Core Java - Exception Handling in Java

What is an Exception?

Ans) The exception is said to be thrown whenever an exceptional event occurs in java which signals that something is not correct with the code written and may give unexpected result. An exceptional event is a occurrence of condition which alters the normal program flow. Exceptional handler is the code that does something about the exception.

Exceptions are defined in which java package?

Ans) All the exceptions are subclasses of java.lang.Exception

How are the exceptions handled in java?

Ans) When an exception occurs the execution of the program is transferred to an appropriate exception handler. The try-catch-finally block is used to handle the exception.

The code in which the exception may occur is enclosed in a try block, also called as a guarded region.

The catch clause matches a specific exception to a block of code which handles that exception.

And the clean up code which needs to be executed no matter the exception occurs or not is put inside the finally block

Explain the exception hierarchy in java.

Ans) The hierarchy is as follows:

Throwable is a parent class of all Exception classes. There are two types of Exceptions: Checked exceptions and UncheckedExceptions or RunTimeExceptions. Both type of exceptions extends Exception class.

What is Runtime Exception or unchecked exception?

Ans) Runtime exceptions represent problems that are the result of a programming problem. Such problems include arithmetic exceptions, such as dividing by zero; pointer exceptions: such as trying to access an object through a null reference; and indexing exceptions: such as attempting to access an array element through an index that is too large or too small.

Runtime exceptions need not be explicitly caught in try catch block as it can occur anywhere in a program, and in a typical one they can be very numerous. Having to add runtime exceptions in every method declaration would reduce a program's clarity. Thus, the compiler does not require that you catch or specify runtime exceptions (although you can). The solution to rectify is to correct the programming logic where the exception has occurred or provide a check.

What is checked exception?

Ans) Checked exception are the exceptions which forces the programmer to catch them explicitly in try-catch block. It is a subClass of Exception. Example: IOException.

What is difference between Error and Exception?

Ans) An error is an irrecoverable condition occurring at runtime. Such as OutOfMemory error. These JVM errors you can not repair them at runtime.Though error can be caught in catch block but the execution of application will come to a halt and is not recoverable.

While exceptions are conditions that occur because of bad input or human error 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.)

 

Difference Between ClassNotFoundException Vs NoClassDefFoundError In Java

In Java, both ClassNotFoundException and NoClassDefFoundError occur when a particular class is not found at run time. But, they occur at different scenarios. ClassNotFoundException is an exception which occurs when you try to load a class at run time using Class.forName() orloadClass() methods and mentioned classes are not found in the classpath. On the other hand, NoClassDefFoundError is an error which occurs when a particular class is present at compile time but it was missing at run time. In this tutorial, we will see the differences between ClassNotFoundException Vs NoClassDefFoundError in java and when they occur.

ClassNotFoundException In Java :

ClassNotFoundException is a run time exception which is thrown when an application tries to load a class at run time using Class.forName() orloadClass() or findSystemClass() methods and the class with specified name are not found in the classpath. For example, you may have come across this exception when you try to connect to MySQL or Oracle databases and you have not updated the classpath with required JAR files. In most of time, this exception occurs when you try to run an application without updating the classpath with required JAR files.

For example, below program will throw ClassNotFoundException if the mentioned class “oracle.jdbc.driver.OracleDriver” is not found in the classpath.

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public class MainClass {     public static void main(String[] args)     {         try         {             Class.forName("oracle.jdbc.driver.OracleDriver");         }         catch (ClassNotFoundException e)         {             e.printStackTrace();         }     } }

If you run the above program without updating the classpath with required JAR files, you will get the exception like below,

java.lang.ClassNotFoundException: oracle.jdbc.driver.OracleDriver     at java.net.URLClassLoader.findClass(Unknown Source)     at java.lang.ClassLoader.loadClass(Unknown Source)     at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)     at java.lang.ClassLoader.loadClass(Unknown Source)     at java.lang.Class.forName0(Native Method)     at java.lang.Class.forName(Unknown Source)     at pack1.MainClass.main(MainClass.java:17)

 

NoClassDefFoundError In Java :

NoClassDefFoundError is an error which is thrown when Java Runtime System tries to load the definition of a class and class definition is no longer available. The required class definition was present at compile time but it was missing at run time. For example, compile the below program.

class A { } public class B {     public static void main(String[] args)     {         A a = new A();     } }

When you compile the above program, two .class files will be generated. One is A.class and another one is B.class. If you remove the A.class file and run the B.class file, Java Runtime System will throw NoClassDefFoundError like below,

Exception in thread "main" java.lang.NoClassDefFoundError: A         at MainClass.main(MainClass.java:10) Caused by: java.lang.ClassNotFoundException: A         at java.net.URLClassLoader.findClass(URLClassLoader.java:381)         at java.lang.ClassLoader.loadClass(ClassLoader.java:424)         at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)         at java.lang.ClassLoader.loadClass(ClassLoader.java:357