Basic Java Questions - 2

This is the second part of Basic Questions asked on Core Java. Please leave a comment if you think there is something wrong with an answer as it will help me greatly.
Question 1: What is data encapsulation in Java?
Data Encapsulation is the process of hiding the internal information from the external world. The information can only be manipulated using a set of operations exposed by the objects known as methods. This is also called as data hiding.

Question 2: What is the difference between Abstract Class and Interface?
Interface is a pure abstract class, means all the methods in interface are abstract whereas in Abstract class we can have non-abstract methods. All the methods in an interface are public by default, whereas Abstract class can have methods with other access modifier. All the method in an interface have only definition and no declaration, whereas an Abstract class can have method with the default implementation. All the member variables in the interface are final, whereas Abstract class can have non-final variables. An interface needs to be implemented using implements whereas an Abstract class needs to be extended using extends. A Java class can implement multiple interfaces but can only extend one Abstract class.

Question 3: Can I have an Abstract Class without any abstract method and vice-versa?
You can have an abstract class without any abstract method, if you don't declare a class as Abstract that have abstract methods, the class will give compilation error.

Question 4: What is the difference between Exception and Error?
Basically a user can recover from an exception at runtime but not from an Error. For example, a user program may recover from an FileNotFoundException, however, it wont be able to recover from an OutOfMemoryError. In other words, Exceptions are programmer generated and should be handled at the application level, whereas Errors are system generated and should be handled at the system level, if possible.

Errors are also a sub class of the Throwable, however we should try to avoid catching errors. If you have a catch Throwable, the block will also catch the Error, which is not a good practice.

Question 5: What is the difference between ClassNotFoundException and NoClassDefFoundError?
ClassNotFoundException is thrown when the class is loaded using one of the following methods:

• The forName method in class Class.
• The findSystemClass method in class ClassLoader .
• The loadClass method in class ClassLoader.

Whereas, NoClassDefFoundError is encountered, when the JVM is trying to load the class as part of a normal method call or as part of creating a new instance using the new expression.

Question 6: Under what condition does finally block will not execute?
Finally block is a block in Java program, which gets executed most of the times. There are only few conditions under which the finally clock will not get executed.
1. If the Application executing the try or catch block exits by calling System.exit or by shutting down JVM.
2. If a thread executing the try or catch block is killed or interrupted, the finally block will not execute.

Question 7: Can you have a finally block without catch?
Yes, you can have finally block with the try statement, without a catch statement.