The Java Job Interview

By | July 20, 2012

Typically, on Friday’s we have an interview and spotlight on one of our users and their application. But today, we wanted to look at the interview from another angle: The Java Job Interview.

So, let’s say you are looking for a job. You have all kinds of interviews, but you want to make sure that you are up to speed on things and just want to double check your knowledge. Well, go no further. Here is a quick run-down on some most-know stuff for any interview for a Java job in an easy Q&A format.

Interview Questions

Q: What is the difference between an Interface and an Abstract class?




Q: Describe synchronization in respect to multithreading.


Q: Explain different way of using thread?

A: The thread could be implemented by using runnable interface or by inheriting from the Thread class. The former is more advantageous, ’cause when you are going for multiple inheritance..the only interface can help.

Q: What are pass by reference and passby value?


Q: What is HashMap and Map?

A: Map is Interface and Hashmap is class that implements that.

Q: Difference between HashMap and HashTable?


Q: Difference between Vector and ArrayList?

A: Vector is synchronized whereas arraylist is not.

Q: Difference between Swing and Awt?


Q: What is the difference between a constructor and a method?


Q: What is an Iterator?







Q: What is an abstract class?



Q: What is static in java?

A: Static means one per class, not one for each object no matter how many instance of a class might exist. This means that you can use them without creating an instance of a class.Static methods are implicitly final, because overriding is done based on the type of the object, and static methods are attached to a class, not an object. A static method in a superclass can be shadowed by another static method in a subclass, as long as the original method was not declared final. However, you can’t override a static method with a nonstatic method. In other words, you can’t change a static method into an instance method in a subclass.

Q: What is final?

A: A final class can’t be extended ie., final class may not be subclassed. A final method can’t be overridden when its class is inherited. You can’t change value of a final variable (is a constant).

Q: What if the main method is declared as private?

A: The program compiles properly but at runtime it will give “Main method not public.” message.

Q: What if the static modifier is removed from the signature of the main method?

A: Program compiles. But at runtime throws an error “NoSuchMethodError”.

Q: What if I write static public void instead of public static void?

A: Program compiles and runs properly.

Q: What if I do not provide the String array as the argument to the method?

A: Program compiles but throws a runtime error “NoSuchMethodError”.

Q: What is the first argument of the String array in main method?



A: It is empty. But not null.

Q: How can one prove that the array is not null but empty using one line of code?



A: CLASSPATH and PATH are the two variables.

Q: Can an application have multiple classes having main method?


Q: Can I have multiple main methods in the same class?


Q: Do I need to import java.lang package any time? Why ?

A: No. It is by default loaded internally by the JVM.

Q: Can I import same package/class twice? Will the JVM load the package twice at runtime?


Q: What are Checked and UnChecked Exception?


Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by’s read() method·


exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String’s charAt() method· Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.

Q: What is Overriding?



Q: What are different types of inner classes?

A: Nested top-level classes, Member classes, Local classes, Anonymous classes






Anonymous classes – Anonymous inner classes extend local inner classes one level further. As anonymous classes have no name, you cannot provide a constructor.

Q: Are the imports checked for validity at compile time? e.g. will the code containing an import such as java.lang.ABCD compile?

A: Yes the imports are checked for the semantic validity at compile time. The code containing above line of import will not compile. It will throw an error saying,can not resolve symbol

symbol : class ABCD

location: package io


Q: Does importing a package imports the subpackages as well? e.g. Does importing com.MyTest.* also import com.MyTest.UnitTests.*?

A: No you will have to import the subpackages explicitly. Importing com.MyTest.* will import classes in the package MyTest only. It will not import any class in any of it’s subpackage.

Q: What is the difference between declaring a variable and defining a variable?

A: In declaration we just mention the type of the variable and it’s name. We do not initialize it. But defining means declaration + initialization.

e.g String s; is just a declaration while String s = new String (“abcd”); Or String s = “abcd”; are both definitions.

Q: What is the default value of an object reference declared as an instance variable?

A: null unless we define it explicitly.

Q: Can a top level class be private or protected?

A: No. A top level class can not be private or protected. It can have either “public” or no modifier. If it does not have a modifier it is supposed to have a default access.If a top level class is declared as private the compiler will complain that the “modifier private is not allowed here”. This means that a top level class can not be private. Same is the case with protected.

Q: What type of parameter passing does Java support?

A: In Java the arguments are always passed by value .

Q: Primitive data types are passed by reference or pass by value?

A: Primitive data types are passed by value.

Q: Objects are passed by value or by reference?

A: Java only supports pass by value. With objects, the object reference itself is passed by value and so both the original reference and parameter copy both refer to the same object .

Q: What is serialization?


Q: How do I serialize an object to a file?

A: The class whose instances are to be serialized should implement an interface Serializable. Then you pass the instance to the ObjectOutputStream which is connected to a fileoutputstream. This will save the object to a file.

Q: Which methods of Serializable interface should I implement?

A: The serializable interface is an empty interface, it does not contain any methods. So we do not implement any methods.

Q: How can I customize the seralization process? i.e. how can one have a control over the serialization process?


Q: What is the common usage of serialization?


Q: What is Externalizable interface?


Q: When you serialize an object, what happens to the object references included in the object?


Q: What one should take care of while serializing the object?


Q: What happens to the static fields of a class during serialization?

A: There are three exceptions in which serialization doesnot necessarily read and write to the stream. These are



3. Transient fields.

Q: Does Java provide any construct to find out the size of an object?


Q: Give a simplest way to find out the time a method takes for execution without using any profiling tool?


To put it in code…

long start = System.currentTimeMillis ();

method ();

long end = System.currentTimeMillis ();



Q: What are wrapper classes?


Q: Why do we need wrapper classes?


Q: What are checked exceptions?

A: Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.

Q: What are runtime exceptions?


Q: What is the difference between error and an exception?


Q: How to create custom exceptions?

A: Your class should extend class Exception, or some more specific type thereof.

Q: If I want an object of my class to be thrown as an exception object, what should I do?

A: The class should extend from Exception class. Or you can extend your class from some more precise exception type also.

Q: If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?

A: One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

Q: How does an exception permeate through the code?

A: An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

Q: What are the different ways to handle exceptions?

A: There are two ways to handle exceptions,


2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.

Q: What is the basic difference between the 2 approaches to exception handling.

1> try catch block and

2> specifying the candidate exceptions in the throws clause?

When should you use which approach?

A: In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it’s own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

Q: Is it necessary that each try block must be followed by a catch block?

A: It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

Q: If I write return at the end of the try block, will the finally block still execute?




Q: How are Observer and Observable used?


Q: What is synchronization and why is it important?


Q: How does Java handle integer overflows and underflows?


Q: Does garbage collection guarantee that a program will not run out of memory?


Q: What is the difference between preemptive scheduling and time slicing?


Q: When a thread is created and started, what is its initial state?

A: A thread is in the ready state after it has been created and started.

Q: What is the purpose of finalization?


Q: What is the Locale class?


Q: What is the difference between a while statement and a do statement?

A: A while statement checks at the beginning of a loop to see whether the next loop iteration should occur. A do statement checks at the end of a loop to see whether the next iteration of a loop should occur. The do statement will always execute the body of a loop at least once.

Q: What is the difference between static and non-static variables?


Q: How are this() and super() used with constructors?

A: This() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

Q: What are synchronized methods and synchronized statements?

A: Synchronized methods are methods that are used to control access to an object. A thread only executes a synchronized method after it has acquired the lock for the method’s object or class. Synchronized statements are similar to synchronized methods. A synchronized statement can only be executed after a thread has acquired the lock for the object or class referenced in the synchronized statement.

Q: What is daemon thread and which method is used to create the daemon thread?


Q: Can applets communicate with each other?

A: At this point in time applets may communicate with other applets running in the same virtual machine. If the applets are of the same class, they can communicate via shared static variables. If the applets are of different classes, then each will need a reference to the same class with static variables. In any case the basic idea is to pass the information back and forth through a static variable.

An applet can also get references to all other applets on the same page using the getApplets() method of java.applet.AppletContext. Once you get the reference to an applet, you can communicate with it by using its public members.

It is conceivable to have applets in different virtual machines that talk to a server somewhere on the Internet and store any data that needs to be serialized there. Then, when another applet needs this data, it could connect to this same server. Implementing this is non-trivial.

Q: What are the steps in the JDBC connection?

A: While making a JDBC connection we go through the following steps :

Step 1 : Register the database driver by using :

Class.forName(” driver classs for that specific database” );

Step 2 : Now create a database connection using :

Connection con = DriverManager.getConnection(url,username,password);

Step 3: Now Create a query using :

Statement stmt = Connection.Statement(“select * from TABLE NAME”);

Step 4 : Exceute the query :


Q: How does a try statement determine which catch clause should be used to handle an exception?

A: When an exception is thrown within the body of a try statement, the catch clauses of the try statement are examined in the order in which they appear. The first catch clause that is capable of handling the exceptions executed. The remaining catch clauses are ignored.

Obviously, this list isn’t complete, nor is an interview just questions, but this is a good start to test your knowledge. If you want a more comprehensive list, you can continue where we leave off here after the jump.

Source: All App Labs : Java Interview Questions

Subscribe to get the latest updates