Java Interview Questions

You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList?

ArrayList

How can you minimize the need of garbage collection and make the memory use more effective?

Use object pooling and weak object references.

There are two classes: A and B. The class B need to inform a class A when some important event has happened. What Java technique would you use to implement it?

If these classes are threads I’d consider notify() or notifyAll(). For regular classes you can use the Observer interface.

What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?

You do not need to specify any access level, and Java will use a default package access level.

If Runnable interface is better than Thread class, than why we are using Thread class? What is the need for Thread class?

What are Encapsulation, Inheritance and Polymorphism?

Encapsulation is the mechanism that binds together code and data it manipulates and keeps both safe from outside interference and misuse.

Inheritance is the process by which one object acquires the properties of another object.

Polymorphism is the feature that allows one interface to be used for general class actions.

What is the difference between Assignment and Initialization?

Assignment can be done as many times as desired whereas initialization can be done only once.

What are Class, Constructor and Primitive data types?

Class is a template for multiple objects with similar features and it is a blue print for objects. It defines a type of object according to the data the object can hold and the operations the object can perform.

Constructor is a special kind of method that determines how an object is initialized when created.

Primitive data types are 8 types and they are: byte, short, int, long, float, double, boolean, char.

What is the difference between an argument and a parameter?

While defining method, variables passed in the method are called parameters. While using those methods, values passed to those variables are called arguments.

What is final, finalize() and finally?

final : final keyword can be used for class, method and variables. A final class cannot be subclassed and it prevents other programmers from subclassing a secure class to invoke insecure methods. A final method can’t be overridden. A final variable can’t change from its initialized value.

finalize() : finalize() method is used just before an object is destroyed and can be called just prior to garbage collection.

finally : finally, a key word used in exception handling, creates a block of code that will be executed after a try/catch block has completed and before the code following the try/catch block.

The finally block will execute whether or not an exception is thrown.

What is the difference between this() and super()?

this() can be used to invoke a constructor of the same class whereas super() can be used to invoke a super class constructor.

What is the difference between superclass and subclass?

A super class is a class that is inherited whereas sub class is a class that does the inheriting.

What are inner class and anonymous class?

Inner class : classes defined in other classes, including those defined in methods are called inner classes. An inner class can have any accessibility including private.

Anonymous class : Anonymous class is a class defined inside a method without a name and is instantiated and declared in the same place and cannot have explicit constructors.

What is the difference between Integer and int?

a) Integer is a class defined in the java. lang package, whereas int is a primitive data type defined in the Java language itself. Java does not automatically convert from one to the other.

b) Integer can be used as an argument for a method that requires an object, whereas int can be used for calculations.

What is a cloneable interface and how many methods does it contain?

It is not having any method because it is a TAGGED or MARKER interface.

What is the difference between abstract class and interface?

a) All the methods declared inside an interface are abstract whereas abstract class must have at least one abstract method and others may be concrete or abstract.

b) In abstract class, key word abstract must be used for the methods whereas interface we need not use that keyword for the methods.

c) Abstract class must have subclasses whereas interface can’t have subclasses.

What is the difference between exception and error?

What is the difference between process and thread?

Process is a program in execution whereas thread is a separate path of execution in a program.

What is synchronization?

Synchronization is the mechanism that ensures that only one thread is accessed the resources at a time.

Are there any global variables in Java, which can be accessed by other part of your program?

No, it is not the main method in which you define variables. Global variables is not possible because concept of encapsulation is eliminated here.