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J2SDK 1.5 and J2SDK 5.0
What's the difference between J2SDK 1.5 and J2SDK 5.0? There is no difference, Sun Microsystems just re-branded this version. 2007-03-03, 7736👍, 0💬

Acquiring Locks on Classes
Can a lock be acquired on a class? Yes, a lock can be acquired on a class. This lock is acquired on the class's Class object. 2007-03-03, 6079👍, 0💬

Waiting State in a Thread
What are three ways in which a thread can enter the waiting state? A thread can enter the waiting state by invoking its sleep() method, by blocking on IO, by unsuccessfully attempting to acquire an object's lock, or by invoking an object's wait() method. It can also enter the waiting state by invoki... 2007-03-03, 6772👍, 0💬

Synchronized Methods and Synchronized Statements
What are synchronized methods and synchronized statements? Synchronized methods are methods that are used to control access to a method or 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 s... 2007-03-03, 6157👍, 0💬

Importance of Synchronization
What is synchronization and why is it important? With respect to multithreading, synchronization is the capability to control the access of multiple threads to shared resources. Without synchronization, it is possible for one thread to modify a shared object while another thread is in the process of... 2007-03-03, 7291👍, 0💬

Observer Interface and Observable Class
How are Observer and Observable used? Objects that subclass the Observable class maintain a list of observers. When an Observable object is updated, it invokes the update() method of each of its observers to notify the observers that it has changed state. The Observer interface is implemented by obj... 2007-03-03, 5978👍, 0💬

Transient Variables
What is a transient variable? A transient variable is a variable that may not be serialized. If you do not want some variables to be serialized, you can mark those variables transient or static. 2007-03-03, 6015👍, 0💬

Serializalble and Externalizable Interfaces
What is the difference between Serializalble and Externalizable interface? When you use Serializable interface, your class is serialized automatically by default. But you can override writeObject() and readObject() two methods to control more complex object serailization process. When you use Extern... 2007-03-03, 5858👍, 0💬

Externalizable Interface: readExternal() and readExternal()
How many methods in the Externalizable interface? There are two methods in the Externalizable interface. You have to implement these two methods in order to make your class externalizable. These two methods are readExternal() and readExternal(). 2007-03-03, 5984👍, 0💬

Serializable Interface
How many methods in the Serializable interface? There is no method in the Serializable interface. The Serializable interface acts as a marker, telling the object serialization tools that your class is serializable. 2007-03-03, 6008👍, 0💬

CLASSPATH Environment Variable
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 the directory or the jar file that contains the package to the CLASSPATH environment variable. Let's say a class Employee belongs to a package com.xyz.hr; and is located in the ... 2007-03-03, 5776👍, 0💬

Defining an Interface
How to define an Interface? Java Interface defines methods but does not implement them. Interface can include constants. A class that implements an interface is required to implement all the methods defined in the interface. Here is an example of interface definition: public interface MyInterface { ... 2007-03-03, 6250👍, 0💬

Defining Abstract Classes
How to define an Abstract class? Defining an abstract class is done using the keyword "abstract". Here is an example of abstract class: abstract class MyAbstract { protected String myString; public String getMyString() { return myString; } public abstract string getMyStringInYourWay(); } 2007-03-03, 5979👍, 0💬

Iterator Interface
Is Iterator a Class or Interface? What is its use? Iterator is an interface which is used to step through the elements of a Collection. 2007-03-03, 5953👍, 0💬

What is Collection API
What is Collection API? The Collection API is a set of classes and interfaces that support operation on collections of objects. These classes and interfaces are more flexible, more powerful, and more regular than the vectors, arrays, and hashtables. Example of classes: HashSet, HashMap, ArrayList, L... 2007-03-03, 6415👍, 0💬

What Is Synchronization
What do you understand by Synchronization? Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner that only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modif... 2007-03-03, 5835👍, 0💬

Swing BorderLayout and Contrainer Classes
Name the containers which uses Border Layout as their default layout? Swing container classes which uses BorderLayout as their default are: Window, Frame and Dialog classes. 2007-03-03, 6047👍, 0💬

Differences between Interfaces and Abstract Classes
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 implemen... 2007-03-03, 6200👍, 0💬

System.setOut() - Setting Standard Console Output to
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 connected to the system console. But... 2007-03-03, 9067👍, 0💬

System.loadLibrary() - Loading DLL for JNI Interface
An application needs to load a library before it starts to run, how to code? One option is to use a static block to load a library before anything is called. For example, class Test { static { System.loadLibrary("myApp.dll" );} .... } When you call new Test(), the static block will be called first b... 2007-03-03, 9048👍, 0💬

Troubleshooting NullPointerException
What is NullPointerException and how to handle it? When an object is not initialized, the default value is null. When the following things happen, the NullPointerException is thrown: Calling the instance method of a null object. Accessing or modifying the field of a null object. Taking the length of... 2007-03-03, 9147👍, 0💬

Runable Interface and Thread
What is more advisable to create a thread, by implementing a Runnable interface or by extending Thread class? Strategically speaking, threads created by implementing Runnable interface are more advisable. If you create a thread by extending a thread class, you cannot extend any other class. If you c... 2007-03-03, 7287👍, 0💬

Clone of Map Objects
Jack developed a program by using a Map container to hold key/value pairs. He wanted to make a change to the map. He decided to make a clone of the map in order to save the original data on side. What do you think of it? If Jack made a clone of the map, any changes to the clone or the original map w... 2007-03-03, 7798👍, 0💬

Virtual Functions in Java
Can you have virtual functions in Java? Yes, all functions in Java are virtual by default. This is actually a pseudo trick question because the word "virtual" is not part of the naming convention in Java (as it is in C++, C-sharp and VB.NET), so this would be a foreign concept for someone who has on... 2007-03-03, 7180👍, 0💬

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.NET C C++ CSS DBA General HTML Java JavaScript JSP JUnit MySQL Networking Oracle Perl Perl PHP PL/SQL RSS Software QA SQL Server Struts Unix Windows XHTML XML