Java Interview Questions Part 14

1. Is String a primitive data type in Java?
No String is not a primitive data type in Java, even though it is one of the most extensively used object. Strings in Java are instances of String class defined in java.lang package.

2. Is main a keyword in Java?
No, main is not a keyword in Java.

3. Is next a keyword in Java?
No, next is not a keyword.

4. Is delete a keyword in Java?
No, delete is not a keyword in Java. Java does not make use of explicit destructors the way C++ does.


5. Is exit a keyword in Java?
No. To exit a program explicitly you use exit method in System object.


6. What happens if you dont initialize an instance variable of any of the primitive types in Java?
Java by default initializes it to the default value for that primitive type. Thus an int will be initialized to 0, a boolean will be initialized to false.


7. What will be the initial value of an object reference which is defined as an instance variable?
The object references are all initialized to null in Java. However in order to do anything useful with these references, you must set them to a valid object, else you will get NullPointerExceptions everywhere you try to use such default initialized references.

Java Interview Questions Part 9

1. If my class already extends from some other class what should I do if I want an instance of my class to be thrown as an exception object?
One can not do anytihng in this scenarion. Because Java does not allow multiple inheritance and does not provide any exception interface as well.

2. How does an exception permeate through the code?
An unhandled exception moves up the method stack in search of a matching When an exception is thrown from a code which is wrapped in a try block followed by one or more catch blocks, a search is made for matching catch block. If a matching type is found then that block will be invoked. If a matching type is not found then the exception moves up the method stack and reaches the caller method. Same procedure is repeated if the caller method is included in a try catch block. This process continues until a catch block handling the appropriate type of exception is found. If it does not find such a block then finally the program terminates.

3. What are the different ways to handle exceptions?
There are two ways to handle exceptions,

1. By wrapping the desired code in a try block followed by a catch block to catch the exceptions.
2. List the desired exceptions in the throws clause of the method and let the caller of the method hadle those exceptions.

4. What is the basic difference between the 2 approaches to exception handling.

• try catch block and
• specifying the candidate exceptions in the throws clause

5. When should you use which approach?
In the first approach as a programmer of the method, you urself are dealing with the exception. This is fine if you are in a best position to decide should be done in case of an exception. Whereas if it is not the responsibility of the method to deal with it's own exceptions, then do not use this approach. In this case use the second approach. In the second approach we are forcing the caller of the method to catch the exceptions, that the method is likely to throw. This is often the approach library creators use. They list the exception in the throws clause and we must catch them. You will find the same approach throughout the java libraries we use.

6. Is it necessary that each try block must be followed by a catch block?
It is not necessary that each try block must be followed by a catch block. It should be followed by either a catch block OR a finally block. And whatever exceptions are likely to be thrown should be declared in the throws clause of the method.

7. If I write return at the end of the try block, will the finally block still execute?
Yes even if you write return as the last statement in the try block and no exception occurs, the finally block will execute. The finally block will execute and then the control return.
 

Java Interview Questions Part 8

1. What are wrapper classes?
Java provides specialized classes corresponding to each of the primitive data types. These are called wrapper classes. They are e.g. Integer, Character, Double etc.

2. Why do we need wrapper classes?
It is sometimes easier to deal with primitives as objects. Moreover most of the collection classes store objects and not primitive data types. And also the wrapper classes provide many utility methods also. Because of these resons we need wrapper classes. And since we create instances of these classes we can store them in any of the collection classes and pass them around as a collection. Also we can pass them around as method parameters where a method expects an object.

3. What are checked exceptions?
Checked exception are those which the Java compiler forces you to catch. e.g. IOException are checked Exceptions.

4. What are runtime exceptions?
Runtime exceptions are those exceptions that are thrown at runtime because of either wrong input data or because of wrong business logic etc. These are not checked by the compiler at compile time.

5. What is the difference between error and an exception?
An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors and you can not repair them at runtime. While exceptions are conditions that occur because of bad input etc. e.g. FileNotFoundException will be thrown if the specified file does not exist. Or a NullPointerException will take place if you try using a null reference. In most of the cases it is possible to recover from an exception (probably by giving user a feedback for entering proper values etc.).

6. How to create custom exceptions?
Your class should extend class Exception, or some more specific type thereof.

7. If I want an object of my class to be thrown as an exception object, what should I do?
The class should extend from Exception class. Or you can extend your class from some more precise exception type also. 

Java Interview Questions Part 4

1. How can one prove that the array is not null but empty using one line of code?
Print args.length. It will print 0. That means it is empty. But if it would have been null then it would have thrown a NullPointerException on attempting to print args.length.

2. What environment variables do I need to set on my machine in order to be able to run Java programs?
CLASSPATH and PATH are the two variables.

3. Can an application have multiple classes having main method?
Yes it is possible. While starting the application we mention the class name to be run. The JVM will look for the Main method only in the class whose name you have mentioned. Hence there is not conflict amongst the multiple classes having main method.

4. Can I have multiple main methods in the same class?
No the program fails to compile. The compiler says that the main method is already defined in the class.

5. Do I need to import java.lang package any time? Why ?
No. It is by default loaded internally by the JVM.

6. Can I import same package/class twice? Will the JVM load the package twice at runtime?
One can import the same package or same class multiple times. Neither compiler nor JVM complains abt it. And the JVM will internally load the class only once no matter how many times you import the same class.

7. What are Checked and UnChecked Exception?
A checked exception is some subclass of Exception (or Exception itself), excluding class RuntimeException and its subclasses. Making an exception checked forces client programmers to deal with the possibility that the exception will be thrown. eg, IOException thrown by java.io.FileInputStream's read() method. Unchecked exceptions are RuntimeException and any of its subclasses. Class Error and its subclasses also are unchecked. With an unchecked exception, however, the compiler doesn't force client programmers either to catch the exception or declare it in a throws clause. In fact, client programmers may not even know that the exception could be thrown. eg, StringIndexOutOfBoundsException thrown by String's charAt() method. Checked exceptions must be caught at compile time. Runtime exceptions do not need to be. Errors often cannot be.