In Java, Exception Handling is one of the effective means to handle runtime errors so that the regular flow of the application can be preserved. Java Exception Handling is a mechanism to handle runtime errors such as ClassNotFoundException, IOException, SQLException, RemoteException, etc.
In this article, we will learn about throw and throws in Java which can handle exceptions in Java.
Java throw
The throw keyword in Java is used to explicitly throw an exception from a method or any block of code. We can throw either checked or unchecked exception. The throw keyword is mainly used to throw custom exceptions.
Syntax in Java throw
throw Instance
Example:
throw new ArithmeticException("/ by zero");
But this exception i.e., Instance must be of type Throwable or a subclass of Throwable.
For example, an Exception is a sub-class of Throwable and user-defined exceptions typically extend the Exception class. Unlike C++, data types such as int, char, floats, or non-throwable classes cannot be used as exceptions.
The flow of execution of the program stops immediately after the throw statement is executed and the nearest enclosing try block is checked to see if it has a catch statement that matches the type of exception. If it finds a match, controlled is transferred to that statement otherwise next enclosing try block is checked, and so on. If no matching catch is found then the default exception handler will halt the program.
Java throw Examples
Example 1:
Java
// Java program that demonstrates the use of throwclass ThrowExcep { static void fun() { try { throw new NullPointerException("demo"); } catch (NullPointerException e) { System.out.println("Caught inside fun()."); throw e; // rethrowing the exception } } public static void main(String args[]) { try { fun(); } catch (NullPointerException e) { System.out.println("Caught in main."); } }} |
Output
Caught inside fun(). Caught in main.
Example 2
Java
// Java program that demonstrates// the use of throwclass Test { public static void main(String[] args) { System.out.println(1 / 0); }} |
Output
Exception in thread "main" java.lang.ArithmeticException: / by zero
Java throws
throws is a keyword in Java that is used in the signature of a method to indicate that this method might throw one of the listed type exceptions. The caller to these methods has to handle the exception using a try-catch block.
Syntax of Java throws
type method_name(parameters) throws exception_list exception_list is a comma separated list of all the exceptions which a method might throw.
In a program, if there is a chance of raising an exception then the compiler always warns us about it and compulsorily we should handle that checked exception, Otherwise, we will get compile time error saying unreported exception XXX must be caught or declared to be thrown. To prevent this compile time error we can handle the exception in two ways:
- By using try catch
- By using the throws keyword
We can use the throws keyword to delegate the responsibility of exception handling to the caller (It may be a method or JVM) then the caller method is responsible to handle that exception.
Java throws Examples
Example 1
Java
// Java program to illustrate error in case// of unhandled exceptionclass tst { public static void main(String[] args) { Thread.sleep(10000); System.out.println("Hello Geeks"); }} |
Output
error: unreported exception InterruptedException; must be caught or declared to be thrown
Explanation
In the above program, we are getting compile time error because there is a chance of exception if the main thread is going to sleep, other threads get the chance to execute the main() method which will cause InterruptedException.
Example 2
Java
// Java program to illustrate throwsclass tst { public static void main(String[] args) throws InterruptedException { Thread.sleep(10000); System.out.println("Hello Geeks"); }} |
Output
Hello Geeks
Explanation
In the above program, by using the throws keyword we handled the InterruptedException and we will get the output as Hello Geeks
Example 3
Java
// Java program to demonstrate working of throwsclass ThrowsExecp { static void fun() throws IllegalAccessException { System.out.println("Inside fun(). "); throw new IllegalAccessException("demo"); } public static void main(String args[]) { try { fun(); } catch (IllegalAccessException e) { System.out.println("caught in main."); } }} |
Output
Inside fun(). caught in main.
Important Points to Remember about throws Keyword
- throws keyword is required only for checked exceptions and usage of the throws keyword for unchecked exceptions is meaningless.
- throws keyword is required only to convince the compiler and usage of the throws keyword does not prevent abnormal termination of the program.
- With the help of the throws keyword, we can provide information to the caller of the method about the exception.
