Prepare 4 interview - Recent questions and answers in Java Interview Questions and Answers

Answered: What access level do you need to specify in the class declaration?

Sun, 05 Sep 2010 10:56:59 +0000

You do not need to specify any access level, and Java will use a default package access level.

Answered: What Java technique would you use to implement it?

Sun, 05 Sep 2010 10:55:31 +0000

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

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

Sun, 05 Sep 2010 10:54:12 +0000

Use object pooling and weak object references.

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

Sun, 05 Sep 2010 10:52:58 +0000

Have this class implement Clone-able interface and call its method clone().

Answered: Which of the two Java collections should you use: ArrayList or LinkedList?

Sun, 05 Sep 2010 10:52:08 +0000

You are planning to do an indexed search in a list of objects. Which of the two Java collections should you use: ArrayList or LinkedList?

Answer: ArrayList

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

Sun, 05 Sep 2010 10:50:41 +0000

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

hashCode()

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

Sun, 05 Sep 2010 10:49:24 +0000

Object cloning.

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

Sun, 05 Sep 2010 10:48:05 +0000

Garbage collection.

Answered: Can you use abstract classes instead of interfaces?

Sun, 05 Sep 2010 10:47:08 +0000

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

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

Sun, 05 Sep 2010 10:43:12 +0000

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

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

Sun, 05 Sep 2010 10:41:48 +0000

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.

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

Sun, 05 Sep 2010 10:40:19 +0000

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

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

Sun, 05 Sep 2010 10:39:09 +0000

Java Vector class is internally synchronized and ArrayList is not.

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

Sun, 05 Sep 2010 10:38:10 +0000

A single ampersand here would lead to a NullPointerException.

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

Sun, 05 Sep 2010 10:37:22 +0000

It's possible if these variables are final.

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

Sun, 05 Sep 2010 10:35:33 +0000

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

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

Sun, 05 Sep 2010 10:28:02 +0000

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.

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

Sun, 05 Sep 2010 10:27:07 +0000

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

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

Sun, 05 Sep 2010 10:26:11 +0000

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

Answered: Explain the usage of Java packages.

Sun, 05 Sep 2010 10:24:46 +0000

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.

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

Sun, 05 Sep 2010 10:23:37 +0000

Yes. Use this() syntax.

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

Sun, 05 Sep 2010 10:22:43 +0000

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.

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

Sun, 05 Sep 2010 10:21:42 +0000

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

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

Sun, 05 Sep 2010 10:20:52 +0000

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.

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

Sun, 05 Sep 2010 10:19:32 +0000

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.

Answered: How can you force garbage collection?

Sun, 05 Sep 2010 10:17:43 +0000

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

Answered: Explain the usage of the keyword transient?

Sun, 05 Sep 2010 10:16:21 +0000

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).

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

Sun, 05 Sep 2010 10:15:14 +0000

Synchronized blocks place locks for shorter periods than synchronized methods.

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

Sun, 05 Sep 2010 10:13:29 +0000

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.

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

Sun, 05 Sep 2010 10:06:46 +0000

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);

Answered: What is finalize() method?

Thu, 12 Aug 2010 07:57:22 +0000

finalize () method is used just before an object is destroyed and can be called just prior to garbage collection.

Answered: What if the main method is declared as private?

Thu, 12 Aug 2010 07:40:06 +0000

The program compiles properly but at runtime it will give "Main method not public." message.