Java Interview Questions

Q 01: Give a few reasons for using Java?
A 01: Java is a fun language. Let’s look at some of the reasons:

Built-in support for multi-threading, socket communication, and memory management (automatic garbage collection).

Object Oriented (OO).

Better portability than other languages across operating systems.

Supports Web based applications (Applet, Servlet, and JSP), distributed applications (sockets, RMI. EJB etc) and network protocols (HTTP, JRMP etc) with the help of extensive standardised APIs (Application Program Interfaces).
Q 02: What is the difference between C++ and Java?
A 02: Both C++ and Java use similar syntax and are Object Oriented, but:

Java does not support pointers. Pointers ar e inherently tricky to use and troublesome.

Java does not support multiple inheritances because it causes more problems than it solves. Instead Java supports multiple interface inheritance, which allows an object to inherit many method signatures from different interfaces with the condition that the inheriting object must implement those inherited methods. The multiple interface inheritance also allows an object to behave polymorphically on those methods.

Java does not support destructors but rather adds a finalize() method. Finalize methods are invoked by the garbage collector prior to reclaiming the memory occupied by the object, which has the finalize() method. This means you do not know when the objects are going to be finalized. Avoid using finalize() method to release non-memory resources like file handles, sockets, database connections etc because Java has only a finite number of these resources and you do not know when the garbage collection is going to kick in to
release these resources through the finalize() method.

Java does not include structures or unions because the traditional data structures are implemented as an object oriented framework
All the code in Java program is encapsulated within cl asses therefore Java does not have global variables or functions.

C++ requires explicit memory management, while Java includes automatic garbage collection.

Q 03: How do you express an ‘is a’ relationship and a ‘has a’ relationship or explain inheritance and composition? What is the difference between composition and aggregation? 
A 03: The ‘is a ’ relationship is expressed with inheritance and ‘has a’ relationship is expressed with composition . Both inheritance and composition allow you to place sub-objects inside your new class. Two of the main techniques for code reuse are class inheritance and object composition.
Inheritance is uni-directional. For example House is a Building. But Building is not a House . Inheritance uses extends key word. Composition: is used when House has a Bathroom. It is incorrect to say House is a Bathroom. Composition simply means using instance variables that refer to other objects. The class House will have an instance variable, which refers to a Bathroom object.

What is the difference between aggregation and composition?

Aggregation is an association in which one class belongs to a collection. This is a part of a whole relationship where a part can exist without a whole. For example a line item is a whole and product is a part. If a line item is deleted then corresponding product need not be deleted. So aggregation has a weaker relationship.

Composition is an association in which one class belongs to a collection. This is a part of a whole relationship where a part cannot exist without a whole. If a whole is deleted then all parts are deleted. For example An order is a whole and line items are parts. If an order deleted then all corresponding line items for that order should be deleted. So composition has a stronger relationship.


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