Abstract classes may not be instantiated, and require subclasses to provide implementations for the abstract methods.
- You can’t mark a class as both abstract and final.
26. Can we instantiate an abstract class?
An abstract class can never be instantiated. Its sole purpose is to be extended (sub-classed).
27. When should I use abstract classes and when should I use interfaces?
Use Interfaces when…
- You see that something in your design will change frequently.
- If various implementations only share method signatures then it is better to use Interfaces.
- you need some classes to use some methods which you don't want to be included in the class, then you go for the interface, which makes it easy to just implement and make use of the methods defined in the interface.
- If various implementations are of the same kind and use common behavior or status then abstract class is better to use.
- When you want to provide a generalized form of abstraction and leave the implementation task with the inheriting subclass.
- Abstract classes are an excellent way to create planned inheritance hierarchies. They're also a good choice for non-leaf classes in class hierarchies.
28. When you declare a method as abstract, can other non-abstract methods access it?
Yes, other non-abstract methods can access a method that you declare as abstract.
29. Can there be an abstract class with no abstract methods in it?
Yes, there can be an abstract class without abstract methods.
