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Question 51 :

What are wrapper classes?

Answer :

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.

Question 52 :

Give a simplest way to find out the time a method takes for execution without using any profiling tool?

Answer :

Read the system time just before the method is invoked and immediately after method returns. Take the time difference, which will give you the time taken by a method for execution.
To put it in code...
long start = System.currentTimeMillis ();
method ();
long end = System.currentTimeMillis ();

System.out.println ("Time taken for execution is " + (end - start));

Remember that if the time taken for execution is too small, it might show that it is taking zero milliseconds for execution. Try it on a method which is big enough, in the sense the one which is doing considerable amount of processing.

Question 53 :

Why do we need wrapper classes?

Answer :

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 reasons 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.

Question 54 :

What are checked exceptions?

Answer :

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

Question 55 :

What is the difference between error and an exception?

Answer :

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.).