Introduction to Session Management

Session tracking is the process of maintaining information, or state, about Web site visitors as they move from page to page. It requires some work on the part of the Web developer since there's no built-in mechanism for it. The connection from a browser to a Web server occurs over the stateless Hypertext Transfer Protocol (HTTP).

There are a number of ways to handle session tracking, but our focus is on the easy-to-use yet powerful HttpSession interface provided by the Java Servlet specification. Before we get into the HttpSession interface, let's look at some other ways of maintaining state.

1. Session-Tracking Techniques

At one time Web developers used website visitors' IP addresses to track the sessions. This approach was inflexible and had many flaws. The main problem was that proxy servers eliminated the use of individual IP addresses. Users no longer had unique addresses, so this technique couldn't work properly.

2. Another way of handling session tracking is the use of the HTML hidden field:

<INPUT TYPE="hidden" NAME="user"VALUE="Sumit">

This technique required server-side scripting that would dynamically generate the HTML code that contained the "user" field. Server-side code was also required to read the field and match it to information about this user on the server.

3.  Another session-tracking technique is URL rewriting. In this approach, identification field(s) are appended to the end of each URL for a Web site. The following HTML code demonstrates this method:

<A HREF="/orderform.htm?user=Sumit">Order Now!</A>

This approach is similar to hidden fields. The difference is that hidden fields can only be used in a form.

A common way of session tracking is the use of cookies. A cookie is information that's stored as a name/value pair and transmitted from the server to the browser. Cookies containing unique user information can be used to tie specific visitors to information about them on the server. The Java Servlet specification provides a simple cookie API that allows you to write and retrieve cookies. The complete API can be found on Sun's Java Web site, http://java.sun.com. The following code shows how to create a new cookie:

Cookie user = new Cookie("user","Sumit");
user.setMaxAge(3600);
response.addCookie(user);

This code creates a cookie with a name of "user" and a value of "Sumit". The cookie's expiration date is set with the setMaxAge() method to 3,600 seconds from the time the browser receives the cookie. The following code demonstrates how you would retrieve the value for a specific cookie:

String user = "";
Cookie[] cookies = request.getCookies();
if (cookies != null) {
for (int i = 0; i < cookies.length; i++) {
if (cookies[i].getName().equals("user")) user =
cookies[i].getValue();
}
}

In this code, an array of cookies is retrieved from the HttpServletRequest object using the getCookies() method. The array is walked through until a cookie with the name of "user" is returned by the getName() method. Once the cookie is found, the getValue() method is called to retrieve the value of the cookie.

The use of cookies provides a flexible and easy option for handling session tracking; however, it does present some problems. The information in the cookie is stored on the client's browser in a text file that can be easily read and manipulated, and this information is transmitted unsecured across the Internet. But the main problem is that they can be disabled through a setting in the Web browser. Web sites that rely on cookies for session tracking will be unable to track users who have disabled cookies.

Sessions in Java

The session management techniques we have looked at so far all have a common security issue: they transmit data in plain text. A powerful session-tracking solution is needed that's more secure and flexible. This is where the Java HttpSession API comes in. The HttpSession API provides a simple mechanism for storing information about individual users on the application server. The API provides access to a session object that can be used to store other objects. The ability to tie objects to a particular user is important when working in an object-oriented environment. It allows you to quickly and efficiently save and retrieve JavaBeans that you may be using to identify your site's visitors, to hold product information for display on your online store, or to track products that potential customers have placed in their shopping carts.

A session object is created on the application server, usually in a Java servlet or a JavaServer Page. The object is stored on the application server and a unique identifier called a session ID is assigned to it. The session object and session ID are handled by a session manager on the application server. Figure 1 illustrates this relationship. Each session ID assigned by the application server has zero or more key/value pairs tied to it. The values are objects that you place in the session. Assign each of those objects a name, and each name must have an object with it because a null is not allowed.

For this session-tracking technique to work, the session ID must be sent to the client's computer. A cookie is used to store the session ID on the Web site visitor's computer. This is automatically handled by the application server. Simply create the session object and begin using it. The application server will, by default, create the session ID and store it in a cookie. The browser will send the cookie back to the server every time a page is requested. The application server, via the server's session manager, will match the session ID from the cookie to a session object. The session object is then placed in the HttpServletRequest object and you retrieve it with the getSession() method.

CIO, CTO & Developer Resources

As we discussed earlier, some Web site visitors will have cookies disabled in their browsers. To get around this problem and continue using sessions, use URL rewriting in your code. URL rewriting appends the session ID to the URL for every page that's requested. The only problem here is that you must rewrite every link in your HTML code as well as those from servlet to servlet, or servlet to JSP.

The procedure for URL rewriting is quite simple and requires only the use of two methods found in the HttpServletResponse interface. These two methods, encodeURL() and encodeRedirectURL(), are used to append the session ID to the URL. This allows the server to track users as they move through your Web pages, but it requires that every URL be rewritten. The string returned by the methods will have the session ID appended to it only if the server determines that it's required. If the user's browser supports cookies, the returned URL will not be altered. Also, the returned URL won't be altered if the application server is configured to not use URL rewriting. The format of the altered URL will vary based on different application server implementations; however, the common format will be the addition of a parameter, such as "sessionID=uniqueIDnumber". The parameter name (in this case "sessionID") is usually controlled through a configuration setting on the server. The value of the parameter ("uniqueIDnumber" in this example) is the unique session ID assigned by the server's session manager and is a long series of letters and numbers. The following line of HTML code from a JSP creates a link to another JSP:

<A HREF="/products/product.jsp">Product Listing</A>

Clicking on this link would send the user to the product.jsp page. Using URL rewriting, the same code would be written as follows:

<A HREF="<%= response.encodeURL("/products/product.jsp")
%>">Product Listing</A>

The returned string from the encodeURL() method would contain the session ID. On a Tomcat 3.2 application server, the result of this line of code would be:

<A HREF="http://www.yourservername.com/products/
product.jsp;$sessionid$xxxx">Product Listing</A>

The xxxx would actually be a unique session ID generated by the server. The other method you can use for rewriting URLs is the encodeRedirectURL(). It's used only in a servlet or JSP that calls the sendRedirect() method of the HttpServlet-Response interface. The following code is a standard redirection statement:

response.sendRedirect("http://www.yourservername.com/
products/sale.jsp");

Using URL rewriting, the code would be:

response.sendRedirect(response.encodeRedirectURL(
"http://www.yourservername.com/products/sale.jsp"));

The application server handles the encodeRedirectURL() method a little differently than the encodeURL() method; however, each method produces the same result.

You should now have a good understanding of how the session ID is tracked and matched to a session object on the server. The first step in using the session object is creating it. The method getSession() is used to create a new session object and to retrieve an already existing one. The getSession() method is passed a Boolean flag of true or false. A false parameter indicates that you want to retrieve a session object that already exists. A true parameter lets the session manager know that a session object needs to be created if one does not already exist. The following line of code demonstrates the use of getSession():

HttpSession session = request.getSession(true);

The getSession() method will return the session object. A new session object is created if one does not already exist. The server uses the session ID to find the session object. If a session ID is not found in a cookie or the URL, a new session object is created. You would probably use only the getSession() method with a true parameter at one point in your Web application. This would be the starting point of your site, possibly after the visitor has successfully logged in. Other servlets in your application should use the getSession(false) method. This will return a current session object or null. It does not generate a new session if one doesn't already exist.

A number of methods are defined in the Java Servlet specification. (A complete API can be found on http://java.sun.com.) The methods you'll use most often and the ones we'll focus on are:

• setAttribute(String name, Object value): Binds an object to this session using the name specified. Returns nothing (void).


• getAttribute(String name): Returns the object bound with the specified name in this session, or null if no object is bound under this name.


• removeAttribute(String name): Removes the object bound with the specified name from this session. Returns nothing (void).


• invalidate(): Invalidates this session and unbinds any objects bound to it. Returns nothing (void).


• isNew(): Returns a Boolean with a value of true if the client does not yet know about the session or if the client chooses not to join the session.For an example in using sessions, we'll look at session management code that could be used for an online banking application that will allow customers to view their account information. The design of the application will follow the Model-View-Controller (MVC) architecture. The model, or data and business logic, will be represented by JavaBeans; the view will be through JavaServer Pages; and the control of the application will be handled by servlets. The ideas in these examples can easily be implemented in other types of Web applications.An online banking application should have an HTML login page where the customer can enter a login name and password in a form. The form will submit (or post) the name and password to a login servlet. The first thing the servlet needs to do is verify the username and password. To stick with the topic at hand (sessions), we'll look only at the code needed to handle the session. After the customer has been verified, a Customer JavaBean can be created. The Customer bean will contain the basic information about this visitor and will be stored in the session. We want to create a new session object, but we also want to invalidate a session that may already exist. To do this, we need to retrieve the existing object (or create a new one) and check if it's a new session using the isNew() method. If it's not a new session object, we need to invalidate it using the invalidate() method. In the servlet, we can accomplish this with the following code:HttpSession session = request.getSession (true);
  if (session.isNew() == false) {
  session.invalidate();
  session = request.getSession(true);
  }The first line of code generates a new session object, or retrieves an existing one. The second line sees if the session is new by checking the value from isNew(). A true tells you the session was just created; a false means this user already had a session and you need to invalidate it. One possible reason the user would have an old session is that he or she has two accounts and logged in on one, then tried to log in on the other.You can now add the Customer JavaBean to the session for future use. The process of placing an object into the session object is known asbinding. The Customer object can be bound to the session using the setAttribute() method as follows:session.setAttribute("CustomerBean", Customer);Since we're working on a bank's Web site, security is a priority. To be secure, every JSP and servlet needs to verify that this user is an authorized customer before displaying any information. To accomplish this, each servlet should contain code that looks in the session for a Customer object and sends any customers who do not have this object to a login page. The following code handles this:
  
  Customer customerBean = (Customer)
  session.getAttribute('CustomerBean');
  if (customerBean == null) {
  response.sendRedirect
  ("https://www.yourservername.com/login.htm");
  return;
  }
  
  The customer will have a valid Customer JavaBean in the session if he or she logged in properly. A getAttribute() on a name that does not exist in the session will always return null. Visitors with a null value need to log in, so we redirect them to a login page. This code should be placed at the top of the JSPs to prevent unauthorized use of the site. The code should also be placed in the servlets. Remember that the JSP creates the session variable for you, and in a servlet you must create it yourself. Keep in mind that this is just one way of handling security for a Web site. Some Web application servers will handle authentication and authorization for you. This example is just a simple demonstration on session management. Your Web application may require more advanced security measures.
  
  You may have noticed that the object returned by the getAttribute() method is cast into a Customer object. This is necessary for any object bound to the session. The object is stored in the session as an Object type. To use the object in your code, you must convert it (or cast it) back to the type of object it is.
  
  As customers move through your site, they may wish to retrieve various pieces of information about their accounts. For example, they may be able to view their checking account balance by clicking on a menu option. Following the MVC architecture, the link would send them to a servlet that would verify who they are and then create a CheckingAccount JavaBean. This object would then be stored in the session using the setAttribute() method, then the servlet would send the customer to a JSP that uses the CheckingAccount JavaBean to display the information about the customer's account.
  
  There may be times when you want to store something in the session other than a JavaBean, such as a text string or a number. You need to remember that you can only bind objects to the session. Text may be stored as a String object. You can put a number in the session as an Integer object. The following code demonstrates how to save a line of text and a number in the session:
  
  session.setAttribute("text","A line of text.");
  session.setAttribute("number", new Integer(10750));
  
  Remember to cast the objects when they are retrieved:
  
  String myText = (String) session.getAttribute("text");
  int myNumber = ((Integer) session.getAttribute
  ('number')).intValue();
  
  Because the session is so easy to use, you may overuse it. Simple messages from a servlet to a JSP can be placed in the session as String objects, but this is not the most efficient way. Soon you'll find that your session is loaded with messages, and most of them are no longer needed. If you find that you're passing simple strings back and forth in the session, perhaps you should consider wrapping up those messages in a special JavaBean. This would keep the session more organized. The session objects for each user of a Web site are stored in memory on the server. Throwing unnecessary information into the session reduces the server's memory resources. Store only essential information in the session and use the removeAttribute() method to clean out objects after you're finished with them.
  
  The use of the session will not only make it easier for you to program a site, but it should also help make the Web visit better for the user. The use of the beans and the session allows you to write JSPs that are customized for each user. For our bank scenario, we could use the information in the Customer bean to create personalized pages for each visitor. We can also use the bean to prepopulate forms for the customer. For example, you could use a JavaBean to store the results from a form that a visitor fills out to request information about services offered. If the user forgot to fill in a required field, you could use the JavaBean to store error messages from a servlet and then display them in the JSP. You could also populate the fields that the visitor just filled in instead of making the visitor fill out the form again.
  
  The session is not intended to be used as a persistent place to store information about a visitor. Each visitor will be assigned a new session every time he or she logs in. A back-end database will be needed if you want to store information about individual users. The session should just be used to track the user during one visit to your site. In cases where you have a large Web site and are running multiple, redundant application servers, you'll need an application server that can handle sessions across servers. This is usually handled by placing session information into a database instead of local memory so each application server can access the information. And many of the commercial application servers will be able to do this for you.
  
  Conclusion
  The use of session tracking is an important design issue because of the complexity of today's Web sites. As Java developers, we have access to a powerful and robust session manager through the use of the HttpSession API. The session examples that we went over in this article cover the main capabilities of the session API. Learning all of Java's session management features will make your job as a Web developer easier and help you create a better experience for your Web site visitors.

Java Interview Questions(Basic): Part 1

1. What is the difference between a constructor and a method?

A constructor is a special member function of a class that is used to create objects of that class. It has the same name as the class itself, has no return type, and is invoked using the new operator.

A method is an ordinary member function of a class. It has its own name, a return type (which may be void), and is invoked using the dot operator.

2. What is the purpose of garbage collection in Java, and when is it used?

The purpose of garbage collection is to identify and discard objects that are no longer needed by a program so that their resources can be reclaimed and reused.

A Java object is subject to garbage collection when it becomes unreachable to the program in which it is used.

3. Describe synchronization in respect to Multithreading?

With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources.

Without synchonization, it is possible for one thread to modify a shared variable while another thread is in the process of using or updating same shared variable. This usually leads to significant errors.

4. What is an abstract class?

Abstract class must be extended/subclassed (to be useful). It serves as a template. A class that is abstract can not be instantiated (ie. you can not call its constructor), abstract class may contain static data.

Any class with an abstract method is automatically abstract itself, and must be declared as such. A class may be declared abstract even if it has no abstract methods. This prevents it from being instantiated.

5. What is the difference between an Interface and an Abstract class?

An abstract class can have instance methods that implement a default behavior. An Interface can only declare constants and instance methods, but cannot implement default behavior and all methods are implicitly abstract.

An interface has all public members and no implementation. An abstract class is a class which may have the usual flavors of class members (private, protected, etc.), but has some abstract methods.

6. Explain different way of using thread?

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

7. What is an Iterator?

Some of the collection classes provide traversal of their contents via a java.util.Iterator interface. This interface allows you to walk through a collection of objects, operating on each object in turn.

Remember when using Iterators that they contain a snapshot of the collection at the time the Iterator was obtained; generally it is not advisable to modify the collection itself while traversing an Iterator.

8. State the significance of public, private, protected, default modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.

public: Public class is visible in other packages, field is visible everywhere (class must be public too)

private : Private variables or methods may be used only by an instance of the same class that declares the variable or method, A private feature may only be accessed by the class that owns the feature.

protected : Is available to all classes in the same package and also available to all subclasses of the class that owns the protected feature. This access is provided even to subclasses that reside in a different package from the class that owns the protected feature.

What you get by default ie, without any access modifier (ie, public private or protected). It means that it is visible to all within a particular package.

9. What is static in java?

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.

10. What is final class?

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).

11. What if the main() method is declared as private?*

The program compiles properly but at runtime it will give "Main method not found in class <classname>." message.

12. What if the static modifier is removed from the signature of the main() method?

Program compiles. But at runtime throws an error "NoSuchMethodError".

13. What if I write static public void instead of public static void?

Program compiles and runs properly.

14. What if I do not provide the String array as the argument to the method?

Program compiles but throws a runtime error "NoSuchMethodError".

15. What is the first argument of the String array in main() method?

The String array is empty. It does not have any element. This is unlike C/C++ where the first element by default is the program name.

16. If I do not provide any arguments on the command line, then the String array of main() method will be empty or null?*

It is empty. But not null.

17. How can one prove that the array is not null but empty using one line of code? *

Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.

18. What environment variables do I need to set on my machine in order to be able to run Java programs?

CLASSPATH and PATH are the two variables.

19. Can an application have multiple classes having main() method?

Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned.

Hence there is not conflict amongst the multiple classes having main() method.

20. Can I have multiple main() methods in the same class?

No the program fails to compile. The compiler says that the main() method is already defined in the class.

Java Interview Questions(Basic): Part 2

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

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

22. Can I import same package/class twice? Will the JVM load the package twice at runtime?

One can import the same package or same class multiple times. Neither compiler nor JVM complains about it. And the JVM will internally load the class only once no matter how many times you import the same class.

23. What are Checked and UnChecked Exception?

A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown.

Example: IOException thrown by java.io.FileInputStream's read() method·

Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown.

Example: 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.

24. What is Overriding?

When a class defines a method using the same name, return type, and arguments as a method in its superclass, the method in the class overrides the method in the superclass.

When the method is invoked for an object of the class, it is the new definition of the method that is called, and not the method definition from superclass. Methods may be overridden to be more public, not more private.

25. Are the imports checked for validity at compile time? Example: will the code containing an import such as java.lang.ABCD compile? *

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

import java.io.ABCD;

26. Does importing a package imports the subpackages as well? Example: Does importing com.MyTest.* also import com.MyTest.UnitTests.*?

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.

27. What is the difference between declaring a variable and defining a variable?

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

Example: String s; is just a declaration while String s = new String ("abcd"); Or String s = "abcd"; are both definitions.

28. What is the default value of an object reference declared as an instance variable?

The default value will be null unless we define it explicitly.

29. Can a top level class be private or protected?

No. A top level class cannot 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.

30. What type of parameter passing does Java support? *

In Java the arguments are always passed by value.

31. Primitive data types are passed by reference or pass by value?

Primitive data types are passed by value.

32. Objects are passed by value or by reference?

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.

33. What is serialization?

Serialization is a mechanism by which you can save the state of an object by converting it to a byte stream.

34. How do I serialize an object to a file?

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.

35. Which methods of Serializable interface should I implement?

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

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

Yes it is possible to have control over serialization process. The class should implement Externalizable interface. This interface contains two methods namely readExternal and writeExternal.

You should implement these methods and write the logic for customizing the serialization process.

37. What is the common usage of serialization?

Whenever an object is to be sent over the network, objects need to be serialized. Moreover if the state of an object is to be saved, objects need to be serilazed.

38. What is Externalizable interface?

Externalizable is an interface which contains two methods readExternal and writeExternal. These methods give you a control over the serialization mechanism.

Thus if your class implements this interface, you can customize the serialization process by implementing these methods.

39. When you serialize an object, what happens to the object references included in the object?

The serialization mechanism generates an object graph for serialization. Thus it determines whether the included object references are serializable or not. This is a recursive process.

Thus when an object is serialized, all the included objects are also serialized alongwith the original object.

40. What one should take care of while serializing the object? *

One should make sure that all the included objects are also serializable. If any of the objects is not serializable then it throws a NotSerializableException.

Java Interview Questions(Basic): Part 4

61. When a thread is created and started, what is its initial state?A thread is in the ready state after it has been created and started. 62. What is the purpose of finalization? The purpose of finalization is to give an unreachable object the opportunity to perform any cleanup processing before the object is garbage collected. 63. What is the Locale class? The Locale class is used to tailor program output to the conventions of a particular geographic, political, or cultural region. 64. What is the difference between a while statement and a do statement? 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. 65. What is the difference between static and non-static variables? A static variable is associated with the class as a whole rather than with specific instances of a class. Non-static variables take on unique values with each object instance. 66. How are this() and super() used with constructors? this() is used to invoke a constructor of the same class. super() is used to invoke a superclass constructor.

67. What is daemon thread and which method is used to create the daemon thread?

Daemon thread is a low priority thread which runs intermittently in the back ground doing the garbage collection operation for the java runtime system.setDaemon method is used to create a daemon thread.

68. Can applets communicate with each other?

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.

69. What are the steps in the JDBC connection?

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 :

stmt.exceuteUpdate();

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

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 exceptionis executed. The remaining catch clauses are ignored.

71. Can an unreachable object become reachable again?

An unreachable object may become reachable again. This can happen when the object's finalize() method is invoked and the object performs an operation which causes it to become accessible to reachable objects.

72. What method must be implemented by all threads?

All tasks must implement the run() method, whether they are a subclass of Thread or implement the Runnable interface.

73. What are synchronized methods and synchronized statements?

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.

74. What is Externalizable?

Externalizable is an Interface that extends Serializable Interface. And sends data into Streams in Compressed Format. It has two methods, writeExternal(ObjectOuput out) and readExternal(ObjectInput in).

75. What modifiers are allowed for methods in an Interface?

Only public and abstract modifiers are allowed for methods in interfaces.

76. What are some alternatives to inheritance?

Delegation is an alternative to inheritance.

Delegation means that you include an instance of another class as an instance variable, and forward messages to the instance. It is often safer than inheritance because it forces you to think about each message you forward, because the instance is of a known class, rather than a new class, and because it doesn't force you to accept all the methods of the super class: you can provide only the methods that really make sense. On the other hand, it makes you write more code, and it is harder to re-use (because it is not a subclass).

77. What does it mean that a method or field is "static"?

Static variables and methods are instantiated only once per class. In other words they are class variables, not instance variables. If you change the value of a static variable in a particular object, the value of that variable changes for all instances of that class.

Static methods can be referenced with the name of the class rather than the name of a particular object of the class (though that works too). That's how library methods like System.out.println() work out is a static field in the java.lang.System class.

78. What is the difference between preemptive scheduling and time slicing?

Under preemptive scheduling, the highest priority task executes until it enters the waiting or dead states or a higher priority task comes into existence. Under time slicing, a task executes for a predefined slice of time and then reenters the pool of ready tasks.

The scheduler then determines which task should execute next, based on priority and other factors.

79. What is the catch or declare rule for method declarations?

If a checked exception may be thrown within the body of a method, the method must either catch the exception or declare it in its throws clause.

80. Is Empty .java file a valid source file?

Yes. An empty .java file is a perfectly valid source file.

Java Interview Questions(Basic): Part 5

81. Can a .java file contain more than one java classes?

Yes. A .java file contain more than one java classes, provided at the most one of them is a public class.

82. Is String a primitive data type in Java?

No. String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.

83. Is main a keyword in Java?

No. main is not a keyword in Java.

84. Is next a keyword in Java?

No. next is not a keyword.

85. Is delete a keyword in Java?

No. delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.

86. Is exit a keyword in Java?

No. To exit a program explicitly you use exit method in System object.

87. What happens if you dont initialize an instance variable of any of the primitive types in Java?

Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0(zero), a boolean will be initialized to false.

88. What will be the initial value of an object reference which is defined as an instance variable?

The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references.

89. What are the different scopes for Java variables?

The scope of a Java variable is determined by the context in which the variable is declared. Thus a java variable can have one of the three scopes at any given point in time.

1. Instance : - These are typical object level variables, they are initialized to default values at the time of creation of object, and remain accessible as long as the object accessible.

2. Local : - These are the variables that are defined within a method. They remain accessbile only during the course of method excecution. When the method finishes execution, these variables fall out of scope.

3. Static: - These are the class level variables. They are initialized when the class is loaded in JVM for the first time and remain there as long as the class remains loaded. They are not tied to any particular object instance.

90. What is the default value of the local variables?

The local variables are not initialized to any default value, neither primitives nor object references. If you try to use these variables without initializing them explicitly, the java compiler will not compile the code. It will complain abt the local varaible not being initilized.

91. How many objects are created in the following piece of code?
MyClass c1, c2, c3;
c1 = new MyClass ();
c3 = new MyClass ();

Only 2 objects are created, c1 and c3. The reference c2 is only declared and not initialized.

92. Can a public class MyClass be defined in a source file named YourClass.java?

No. The source file name, if it contains a public class, must be the same as the public class name itself with a .java extension.

93. Can main() method be declared final?

Yes, the main() method can be declared final, in addition to being public static.

94. What is HashMap and Map?

Map is an Interface and Hashmap is the class that implements Map.

95. Difference between HashMap and HashTable?

The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls. (HashMap allows null values as key and value whereas Hashtable doesnt allow).

HashMap does not guarantee that the order of the map will remain constant over time. HashMap is unsynchronized and Hashtable is synchronized.

96. Difference between Vector and ArrayList?

Vector is synchronized whereas arraylist is not.

97. Difference between Swing and Awt?

AWT are heavy-weight componenets. Swings are light-weight components. Hence swing works faster than AWT.

98. What will be the default values of all the elements of an array defined as an instance variable?

If the array is an array of primitive types, then all the elements of the array will be initialized to the default value corresponding to that primitive type.

Example: All the elements of an array of int will be initialized to 0(zero), while that of boolean type will be initialized to false. Whereas if the array is an array of references (of any type), all the elements will be initialized to null.