Java: Interview Questions Part 1 - Online Article

1. What is the difference between private, protected, and public ?

These keywords are for allowing privileges to components such as java methods and variables.

Public: accessible to all classes
Private: accessible only to the class to which they belong
Protected: accessible to the class to which they belong and any subclasses.

Access specifiers are keywords that determines the type of access to the member of a class. These are:

  • Public
  • Protected
  • Private
  • Defaults

2. What's the difference between an interface and an abstract class ? Also discuss the similarities.

Abstract class is a class which contain one or more abstract methods, which has to be implemented by sub classes. Interface is a Java Object containing method declaration and doesn't contain implementation. The classes which have implementing the Interfaces must provide the method definition for all the methods. Abstract class is a Class prefix with a abstract keyword followed by Class definition. Interface is a Interface which starts with interface keyword.

Abstract class contains one or more abstract methods. where as Interface contains all abstract methods and final declarations. Abstract classes are useful in a situation that Some general methods should be implemented and specialization behavior should be implemented by child classes. Interfaces are useful in a situation that all properties should be implemented.

Differences are as follows:

  • 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 to find corresponding method in in the actual class. Abstract classes are fast.

Similarities:

  • Neither Abstract classes or Interface can be instantiated.

How to define an Abstract class ?

A class containing abstract method is called Abstract class. An Abstract class can't be instantiated.

Example of Abstract class:

abstract class testAbstractClass {
   protected String myString;
   public String getMyString() {
   return myString;
}
public abstract string anyAbstractFunction();
}

How to define an Interface 

Answer: In Java Interface defines the methods but does not implement them. Interface can include constants. A class that implements the interfaces is bound to implement all the methods defined in Interface.

Example of Interface:

public interface sampleInterface {
   public void functionOne();
   public long CONSTANT_ONE = 1000;
}

3. How you can force the garbage collection ?


Garbage collection automatic process and can't be forced. You could request it by calling System.gc(). JVM does not guarantee that GC will be started immediately.

Garbage collection is one of the most important feature of Java, Garbage collection is also called automatic memory management as JVM automatically removes the unused variables/objects (value is null) from the memory. User program can't directly free the object from memory, instead it is the job of the garbage collector to automatically free the objects that are no longer referenced by a program. Every class inherits finalize() method from java.lang.Object, the finalize() method is called by garbage collector when it determines no more references to the object exists. In Java, it is good idea to explicitly assign null into a variable when no more in use. I Java on 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.

4. What's the difference between constructors and normal methods ?


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 and it can return a value or can be void.

5. Can you call one constructor from another if a class has multiple constructors


Yes. Use this() to call a constructor from an other constructor.

6. Explain the usage of Java packages.


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.

7. Explain in your own words the "bottom line" benefits of the use of an interface.


The interface makes it possible for a method in one class to invoke methods on objects of other classes, without the requirement to know the true class of those objects, provided that those objects are all instantiated from classes that implement one or more specified interfaces. In other words, objects of classes that implement specified interfaces can be passed into methods of other objects as the generic type Object, and the methods of the other objects can invoke methods on the incoming objects by first casting them as the interface type.

8. What are some advantages and disadvantages of Java Sockets ?


Some advantages of Java Sockets:

Sockets are flexible and sufficient. Efficient socket based programming can be easily implemented for general communications. Sockets cause low network traffic. Unlike HTML forms and CGI scripts that generate and transfer whole web pages for each new request, Java applets can send only necessary updated information.

Some disadvantages of Java Sockets:

Security restrictions are sometimes overbearing because a Java applet running in a Web browser is only able to establish connections to the machine where it came from, and to nowhere else on the network   Despite all of the useful and helpful Java features, Socket based communications allows only to send packets of raw data between applications. Both the client-side and server-side have to provide mechanisms to make the data useful in any way.

9. Explain the usage of the keyword transient ?


Transient 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).

10. What's the difference between the methods sleep() and wait()


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.

11. What would you use to compare two String variables - the operator == or the method equals() ?


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.

12. Why would you use a synchronized block vs. synchronized method ?


Synchronized blocks place locks for shorter periods than synchronized methods.

13. 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.

14. Can an inner class declared inside of a method access local variables of this method ?


It's possible if these variables are final.

15. What can go wrong if you replace && with & in the following code:

String a=null; if (a!=null && a.length()>10) {...}


A single ampersand here would lead to a NullPointerException.

16. What's the main difference between a Vector and an ArrayList ?


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

17. Describe the wrapper classes in Java.


Wrapper class is wrapper around a primitive data type. An instance of a wrapper class contains, or wraps, a primitive value of the corresponding type.

Following table lists the primitive types and the corresponding wrapper classes:

  • Primitive Wrapper
  • boolean  - java.lang.Boolean
  • byte - java.lang.Byte
  • char - java.lang.Character
  • double - java.lang.Double
  • float - java.lang.Float
  • int - java.lang.Integer
  • long - java.lang.Long
  • short - java.lang.Short
  • void - java.lang.Void


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


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);

19. How do you know if an explicit object casting is needed ?


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;


20. When you assign a subclass to a variable having a supeclass type, the casting is performed automatically. Can you write a Java class that could be used both as an applet as well as an application ?


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

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


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 com.xyz.hr; and is located in the file c:\dev\com\xyz\hr\Employee.javIn 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:

c:\>java com.xyz.hr.Employee

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Comments

m rani on 2012-10-11 12:11:45 wrote,

nice questions but pls provide more ques