Java Interview questions

1. 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 (“techinterviews_com.txt”));
System.setErr(st);
System.setOut(st);

2. What’s the difference between an interface and an abstract class?

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.

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

Synchronized blocks place locks for shorter periods than synchronized methods.

4. Explain the usage of the keyword transient?

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).

5. How can you force garbage collection?

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

6. 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;

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

7. 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.

8. 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.

9. What’s the difference between constructors and other 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.

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

Yes. Use this() syntax.

11. 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.

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

13. What’s the difference between J2SDK 1.5 and J2SDK 5.0?

There’s no difference, Sun Microsystems just re-branded this version.

14. 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.

15. Does it matter in what order catch statements for FileNotFoundException and IOExceptipon are written?

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

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

It’s possible if these variables are final.

17. 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.

18. What’s the main difference between a Vector and an ArrayList

Java Vector class is internally synchronized and ArrayList is not.

19. When should the method invokeLater()be used?

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

20. How can a subclass call a method or a constructor defined in a superclass?

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.

21. What’s the difference between a queue and a stack?

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

22. 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?

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

23. What comes to mind when you hear about a young generation in Java?

Garbage collection.

24. What comes to mind when someone mentions a shallow copy in Java?

Object cloning.

25. If you’re overriding the method equals() of an object, which other method you might also consider?

hashCode()

26. 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?

ArrayList

27. How would you make a copy of an entire Java object with its state?

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

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

Use object pooling and weak object references.

29. 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?

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

30. 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.

India to take steps to protect cyber infrastructure

India is set to take steps to protect itscyber infrastructure and designate agencies for carrying out offensive cyber attacks on other countries. The move comes at a time when proof shows countries launching cyber attacks - not only for intelligence gathering - and many nations describing the attacks as an act of war.

According to sources, the National Security Council(NSC) headed by Prime Minister Manmohan Singhwould soon approve the comprehensive plan and designate the Defence Intelligence Agency (DIA) and National Technical Research Organization (NTRO) as agencies for carrying out offensive cyber operations, if necessary. All other intelligence agencies would be authorized to carry out intelligence gathering abroad, but not offensive operations, sources said.

The detailed policy for national cyber infrastructure protection is presently before the NSC awaiting its approval. The policy would identify all government agencies that would have a role in the protection of Indian cyber infrastructure and define their roles.

The move to not just define defensive mechanism but also designate agencies for offensive operations comes as New Delhi tackles repeated waves of cyber intrusions, though all of them are aimed at gathering information from critical networks. But the next stage, of an adversary carrying out offensive cyber attack, of bringing down a power grid, stalling air traffic control systems, or manipulating controls of a dam are now believed to be a real possibility.

Stuxnet, the cyber worm created by US's National Security Agency and Israeli military and specifically targeted at Iran's nuclear enrichment center at Natanz, was found to have infected Indian systems. "It was probably unintentional, but an intentional attack on India's critical infrastructure cannot be ruled out," says a senior official. "We haven't yet seen a cyber attack, but only intelligence gathering. An attack that can debilitate our infrastructure is what we must be prepared for," he said.

CERT-IN (Computer Emergency Response Team India) would be responsible for protection of most of the cyber space, while NTRO would be tasked to protect the critical infrastructure such as important government networks. NTRO would be tasked to create the National Critical Information Infrastructure Protection Centre (NCIPC), which would be a command-and-control centre for monitoring the critical infrastructure. It would be a round-the-clock centre, providing real time response to cyber security breaches.

The proposal before NSC also envisages creation of sectoral CERTs in order to respond quickly to protect power distribution networks, Air Traffic Controls, traffic networks and other areas that heavily dependent on networked systems, and thus are susceptible to attacks.

The policy suggests that the defence forces would be responsible for their own networks' protection.

NTRO and Intelligence Bureau (IB) would primarily be responsible for security of various government networks. While NTRO would operate through NCIPC, IB would be mainly looking at the physical security of networks. State polices, CBI, NIA etc would be tasked to do follow up action, if any intrusions are detected.

60% employers to monitor their employees‘ Facebook, Twitter pages by 2015: Gartner

Many employers already monitor their workers' Facebook, Twitter and other social media pages - but the practice is set to increase, a new report has revealed.

A new report by data analysts Gartner has claimed that by the year 2015, 60 per cent of employers will monitor social media pages of their employees.

The 'Big Brother' monitoring will be driven by security worries about employees leaking information or talking negatively about their workplace.

"The growth in monitoring employee behavior in digital environments is increasingly enabled by new technology and services," the Daily Mail quotedAndrew Walls, research vice president of Gartner, as saying.

"Surveillance of individuals, however, can both mitigate and create risk, which must be managed carefully to comply with ethical and legal standards," Walls said,

Most employers will use their monitoring to prevent security breaches - but simply having the technology at their disposal will be a huge temptation to managers who want to know more about their staff.

"The development of effective security intelligence and control depends on the ability to capture and analyse user actions that take place inside and outside the enterprise IT environment," he said.

Walls predicts that the practice, which is increasingly common in America, of asking for Facebook passwords as part of job interviews, will fade out of fashion.

Earlier this year, Facebook said it has "seen a distressing increase in reports of employers or others seeking to gain inappropriate access to people's Facebook profiles or private information."

Debate over the legality of employers forcing job applicants to hand over their passwords has raged on since the rise of social networking.

It has become common for managers to review publicly available Facebook profiles, Twitter accounts and other sites to learn more about job candidates.

But many users, especially on Facebook, have their profiles set to private, making them available only to selected people or certain networks.

Companies that don't ask for passwords have taken other steps -- such as asking applicants to friend human resource managers or to log in to a company computer during an interview.

Once employed, some workers have been required to sign non-disparagement agreements that ban them from talking negatively about an employer on social media.