M Jolly. - Java Interview Notes: 700 Java Interview Questions Answered

Java Interview Notes

by Jolly

Copyright 2016 by Jolly M.

All rights reserved, including the right to reproduce this book, or any portions of it, in any form.

CONTENT

INTRODUCTION

Introduction

Java Interview Notes cover topics that are frequently discussed during Java technical interview round, along with interview questions for each topic. This book also contains lots of code snippets and figures to explain topics.

To be confident in an interview, thorough understanding of software design and language concepts is very important, because during an interview same question can be asked in many different ways.

Interviewers are interested in hearing not just the correct answer, but also your opinion and thoughts about the topic. Your approach towards understanding the questions and sharing your thoughts plays an extremely important role in an interview success.

Confidence to face such interview can only be achieved when you spend really good amount of time coding, reading technical topics and discussing this with peers. There is no other shortcut available.

I hope this book helps to prepare you not only for your next interview, but also for your day-to-day software development task.

All the best!!!

JAVA

FUNDAMENTALS

Java Program Anatomy

The following code snippet depicts the anatomy of simple Java program.

• Package - Represents logical grouping of similar types into namespaces. It also prevents naming collision and provides access protection.
• Import - Imports the package, so that classes can be used in the code by their unqualified names.
• Class - Represents a type template having properties and methods.
• Field - Represents a member used for holding values.
• Method - Represents an operation/behaviour of the class.
• Modifier - Specifies access control level of a class and its members.
• Parameter - Specifies the variable declared in the method definition.
• Argument - Specifies the data passed to the method parameters.

Questions

• What is package?
• Why do you need to import packages?
• Why do you need to specify access modifiers?
• What is static import?
  • static import enables access to static members of a class without need to qualify it by the class name.
• What is the difference between argument and parameter?

Compiling and Executing Java Code in JVM

Java program compilation and execution steps

1. Java Compiler compiles the Java source code (.java file) into a binary format known as bytecode (.class file). Java bytecode is platform independent instruction set, which contains instructions (opcode) and parameter information.
2. Bytecode is translated by the Operating System specific Java Virtual Machine (JVM) into the platform specific machine code.
3. Class loader in JVM loads the binary representation of the classes into memory.
4. Execution engine in JVM executes the byte code and generates Operating System specific machine instructions. These machine instructions are executed directly by the central processing unit (CPU).

Questions

• Explain the process of Java code compilation and execution?
• What is bytecode?
• What is the difference between bytecode and source code?
• What is machine code?
• What is the difference between bytecode and machine code?
• What is JVM? Is it same or different for different Operating Systems?
• What are the major components of JVM?
• What is the role of class loader in JVM?
• What is the role of Execution Engine in JVM?
• What are machine instructions?

Data Types

Primitive Types

• Primitive types are byte , boolean , char , short , int , float , long and double .
• Primitive types always have a value; if not assigned, will have a default value.
• A long value is suffixed with L (or l) to differentiate it from int .
• A float value is suffixed with F (or f) to differentiate it from double . Similarly double is suffixed with D ( or d )
• A char is unsigned and represent an Unicode values.
• When a primitive type is assigned to another variable, a copy is created.

Reference Types

• All non-primitive types are reference types.
• Reference types are also usually known as objects . A reference type also refers to an object in memory.
• Objects of reference type will have null as default value, when it's unassigned.
• Objects have variables and methods, which define its state and the behaviour.
• When a reference is assigned to another reference, both points to the same object.

Questions

• What are primitive data types?
• If a variable of primitive data type is not assigned, what does it contain?
• Why do we suffix L with long, F with Float and D with double?
• What happens when you assign a variable of primitive data type to another variable of same type?
• What are reference data types?
• What happens when you assign a variable of reference data type to another variable of same reference type?
• What are the differences between primitive data types and reference data types?
• What are the purposes of variables and methods in a reference type?
• If a variable of reference data type is not assigned, what does it contain?

Object class

Every Java class is inherited, directly or indirectly, from java.lang.Object class, which also means that a variable of Object class can reference object of any class.

Due to inheritance, all the following java.lang.Object class methods, which are not final or private , are available for overriding with class specific code.

• - returns hash-code value for the object.
• - compares two objects for equality using identity (==) operator.
• interface and implement clone() method to define the meaning of copy.
• toString() - returns string representation of the object.
• f - called by the Garbage Collector to clean up the resources. java.lang.Object's implementation of finalize() does nothing.

Questions

• What is the base class for all Java classes?
• What are the different methods of java.lang.Object class, which are available for overriding in the derived class.
• What happens if your class does not override equals method from the java.lang.Object class?
  • The equals() method in java.lang.Object class compares whether object references are same, and not the content. To compare content, you need to override equals() method.
• What is the purpose of () method?
• Why should the overriding class define the meaning of clone() method?
• What happens if overriding class does not override clone() method?
  • In case if an object contains references to an external objects, any change made to the referenced object will be visible in the cloned object too.

Access Modifiers

Access modifiers determine the visibility rules whether other classes can access a variable or invoke a method.