Core Java Theory Part 3

Core Java Theory Part 3

Have a look into the most important questions asked in interviews on Core Java Theory.

Garbage Collectors

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

The purpose of garbage collection is to identify and discard those objects that are no longer needed by the application, in order for the resources to be reclaimed and reused.

5.2 What does System.gc() and Runtime.gc() methods do?

These methods can be used as a hint to the JVM, in order to start a garbage collection. However, it is up to the Java Virtual Machine (JVM) to start the garbage collection immediately or later in time.

5.3 When is the finalize() called? What is the purpose of finalization?

The finalize method is called by the garbage collector, just before releasing the object’s memory. It is normally advised to release resources held by the object inside the finalize method.

5.4 If an object reference is set to null, will the Garbage Collector immediately free the memory held by that object?

No, the object will be available for garbage collection in the next cycle of the garbage collector.

5.5 What is the structure of Java Heap? What is Perm Gen space in Heap?

The JVM has a heap that is the runtime data area from which memory for all class instances and arrays is allocated. It is created at the JVM start-up. Heap memory for objects is reclaimed by an automatic memory management system which is known as a garbage collector. Heap memory consists of live and dead objects. Live objects are accessible by the application and will not be a subject of garbage collection. Dead objects are those which will never be accessible by the application but have not been collected by the garbage collector yet. Such objects occupy the heap memory space until they are eventually collected by the garbage collector.

5.6 Difference between Serial and Throughput Garbage collector?

The throughput garbage collector uses a parallel version of the young generation collector and is meant to be used with applications that have medium to large data sets. On the other hand, the serial collector is usually adequate for most small applications (those requiring heaps of up to approximately 100MB on modern processors).

5.7 When does an Object becomes eligible for Garbage collection in Java?

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

5.8 Does Garbage collection occur in permanent generation space in JVM?

Garbage Collection does occur in PermGen space and if PermGen space is full or cross a threshold, it can trigger a full garbage collection. If you look carefully at the output of the garbage collector, you will find that PermGen space is also garbage collected. This is the reason why the correct sizing of PermGen space is important to avoid frequent full garbage collections.

Exception Handling

6.1 Types of Exceptions in Java? Which are the differences between them?

Java has two types of exceptions: checked exceptions and unchecked exceptions. Unchecked exceptions do not need to be declared in a method or a constructor’s throws clause, if they can be thrown by the execution of the method or the constructor, and propagate outside the method or constructor boundary. On the other hand, checked exceptions must be declared in a method or a constructor’s throws clause.

6.2 What is the difference between Exception and Error in java?

Exception and Error classes are both subclasses of the Throwable class. The Exception class is used for exceptional conditions that a user’s program should catch. The Error class defines exceptions that are not excepted to be caught by the user program.

6.3 Difference between throw and throws?

The throw keyword is used to explicitly raise an exception within the program. On the contrary, the throws clause is used to indicate those exceptions that are not handled by a method. Each method must explicitly specify which exceptions does not handle, so the callers of that method can guard against possible exceptions. Finally, multiple exceptions are separated by a comma.

6.4 Importance of finally block in exception handling?

A finally block will always be executed, whether or not an exception is actually thrown. Even in the case where the catch statement is missing and an exception is thrown, the finally block will still be executed. The last thing to mention is that the finally block is used to release resources like I/O buffers, database connections, etc.

6.5 What will happen to the Exception object after exception handling?

The Exception object will be garbage collected in the next garbage collection.

6.6 How does finally block differ from finalize() method?

A finally block will be executed whether or not an exception is thrown and is used to release those resources held by the application. Finalize is a protected method of the Object class, which is called by the Java Virtual Machine (JVM) just before an object is garbage collected.

Java Applets

7.1 What is Applet?

A java applet is a program that can be included in an HTML page and be executed in a java enabled client browser. Applets are used for creating dynamic and interactive web applications.

7.2 Explain the life cycle of an Applet.

An applet may undergo the following states:

• Init: An applet is initialized each time is loaded.
• Start: Begin the execution of an applet.
• Stop: Stop the execution of an applet.
• Destroy: Perform a final cleanup, before unloading the applet.

7.3 What happens when an applet is loaded?

First of all, an instance of the applet’s controlling class is created. Then, the applet initializes itself and finally, it starts running.

7.4 Difference between an Applet and a Java Application?

Applets are executed within a java-enabled browser, but a Java application is a standalone Java program that can be executed outside of a browser. However, they both require the existence of a Java Virtual Machine (JVM). Furthermore, a Java application requires a main method with a specific signature, in order to start its execution. Java applets don’t need such a method to start their execution. Finally, Java applets typically use a restrictive security policy, while Java applications usually use more relaxed security policies.

7.5 What are the restrictions imposed on Java applets?

Mostly due to security reasons, the following restrictions are imposed on Java applets: • An applet cannot load libraries or define native methods.

• An applet cannot ordinarily read or write files on the execution host.
• An applet cannot read certain system properties.
• An applet cannot make network connections except to the host that it came from. • An applet cannot start any program on the host that’s executing it.

7.6 Explain untrusted applets?

Untrusted applets are those Java applets that cannot access or execute local system files. By default, all downloaded applets are considered as untrusted.

7.7 Difference between applets loaded over the internet and applets loaded via the file system?

Regarding the case where an applet is loaded over the internet, the applet is loaded by the applet class loader and is subject to the restrictions enforced by the applet security manager. Regarding the case where an applet is loaded from the client’s local disk, the applet is loaded by the file system loader. Applets loaded via the file system are allowed to read files, write files and to load libraries on the client. Also, applets loaded via the file system are allowed to execute processes and finally, applets loaded via the file system are not passed through the byte code verifier.

7.8 What is the applet class loader, and what does it provide?

When an applet is loaded over the internet, the applet is loaded by the applet classloader. The class loader enforces the Java namespace hierarchy. Also, the class loader guarantees that a unique namespace exists for classes that come from the local file system and that a unique namespace exists for each network source. When a browser loads an applet over the net, that applet’s classes are placed in a private namespace associated with the applet’s origin. Then, those classes loaded by the class loader are passed through the verifier. The verifier checks that the class file conforms to the Java language specification. Among other things, the verifier ensures that there are no stack overflows or underflows and that the parameters to all bytecode instructions are correct.

7.9 Explain applet security manager, and what does it provide?

The applet security manager is a mechanism to impose restrictions on Java applets. A browser may only have one security manager. The security manager is established at startup, and it cannot thereafter be replaced, overloaded, overridden, or extended.

Swing

8.1 Difference between a Choice and a List?

A Choice is displayed in a compact form that must be pulled down, in order for a user to be able to see the list of all available choices. Only one item may be selected from a Choice. A List may be displayed in such a way that several List items are visible. A List supports the selection of one or more List items.

8.2 What is a layout manager?

A layout manager is used to organize the components in a container.

8.3 Difference between a Scrollbar and a JScrollPane ?

A Scrollbar is a Component, but not a Container. A ScrollPane is a Container. A ScrollPane handles its own events and performs its own scrolling.

8.4 Which Swing methods are thread-safe?

There are only three thread-safe methods: repaint, revalidate and invalidate.

8.5 Name three Component subclasses that support painting.

The Canvas, Frame, Panel, and Applet classes support painting.

8.6 What is clipping?

Clipping is defined as the process of confining paint operations to a limited area or shape.

8.7 Difference between a MenuItem and a CheckboxMenuItem?

The CheckboxMenuItem class extends the MenuItem class and supports a menu item that may be either checked or unchecked.

8.8 How are the elements of a BorderLayout organized?

The elements of a BorderLayout are organized at the borders (North, South, East, and West) and the centre of a container.

8.9 How are the elements of a GridBagLayout organized?

The elements of a GridBagLayout are organized according to a grid. The elements are of different sizes and may occupy more than one row or column of the grid. Thus, the rows and columns may have different sizes.

8.10 Difference between a Window and a Frame?

The Frame class extends the Window class and defines the main application window that can have a menu bar.

8.11 Relationship between clipping and repainting?

When a window is repainted by the AWT painting thread, it sets the clipping regions to the area of the window that requires repainting.

8.12 What is the relationship between an event-listener interface and an event-adapter class?

An event-listener interface defines the methods that must be implemented by an event handler for a particular event. An event adapter provides a default implementation of an event-listener interface.

8.13 How can a GUI component handle its own events?

A GUI component can handle its own events, by implementing the corresponding event-listener interface and adding itself as its own event listener.

8.14 Advantages that Java’s layout managers provide over traditional windowing systems?

Java uses layout managers to layout components in a consistent manner, across all windowing platforms. Since layout managers aren’t tied to absolute sizing and positioning, they are able to accommodate platform-specific differences among windowing systems.

8.15 What is the design pattern that Java uses for all Swing components?

The design pattern used by Java for all Swing components is the Model View Controller (MVC) pattern.

JDBC

9.1 Explain is JDBC?

JDBC is an abstraction layer that allows users to choose between databases. JDBC enables developers to write database applications in Java, without having to concern themselves with the underlying details of a particular database.

9.2 Describe the role of the Driver in JDBC.

The JDBC Driver provides vendor-specific implementations of the abstract classes provided by the JDBC API. Each driver must provide implementations for the following classes of the java.sql package: Connection, Statement, PreparedStatement, CallableStatement, ResultSet and Driver.

9.3 Purpose Class.forName method?

This method is used to load the driver that will establish a connection to the database.

9.4 Advantage of PreparedStatement over Statement?

PreparedStatements are precompiled and thus, their performance is much better. Also, PreparedStatement objects can be reused with different input values to their queries.

9.5 What is the use of CallableStatement? Name the method, which is used to prepare a CallableStatement.

A CallableStatement is used to execute stored procedures. Stored procedures are stored and offered by a database. Stored procedures may take input values from the user and may return a result. The usage of stored procedures is highly encouraged because it offers security and modularity. The method that prepares a CallableStatement is the following: CallableStament. prepareCall();

9.6 Define Connection pooling?

The interaction with a database can be costly, regarding the opening and closing of database connections. Especially, when the number of database clients increases, this cost is very high and a large number of resources is consumed. A pool of database connections is obtained at startup by the application server and is maintained in a pool. A request for a connection is served by a connection residing in the pool. At the end of the connection, the request is returned to the pool and can be used to satisfy future requests.

Remote Method Invocation (RMI)

10.1 Explain RMI?

The Java Remote Method Invocation (Java RMI) is a Java API that performs the object-oriented equivalent of remote procedure calls (RPC), with support for direct transfer of serialized Java classes and distributed garbage collection. Remote Method Invocation (RMI) can also be seen as the process of activating a method on a remotely running object. RMI offers location transparency because a user feels that a method is executed on a locally running object. Check some RMI Tips here.

10.2 What is the basic principle of RMI architecture?

The RMI architecture is based on a very important principle which states that the definition of the behaviour and the implementation of that behaviour, are separate concepts. RMI allows the code that defines the behaviour and the code that implements the behaviour to remain separate and to run on separate JVMs.

10.3 Layers of RMI Architecture?

The RMI architecture consists of the following layers:

• Stub and Skeleton layer: This layer lies just beneath the view of the developer. This layer is responsible for intercepting method calls made by the client to the interface and redirect these calls to a remote RMI Service.

• Remote Reference Layer: The second layer of the RMI architecture deals with the interpretation of references made from the client to the server’s remote objects. This layer interprets and manages references made from clients to the remote service objects. The connection is a one-to-one (unicast) link.

• Transport layer: This layer is responsible for connecting the two JVM participating in the service. This layer is based on TCP/IP connections between machines in a network. It provides basic connectivity, as well as some firewall penetration strategies.

10.4 Role of Remote Interface in RMI?

The Remote interface serves to identify interfaces whose methods may be invoked from a non-local virtual machine. Any object that is a remote object must directly or indirectly implement this interface. A class that implements a remote interface should declare the remote interfaces being implemented, define the constructor for each remote object and provide an implementation for each remote method in all remote interfaces.

10.5 What is the role of the java.rmi.Naming Class?

The java.rmi.Naming class provides methods for storing and obtaining references to remote objects in the remote object registry. Each method of the Naming class takes as one of its arguments a name that is a String in URL format.

10.6 Define binding in RMI?

Binding is the process of associating or registering a name for a remote object, which can be used at a later time, in order to look up that remote object. A remote object can be associated with a name using the bind or rebind methods of the Naming class.

10.7 Difference between using bind() and rebind() methods of Naming Class?

The bind method bind is responsible for binding the specified name to a remote object, while the rebind method is responsible for rebinding the specified name to a new remote object. In case a binding exists for that name, the binding is replaced.

10.8 What are the steps involved to make work a RMI program?

The following steps must be involved in order for RMI program to work properly:

• Compilation of all source files.
• Generation of the stubs using rmic. • Start the rmiregistry.
• Start the RMIServer.
• Run the client program.

10.9 Role of stub in RMI?

A stub for a remote object acts as a client’s local representative or proxy for the remote object. The caller invokes a method on the local stub, which is responsible for executing the method on the remote object. When a stub’s method is invoked, it undergoes the following steps:

• It initiates a connection to the remote JVM containing the remote object.
• It marshals the parameters to the remote JVM.
• It waits for the result of the method invocation and execution.
• It unmarshals the return value or an exception if the method has not been successfully executed. • It returns the value to the caller.

10.10 What is DGC? And how does it work?

DGC stands for Distributed Garbage Collection. Remote Method Invocation (RMI) uses DGC for automatic garbage collection. Since RMI involves remote object references across JVM’s, garbage collection can be quite difficult. DGC uses a reference counting algorithm to provide automatic memory management for remote objects.

10.11 Purpose of using RMISecurityManager in RMI?

RMISecurityManager provides a security manager that can be used by RMI applications, which use downloaded code. The class loader of RMI will not download any classes from remote locations, if the security manager has not been set.

10.12 Explain Marshalling and demarshalling.

When an application wants to pass its memory objects across a network to another host or persist it to the storage, the in-memory representation must be converted to a suitable format. This process is called marshalling and the revert operation is called demarshalling.

10.13 What is Serialization and Deserialization.

Java provides a mechanism, called object serialization where an object can be represented as a sequence of bytes and includes the object’s data, as well as information about the object’s type, and the types of data stored in the object. Thus, serialization can be seen as a way of flattening objects, in order to be stored on disk, and later, read back and reconstituted. Deserialisation is the reverse process of converting an object from its flattened state to a live object.

Servlets

11.1 iDescribe Servlet ?

The servlet is a Java programming language class used to process client requests and generate dynamic web content. Servlets are mostly used to process or store data submitted by an HTML form, provide dynamic content and manage state information that does not exist in the stateless HTTP protocol.

11.2 Explain the architecture of a Servlet.

The core abstraction that must be implemented by all servlets is the javax.servlet.Servlet interface. Each servlet must implement it either directly or indirectly, either by extending javax.servlet.GenericServlet or javax.servlet.http.HTTPServlet. Finally, each servlet is able to serve multiple requests in parallel using multithreading.

11.3 Difference between an Applet and a Servlet?

An Applet is a client-side java program that runs within a Web browser on the client machine. On the other hand, a servlet is a server-side component that runs on the webserver. An applet can use the user interface classes, while a servlet does not have a user interface. Instead, a servlet waits for the client’s HTTP requests and generates a response in every request.

11.4 What is the difference between GenericServlet and HttpServlet?

GenericServlet is a generalized and protocol-independent servlet that implements the Servlet and ServletConfig interfaces. Those servlets extending the GenericServlet class shall override the service method. Finally, in order to develop an HTTP servlet for use on the Web that serves requests using the HTTP protocol, your servlet must extend the HttpServlet instead. Check Servlet examples here.

11.5 Explain the life cycle of a Servlet.

On every client’s request, the Servlet Engine loads the servlets and invokes its init methods, in order for the servlet to be initialized. Then, the Servlet object handles all subsequent requests coming from that client, by invoking the service method for each request separately. Finally, the servlet is removed by calling the server’s destroy method.

11.6 Difference between doGet() and doPost() ?

doGET: The GET method appends the name-value pairs on the request’s URL. Thus, there is a limit on the number of characters and subsequently on the number of values that can be used in a client’s request. Furthermore, the values of the request are made visible and thus, sensitive information must not be passed in that way.

doPOST: The POST method overcomes the limit imposed by the GET request, by sending the values of the request inside its body. Also, there are no limitations on the number of values to be sent across. Finally, the sensitive information passed through a POST request is not visible to an external client.

11.7 What is meant by a Web Application?

A Web application is a dynamic extension of a Web or application server. There are two types of web applications: presentation-oriented and service-oriented. A presentation-oriented Web application generates interactive web pages, which contain various types of markup language and dynamic content in response to requests. On the other hand, a service-oriented web application implements the endpoint of a web service. In general, a Web application can be seen as a collection of servlets installed under a specific subset of the server’s URL namespace.

11.8 Explain what is a Server Side Include (SSI)?

Server Side Includes (SSI) is a simple interpreted server-side scripting language, used almost exclusively for the Web, and is embedded with a servlet tag. The most frequent use of SSI is to include the contents of one or more files into a Web page on a Web server. When a Web page is accessed by a browser, the Web server replaces the servlet tag in that Web page with the hypertext generated by the corresponding servlet.

11.9 What is Servlet Chaining ?

Servlet Chaining is the method where the output of one servlet is sent to a second servlet. The output of the second servlet can be sent to a third servlet, and so on. The last servlet in the chain is responsible for sending the response to the client.

11.10 How do you find out what client machine is making a request to your servlet?

The ServletRequest class has functions for finding out the IP address or hostname of the client machine. getRemoteAddr() gets the IP address of the client machine and getRemoteHost() gets the hostname of the client machine. 

11.11 What is the structure of the HTTP response?

The HTTP response consists of three parts:

• Status Code: describes the status of the response. It can be used to check if the request has been successfully completed. In case the request failed, the status code can be used to find out the reason behind the failure. If your servlet does not return a status code, the success status code, HttpServletResponse.SC_OK, is returned by default.

• HTTP Headers: they contain more information about the response. For example, the headers may specify the date/time after which the response is considered stale, or the form of encoding used to safely transfer the entity to the user. See how to retrieve headers in Servlet here.

• Body: it contains the content of the response. The body may contain HTML code, an image, etc. The body consists of the data bytes transmitted in an HTTP transaction message immediately following the headers.

11.12 Explain what is a cookie? What is the difference between session and cookie?

A cookie is a bit of information that the Web server sends to the browser. The browser stores the cookies for each Web server in a local file. In a future request, the browser, along with the request, sends all stored cookies for that specific Web server. The differences between session and a cookie are the following:

• The session should work, regardless of the settings on the client browser. The client may have chosen to disable cookies. However, the sessions still work, as the client has no ability to disable them in the server-side.

• The session and cookies also differ in the amount of information the can store. The HTTP session is capable of storing any Java object, while a cookie can only store String objects.

11.13 Which protocol will be used by browser and servlet to communicate?

The browser communicates with a servlet by using the HTTP protocol.

11.14 What is HTTP Tunneling?

HTTP Tunneling is a technique by which, communications performed using various network protocols are encapsulated using the HTTP or HTTPS protocols. The HTTP protocol, therefore, acts as a wrapper for a channel that the network protocol being tunnelled uses to communicate. The masking of other protocol requests as HTTP requests is HTTP Tunneling.

11.15 Difference between sendRedirect and forward methods?

The sendRedirect method creates a new request, while the forward method just forwards a request to a new target. The previous request scope objects are not available after a redirect, because it results in a new request. On the other hand, the previous request scope objects are available after forwarding. Finally, in general, the sendRedirect method is considered to be slower compared to the forward method.

11.16 Explain URL Encoding and URL Decoding?

The URL encoding procedure is responsible for replacing all the spaces and every other extra special character of a URL, into their corresponding Hex representation. In correspondence, URL decoding is the exact opposite procedure.

JSP

12.1 What is a JSP Page?

A Java Server Page (JSP) is a text document that contains two types of text: static data and JSP elements. Static data can be expressed in any text-based format, such as HTML or XML. JSP is a technology that mixes static content with dynamically- generated content. See JSP example here.

12.2 How are the JSP requests handled?

On the arrival of a JSP request, the browser first requests a page with a .jsp extension. Then, the Web server reads the request and using the JSP compiler, the Web server converts the JSP page into a servlet class. Notice that the JSP file is compiled only on the first request of the page, or if the JSP file has changed.The generated servlet class is invoked, in order to handle the browser’s request. Once the execution of the request is over, the servlet sends a response back to the client. See how to get Request parameters in a JSP.

12.3 What are the advantages of JSP?

The advantages of using JSP technology are shown below:

• JSP pages are dynamically compiled into servlets and thus, the developers can easily make updates to presentation code.
• JSP pages can be pre-compiled.
• JSP pages can be easily combined to static templates, including HTML or XML fragments, with code that generates dynamic content.
• Developers can offer customized JSP tag libraries that page authors access using an XML-like syntax.
• Developers can make logic changes at the component level, without editing the individual pages that use the application’s logic.

12.4 Define Directives? What are the different types of Directives available in JSP?

Directives are instructions that are processed by the JSP engine when the page is compiled to a servlet. Directives are used to set page-level instructions, insert data from external files, and specify custom tag libraries.Directives are defined between< %@ and % >.The different types of directives are shown below:

• Include directive: it is used to include a file and merges the content of the file with the current page. • Page directive: it is used to define specific attributes in the JSP page, like error page and buffer.
• Taglib: it is used to declare a custom tag library that is used on the page.

12.5 What are JSP actions?

JSP actions use constructs in XML syntax to control the behaviour of the servlet engine. JSP actions are executed when a JSP page is requested. They can be dynamically inserted into a file, re-use JavaBeans components, forward the user to another page, or generate HTML for the Java plugin. Some of the available actions are listed below:

• jsp: include – includes a file, when the JSP page is requested.

• jsp:useBean – finds or instantiates a JavaBean.

• jsp:setProperty – sets the property of a JavaBean.

• jsp:getProperty – gets the property of a JavaBean.

• jsp:forward – forwards the requester to a new page. • jsp:plugin – generates browser-specific code.

12.6 Explain Scriptlets?

In Java Server Pages (JSP) technology, a scriptlet is a piece of Java-code embedded in a JSP page. The scriptlet is everything inside the tags. Between these tags, a user can add any valid scriplet.

12.7 What are Decalarations ?

Declarations are similar to variable declarations in Java. Declarations are used to declare variables for subsequent use in expressions or scriptlets. To add a declaration, you must use the sequences to enclose your declarations.

12.8 Define are Expressions?

A JSP expression is used to insert the value of a scripting language expression, converted into a string, into the data stream returned to the client, by the webserver.Expressionsaredefinedbetween<% =and %>tags.

12.9 Explain implicit objects and what are they?

JSP implicit objects are those Java objects that the JSP Container makes available to developers in each page. A developer can call them directly, without being explicitly declared. JSP Implicit Objects are also called pre-defined variables. The following objects are considered implicit in a JSP page:

• application

• page

• request

• response

• session

• exception

• out

• config

• pageContext

Check Core Java Interview Question

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