Saturday, 28 January 2017

Java Interview Questions and Answers

Question: What are access modifiers

Answer: These public, protected and private, these can be applied to class, variables, constructors and methods. But if you don't specify an access modifier then it is considered as Friendly

Question: Can protected or friendly features be accessed from different packages

Answer: No when features are friendly or protected they can be accessed from all the classes in that package but not from classes in another package

Question: How can you access protected features from another package

Answer: You can access protected features from other classes by subclassing the that class in another package, but this cannot be done for friendly features

Question: What are the rules for overriding


Private method can be overridden by private, friendly, protected or public methods
Friendly method can be overridden by friendly, protected or public methods
Protected method can be overridden by protected or public methods
Public method can be overridden by public method

Question: Explain modifier final

Answer: Final can be applied to classes, methods and variables and the features cannot be changed. Final class cannot be subclassed, methods cannot be overridden

Question: Can you change the reference of the final object

Answer: No the reference cannot be change, but the data in that object can be changed

Question: Can abstract modifier be applied to a variable

Answer: No it is applied only to class and methods

Question: Can abstract class be instantiated

Answer: No abstract class cannot be instantiated i.e you cannot create a new object of this class

Question: When does the compiler insist that the class must be abstract


If one or more methods of the class are abstract.
If class inherits one or more abstract methods from the parent abstract class and no implementation is provided for that method
If class implements an interface and provides no implementation for those methods

Question: How is abstract class different from final class

Answer: Abstract class must be subclassed and final class cannot be subclassed

Question: Where can static modifiers be used

Answer: They can be applied to variables, methods and even a block of code, static methods and variables are not associated with any instance of class

Question: When are the static variables loaded into the memory

Answer: During the class load time

Question: When are the non static variables loaded into the memory

Answer: They are loaded just before the constructor is called

Question: How can you reference static variables

Answer: Via reference to any instance of the class

Computer comp = new Computer ();
comp.harddisk where hardisk is a static variable
comp.compute() where compute is a method

Via the class name


Question: Can static method use non static features of there class

Answer: No they are not allowed to use non static features of the class, they can only call static methods and can use static data

Question: What is static initializer code

Answer: A class can have a block of initializer code that is simply surrounded by curly braces and labeled as static e.g.
public class Demo{
static int =10;
System.out.println(?Hello world');

And this code is executed exactly once at the time of class load

Where is native modifier used It can refer only to methods and it indicates that the body of the method is to be found else where and it is usually written in non java language

Question: What are transient variables

Answer: A transient variable is not stored as part of objects persistent state and they cannot be final or static

Question: What is synchronized modifier used for

Answer: It is used to control access of critical code in multithreaded programs

Question: What are volatile variables

Answer: It indicates that these variables can be modified asynchronously 

Question: What is serialization ?

Answer: Serialization is the process of writing complete state of java object into output stream, that stream can be file or byte array or stream associated with TCP/IP socket.

Question: What does the Serializable interface do ?

Answer: Serializable is a tagging interface; it prescribes no methods. It serves to assign the Serializable data type to the tagged class and to identify the class as one which the developer has designed for persistence. ObjectOutputStream serializes only those objects which implement this interface.

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

Answer: To serialize an object into a stream perform the following actions:

- Open one of the output streams, for exaample FileOutputStream
- Chain it with the ObjectOutputStream - Call the method writeObject() providingg the instance of a Serializable object as an argument.
- Close the streams

Java Code --------- try{ fOut= new FileOutputStream("c:\\emp.ser"); out = new ObjectOutputStream(fOut); out.writeObject(employee); //serializing System.out.println("An employee is serialized into c:\\emp.ser"); } catch(IOException e){ e.printStackTrace(); }

Question: How do I deserilaize an Object?

Answer: To deserialize an object, perform the following steps:

- Open an input stream
- Chain it with the ObjectInputStream - Call the method readObject() and cast tthe returned object to the class that is being deserialized.
- Close the streams
Java Code try{ fIn= new FileInputStream("c:\\emp.ser"); in = new ObjectInputStream(fIn); //de-serializing employee Employee emp = (Employee) in.readObject(); System.out.println("Deserialized " + emp.fName + " " + emp.lName + " from emp.ser "); }catch(IOException e){ e.printStackTrace(); }catch(ClassNotFoundException e){ e.printStackTrace(); }

Question: What is Externalizable Interface ?

Answer : Externalizable interface is a subclass of Serializable. Java provides Externalizable interface that gives you more control over what is being serialized and it can produce smaller object footprint. ( You can serialize whatever field values you want to serialize)

This interface defines 2 methods: readExternal() and writeExternal() and you have to implement these methods in the class that will be serialized. In these methods you'll have to write code that reads/writes only the values of the attributes you are interested in. Programs that perform serialization and deserialization have to write and read these attributes in the same sequence.

Question: Explain garbage collection ?

Answer: Garbage collection is an important part of Java's security strategy. Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects from the memory. The name "garbage collection" implies that objects that are no longer needed by the program are "garbage" and can be thrown away. A more accurate and up-to-date metaphor might be "memory recycling." When an object is no longer referenced by the program, the heap space it occupies must be recycled so that the space is available for subsequent new objects. The garbage collector must somehow determine which objects are no longer referenced by the program and make available the heap space occupied by such unreferenced objects. In the process of freeing unreferenced objects, the garbage collector must run any finalizers of objects being freed

Question : How you can force the garbage collection ?

Answer : Garbage collection automatic process and can't be forced. We can call garbage collector in Java by calling System.gc() and Runtime.gc(), JVM tries to recycle the unused objects, but there is no guarantee when all the objects will garbage collected.

Question : What are the field/method access levels (specifiers) and class access levels ?

Answer: Each field and method has an access level:

    private: accessible only in this class
    (package): accessible only in this package
    protected: accessible only in this package and in all subclasses of this class
    public: accessible everywhere this class is available

Similarly, each class has one of two possible access levels:

    (package): class objects can only be declared and manipulated by code in this package

public: class objects can be declared and manipulated by code in any package

 Question: How can i tell what state a thread is in ?

Answer: Prior to Java 5, isAlive() was commonly used to test a threads state. If isAlive() returned false the thread was either new or terminated but there was simply no way to differentiate between the two.

Starting with the release of Tiger (Java 5) you can now get what state a thread is in by using the getState() method which returns an Enum of Thread.States. A thread can only be in one of the following states at a given point in time.

NEW  Fresh thread that has not yet started to execute.

RUNNABLE  A thread that is executing in the Java virtual machine.

BLOCKED A thread that is blocked waiting for a monitor lock.

WAITING A thread that is wating to be notified by another thread.

TIMED_WAITING A thread that is wating to be notified by another thread for a specific amount of time

TERMINATED A thread whos run method has ended.

The folowing code prints out all thread states. public class ThreadStates{ public static void main(String[] args){ Thread t = new Thread(); Thread.State e = t.getState(); Thread.State[] ts = e.values(); for(int i = 0; i < ts.length; i++){ System.out.println(ts[i]); } } }

Question: What methods java providing for Thread communications ?

Answer: Java provides three methods that threads can use to communicate with each other: wait, notify, and notifyAll. These methods are defined for all Objects (not just Threads). The idea is that a method called by a thread may need to wait for some condition to be satisfied by another thread; in that case, it can call the wait method, which causes its thread to wait until another thread calls notify or notifyAll.

Question: What is the difference between notify and notify All methods ?

Answer: A call to notify causes at most one thread waiting on the same object to be notified (i.e., the object that calls notify must be the same as the object that called wait). A call to notifyAll causes all threads waiting on the same object to be notified. If more than one thread is waiting on that object, there is no way to control which of them is notified by a call to notify (so it is often better to use notifyAll than notify).

Question: What is synchronized keyword? In what situations you will Use it?

Answer: Synchronization is the act of serializing access to critical sections of code. We will use this keyword when we expect multiple threads to access/modify the same data. To understand synchronization we need to look into thread execution manner.

Threads may execute in a manner where their paths of execution are completely independent of each other. Neither thread depends upon the other for assistance. For example, one thread might execute a print job, while a second thread repaints a window. And then there are threads that require synchronization, the act of serializing access to critical sections of code, at various moments during their executions. For example, say that two threads need to send data packets over a single network connection. Each thread must be able to send its entire data packet before the other thread starts sending its data packet; otherwise, the data is scrambled. This scenario requires each thread to synchronize its access to the code that does the actual data-packet sending.

If you feel a method is very critical for business that needs to be executed by only one thread at a time (to prevent data loss or corruption), then we need to use synchronized keyword.


Some real-world tasks are better modeled by a program that uses threads than by a normal, sequential program. For example, consider a bank whose accounts can be accessed and updated by any of a number of automatic teller machines (ATMs). Each ATM could be a separate thread, responding to deposit and withdrawal requests from different users simultaneously. Of course, it would be important to make sure that two users did not access the same account simultaneously. This is done in Java using synchronization, which can be applied to individual methods, or to sequences of statements.

One or more methods of a class can be declared to be synchronized. When a thread calls an object's synchronized method, the whole object is locked. This means that if another thread tries to call any synchronized method of the same object, the call will block until the lock is released (which happens when the original call finishes). In general, if the value of a field of an object can be changed, then all methods that read or write that field should be synchronized to prevent two threads from trying to write the field at the same time, and to prevent one thread from reading the field while another thread is in the process of writing it.

Here is an example of a BankAccount class that uses synchronized methods to ensure that deposits and withdrawals cannot be performed simultaneously, and to ensure that the account balance cannot be read while either a deposit or a withdrawal is in progress. (To keep the example simple, no check is done to ensure that a withdrawal does not lead to a negative balance.)
public class BankAccount { private double balance; // constructor: set balance to given amount public BankAccount( double initialDeposit ) { balance = initialDeposit; } public synchronized double Balance( ) { return balance; } public synchronized void Deposit( double deposit ) { balance += deposit; } public synchronized void Withdraw( double withdrawal ) { balance -= withdrawal; } }

Note: that the BankAccount's constructor is not declared to be synchronized. That is because it can only be executed when the object is being created, and no other method can be called until that creation is finished.

There are cases where we need to synchronize a group of statements, we can do that using synchrozed statement.
Java Code Example synchronized ( B ) { if ( D > B.Balance() ) { ReportInsuffucientFunds(); } else { B.Withdraw( D ); } }

 Question: What are different types of inner classes ?

Answer: Inner classes nest within other classes. A normal class is a direct member of a package. Inner classes, which became available with Java 1.1, are four types

    Static member classes
    Member classes
    Local classes
    Anonymous classes

Static member classes - a static member class is a static member of a class. Like any other static method, a static member class has access to all static methods of the parent, or top-level, class.

Member Classes - a member class is also defined as a member of a class. Unlike the static variety, the member class is instance specific and has access to any and all methods and members, even the parent's this reference.

Local Classes - Local Classes declared within a block of code and these classes are visible only within the block.

Anonymous Classes - These type of classes does not have any name and its like a local class
Java Anonymous Class Example public class SomeGUI extends JFrame { ... button member declarations ... protected void buildGUI() { button1 = new JButton(); button2 = new JButton(); ... button1.addActionListener( new java.awt.event.ActionListener() <------ Anonymous Class { public void actionPerformed(java.awt.event.ActionEvent e) { // do something } } );

Question: What are the uses of Serialization?

Answer: In some types of applications you have to write the code to serialize objects, but in many cases serialization is performed behind the scenes by various server-side containers.

These are some of the typical uses of serialization:

    To persist data for future use.
    To send data to a remote computer using such client/server Java technologies as RMI or socket programming.
    To "flatten" an object into array of bytes in memory.
    To exchange data between applets and servlets.
    To store user session in Web applications.
    To activate/passivate enterprise java beans.
    To send objects between the servers in a cluster.

Question: what is a collection ?

Answer: Collection is a group of objects. java.util package provides important types of collections. There are two fundamental types of collections they are Collection and Map. Collection types hold a group of objects, Eg. Lists and Sets where as Map types hold group of objects as key, value pairs Eg. HashMap and Hashtable.

Question: For concatenation of strings, which method is good, StringBuffer or String ?

Answer: StringBuffer is faster than String for concatenation.

Question: What is Runnable interface ? Are there any other ways to make a java program as multithred java program?

Answer: There are two ways to create new kinds of threads:

- Define a new class that extends the Thread class
- Define a new class that implements the Runnable interface, and pass an object of that class to a Thread's constructor.
- An advantage of the second approach is that the new class can be a subclass of any class, not just of the Thread class.
Here is a very simple example just to illustrate how to use the second approach to creating threads: class myThread implements Runnable { public void run() { System.out.println("I'm running!"); } } public class tstRunnable { public static void main(String[] args) { myThread my1 = new myThread(); myThread my2 = new myThread(); new Thread(my1).start(); new Thread(my2).start(); }

Question: How could Java classes direct program messages to the system console, but error messages, say to a file?

Answer: The class System has a variable out that represents the standard output, and the variable err that represents the standard error device. By default, they both point at the system console. This how the standard output could be re-directed:

Stream st = new Stream(new FileOutputStream("output.txt")); System.setErr(st); System.setOut(st); 

Question: What's the difference between an interface and an abstract class?

Answer: An abstract class may contain code in method bodies, which is not allowed in an interface. With abstract classes, you have to inherit your class from it and Java does not allow multiple inheritance. On the other hand, you can implement multiple interfaces in your class.

Question: Why would you use a synchronized block vs. synchronized method?

Answer: Synchronized blocks place locks for shorter periods than synchronized methods.

Question: Explain the usage of the keyword transient?

Answer: This keyword indicates that the value of this member variable does not have to be serialized with the object. When the class will be de-serialized, this variable will be initialized with a default value of its data type (i.e. zero for integers).

Question: How can you force garbage collection?

Answer: You can't force GC, but could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

Question: How do you know if an explicit object casting is needed?

Answer: If you assign a superclass object to a variable of a subclass's data type, you need to do explicit casting. For example:

Object a; Customer b; b = (Customer) a;

When you assign a subclass to a variable having a supeclass type, the casting is performed automatically.

Question: What's the difference between the methods sleep() and wait()

Answer: The code sleep(1000); puts thread aside for exactly one second. The code wait(1000), causes a wait of up to one second. A thread could stop waiting earlier if it receives the notify() or notifyAll() call. The method wait() is defined in the class Object and the method sleep() is defined in the class Thread.

Question: Can you write a Java class that could be used both as an applet as well as an application?

Answer: Yes. Add a main() method to the applet.

Question: What's the difference between constructors and other methods?

Answer: Constructors must have the same name as the class and can not return a value. They are only called once while regular methods could be called many times.

Question: Can you call one constructor from another if a class has multiple constructors

Answer: Yes. Use this() syntax.

Question: Explain the usage of Java packages.

Answer: This is a way to organize files when a project consists of multiple modules. It also helps resolve naming conflicts when different packages have classes with the same names. Packages access level also allows you to protect data from being used by the non-authorized classes.

Question: If a class is located in a package, what do you need to change in the OS environment to be able to use it?

Answer: You need to add a directory or a jar file that contains the package directories to the CLASSPATH environment variable. Let's say a class Employee belongs to a package; and is located in the file c:\dev\com\xyz\hr\ In this case, you'd need to add c:\dev to the variable CLASSPATH. If this class contains the method main(), you could test it from a command prompt window as follows:


Question: What's the difference between J2SDK 1.5 and J2SDK 5.0?

Answer:  There's no difference, Sun Microsystems just re-branded this version.

Question: What would you use to compare two String variables - the operator == or the method equals()?
Answer:  I'd use the method equals() to compare the values of the Strings and the == to check if two variables point at the same instance of a String object.

Question: Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?

Answer: Yes, it does. The FileNoFoundException is inherited from the IOException. Exception's subclasses have to be caught first.

Question: Can an inner class declared inside of a method access local variables of this method?

Answer: It's possible if these variables are final.

Question: What can go wrong if you replace && with & in the following code: String a=null; if (a!=null && a.length()>10) {...}

Answer: A single ampersand here would lead to a NullPointerException.

Question: What's the main difference between a Vector and an ArrayList

Answer: Java Vector class is internally synchronized and ArrayList is not.

Question: When should the method invokeLater()be used?

Answer: This method is used to ensure that Swing components are updated through the event-dispatching thread.

Question: How can a subclass call a method or a constructor defined in a superclass?

Answer: Use the following syntax: super.myMethod(); To call a constructor of the superclass, just write super(); in the first line of the subclass's constructor.

For senior-level developers:

Question: What's the difference between a queue and a stack?

Answer: Stacks works by last-in-first-out rule (LIFO), while queues use the FIFO rule

Question: You can create an abstract class that contains only abstract methods. On the other hand, you can create an interface that declares the same methods. So can you use abstract classes instead of interfaces?

Answer: Sometimes. But your class may be a descendent of another class and in this case the interface is your only option.

Question: What comes to mind when you hear about a young generation in Java?

Answer: Garbage collection.

Question: What comes to mind when someone mentions a shallow copy in Java?

Answer: Object cloning.

Question: If you're overriding the method equals() of an object, which other method you might also consider?

Answer: hashCode()

Question: 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?

Answer: ArrayList

Question: How would you make a copy of an entire Java object with its state?

Answer: Have this class implement Cloneable interface and call its method clone().

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

Answer: Use object pooling and weak object references.

Question: 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?

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

Question: What access level do you need to specify in the class declaration to ensure that only classes from the same directory can access it?
Answer: You do not need to specify any access level, and Java will use a default package access level .

Question: When you declare a method as abstract method ?

Answer: When i want child class to implement the behavior of the method.

Question: Can I call a abstract method from a non abstract method ?

Answer: Yes, We can call a abstract method from a Non abstract method in a Java abstract class        

Question: What is the difference between an Abstract class and Interface in Java ? or can you explain when you use Abstract classes ?

Answer: Abstract classes let you define some behaviors; they force your subclasses to provide others. These abstract classes will provide the basic funcationality of your applicatoin, child class which inherited this class will provide the funtionality of the abstract methods in abstract class. When base class calls this method, Java calls the method defined by the child class.

    An Interface can only declare constants and instance methods, but cannot implement default behavior.
    Interfaces provide a form of multiple inheritance. A class can extend only one other class.
    Interfaces are limited to public methods and constants with no implementation. Abstract classes can have a partial implementation, protected parts, static methods, etc.
    A Class may implement several interfaces. But in case of abstract class, a class may extend only one abstract class.
    Interfaces are slow as it requires extra indirection to find corresponding method in the actual class. Abstract classes are fast.

Question: What is user-defined exception in java ?

Answer: User-defined expections are the exceptions defined by the application developer which are errors related to specific application. Application Developer can define the user defined exception by inherite the Exception class as shown below. Using this class we can throw new exceptions.
Java Example : public class noFundException extends Exception { } Throw an exception using a throw statement: public class Fund { ... public Object getFunds() throws noFundException { if (Empty()) throw new noFundException(); ... } } User-defined exceptions should usually be checked.

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