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What is Exception in Java?

An exception is an unwanted or unexpected event which occurs at the run time of the program, that leads to the disruption of the normal flow of execution of the program.

What is Exception Handling?

Exception Handling is a mechanism to handle runtime errors happen in the program such as ClassNotFoundException, IOException, SQLException, RemoteException, etc, which disruptes the normal execution of the program.

Suppose there are 6 statements in your program and there occurs an exception at statement 3, the rest of the code will not be executed i.e. statement 4 to 6 will not be executed. If we perform exception handling, the rest of the statement will be executed. That is why we use exception handling in Java.

What is the difference between Error and Exception?

Error: An Error indicates serious problem that a reasonable application should not try to catch. Error are used by the Java run-time system(JVM) to indicate errors having to do with the run-time environment itself(JRE) or StackOverflowError or OutOfMemoryError

Exception: Exception indicates conditions that a reasonable application might try to catch. Example of exceptions are IOException, SQLException, etc.

Types of Exceptions

1) Checked Exception
The classes which directly inherit Throwable class except RuntimeException and Error are known as checked exceptions e.g. IOException, SQLException etc. Checked exceptions are checked at compile-time.

2) Unchecked Exception
The classes which inherit RuntimeException are known as unchecked exceptions e.g. ArithmeticException, NullPointerException, ArrayIndexOutOfBoundsException etc. Unchecked exceptions are not checked at compile-time, but they are checked at runtime.

Hierarchy of Java Exception classes

Keywords used in Exception Handling in Java

1. try keyword – It is used to specify a block where we should place exception code. The try block must be followed by either catch or finally. It means, we can’t use try block alone.

2. catch keyword – It is used to handle the exception. It must be preceded by try block which means we can’t use catch block alone. It can be followed by finally block later.

3. finally keyword – It is used to execute the important code of the program irrespective the exception is handled or not.

4. throw keyword – It is used to throw an exception explicitly in the program inside a function or inside a block of code.

5. throws keyword – It is used to declare exceptions. It doesn’t throw an exception. It specifies that there may occur an exception in the method. It is always used with method signature.

Exception Handling Example in Java

Let’s see an example of exception handling where we can use try-catch block and will not use try-catch statements. Below example shows an exception without try-catch block.

package JavaDemo;

public class ExceptionHandlingDemo {

	static String a = null;

	public static void main(String[] args) {

		System.out.println(a.length());
	}

}

Output
Exception in thread "main" java.lang.NullPointerException
	at JavaDemo.ExceptionHandlingDemo.main(ExceptionHandlingDemo.java:9)

Below example shows an exception with try-catch block.

package JavaDemo;

public class ExceptionHandlingDemo {
	static String a = null;

	public static void main(String[] args) {
		// TODO Auto-generated method stub

		try {
			System.out.println(a.length());
		} catch (NullPointerException e) {
			System.out.println(e);
		}
		System.out.println("Exception is handled");
	}

}

Output
java.lang.NullPointerException
Exception is handled