Java – 5 important keywords in Exception handling

February 25, 2017 SJ Exception Handling 1

In this article, we will discuss 5 important keywords related to Java exception handling i.e.;

  1. try
  2. catch
  3. finally
  4. throw
  5. throws

Although we have covered every keyword individually, let us summarize each keyword with few lines and finally one example covering each keyword in a single program

1. try-block:

  • The code which might raise exception must be enclosed within try-block
  • try-block must be followed by either catch-block or finally-block, at the end
  • If both present, it is still valid but the sequence of the try-catch-finally matters the most
  • Otherwise, compile-time error will be thrown for invalid sequence
  • The valid combination like try-catch block or try-catch-finally blocks must reside inside method
  • Note: code inside try-block must always be wrapped inside curly braces, even if it contains just one line of code;
  • Otherwise, compile-time error will be thrown
  • Compile-time Error : “Syntax error on token “)”, Block expected after this token”
  • Read more about try block in detail

2. catch-block:

  • Contains handling code for any exception raised from corresponding try-block and it must be enclosed within catch block
  • catch-block takes one argument which should be of type Throwable or one of its sub-classes i.e.; class-name followed by a variable
  • Variable contains exception information for exception raised from try-block
  • Note: code inside catch-block must always be wrapped inside curly braces, even if it contains just one line of code;
  • Otherwise, compile-time error will be thrown
  • Compile-time Error: “Syntax error on token “)”, Block expected after this token”
  • Read more about catch block in detail

3. finally-block:

  • finally block is used to perform clean-up activities or code clean-up like closing database connection & closing streams or file resources, etc
  • finally block is always associated with try-catch block
  • With finally-block, there can be 2 combinations
  • One is try-block is followed by finally-block and other is try-catch-finally sequence
  • The only other possible combination is try block followed by multiple catch block and one finally block at the end (this is case of multiple catch blocks)
  • Advantage: The beauty of finally block is that, it is executed irrespective of whether exception is thrown or NOT (from try-block)
  • Also, it gets executed whether respective exception is handled or NOT (inside catch-block)
  • Note: finally block won’t get executed if JVM exits with System.exit() or due to some fatal error like code is interrupted or killed
  • Read more about finally block in detail

4. throw clause:

  • Sometimes, programmer can also throw/raise exception explicitly at runtime on the basis of some business condition
  • To raise such exception explicitly during program execution, we need to use throw keyword
  • Syntax:
    • throw instanceOfThrowableType
  • Generally, throw keyword is used to throw user-defined exception or custom exception
  • Although, it is perfectly valid to throw pre-defined exception or already defined exception in Java like IOException, NullPointerException, ArithmeticException, InterruptedExcepting, ArrayIndexOutOfBoundsException, etc.
  • Read more about throw clause or throw keyword in detail
  • Pseudo code: 
try {
 
    // some valid Java statements
    throw new RuntimeException();
}
catch(Throwable th) {
 
    // handle exception here
    // or re-throw caught exception
}

5. throws keyword or throws clause:

  • throws keyword is used to declare the exception that might raise during program execution
  • whenever exception might thrown from program, then programmer doesn’t necessarily need to handle that exception using try-catch block instead simply declare that exception using throws clause next to method signature
  • But this forces or tells the caller method to handle that exception; but again caller can handle that exception using try-catch block or re-declare those exception with throws clause
  • Note: use of throws clause doesn’t necessarily mean that program will terminate normally rather it is the information to the caller to handle for normal termination
  • Any number of exceptions can be specified using throws clause, but they are all need to be separated by commas (,)
  • throws clause is applicable for methods & constructor but strictly not applicable to classes
  • It is mainly used for checked exception, as unchecked exception by default propagated back to the caller (i.e.; up in the runtime stack)
  • Note: It is highly recommended to use try-catch for exception handling instead of throwing exception using throws clause
  • Read more about throws clause or throws keyword in detail

Demo program on Java Exception handling:

  • This demo program covers all 5 keywords related to Java Exception handling

DemoOnTryCatchFinallyThrowThrows.java

package in.bench.resources.exception.handling;

public class DemoOnTryCatchFinallyThrowThrows {

	// main() method - start of JVM execution
	public static void main(String[] args) {

		try {
			// call division() method
			String welcomeMessage = welcomeMessage("SJ");

			// print to console
			System.out.println("The returned welcome message : "
					+ welcomeMessage);
		}
		catch (NullPointerException npex){
			System.out.println("Exception handled : "
					+ npex.toString());
		}
		finally {
			System.out.println("Rest of the clean-up code here");
		}
	}

	// division method returning quotient
	public static String welcomeMessage(String name)
			throws NullPointerException {

		if(name == null) {

			// explicitly throwing Null Pointer Error
			// using throw keyword
			throw new NullPointerException(
					"Invoke method with VALID name");
		}

		// performing String concatenation
		String welcomeMsg = "Welcome " + name;

		/// return concatenated string value
		return welcomeMsg;
	}
}

Output:

The returned welcome message : Welcome SJ
Rest of the clean-up code here

Related Articles:

References:

Happy Coding !!
Happy Learning !!

Java - User-defined Exception or Custom Exception
Java - Exception propagation

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