In Java, the throws keyword is included in method declarations to indicate the potential exceptions that a method could generate while running. As a result, it is essential for the developer to either implement exception handling mechanisms or pass on the exception to ensure the smooth progress of the program.
Exception handling is primarily utilized for managing checked exceptions. When an unchecked exception like NullPointerException arises, it indicates a lapse on the part of the programmer for not validating the code before its execution.
Syntax
return_type method_name() throws exception_class_name{
//method code
}
Advantages of the throws keyword
- Simplifies Exception Handling: Instead of managing exceptions within the method, throws allows the caller to handle them, making the code cleaner.
- Encourages Code Reusability: Methods can be reused without integrating exception-handling logic, which enhances flexibility.
- Supports Multiple Exception Declaration: A method can declare multiple exceptions using commas, which improves clarity.
- Useful for Checked Exceptions: It ensures that checked exceptions are appropriately handled or propagated, preventing unexpected runtime failures.
Java throws Example
We will now examine an illustration of the Java throws clause, which explains how the throws keyword can be used to pass on checked exceptions.
Example
import java.io.IOException;
class Main{
void m()throws IOException{
throw new IOException("device error");//checked exception
}
void n()throws IOException{
m();
}
void p(){
try{
n();
}catch(Exception e){System.out.println("exception handled");}
}
public static void main(String args[]){
Main obj=new Main();
obj.p();
System.out.println("normal flow...");
}
}
Output:
exception handled
normal flow...
Guideline: When invoking a method that specifies an exception, it is necessary to either handle or specify the exception.
There are the following two cases:
Scenario 1: The exception has been captured, indicating that it has been managed through the implementation of a try/catch block.
Scenario 2: An exception has been defined by including the throws keyword with the method declaration.
Case 1: Handle Exception Using try-catch Block
If we properly manage the exception, the code will run smoothly regardless of whether an exception occurs during the program execution or not.
Example
import java.io.*;
class M{
void method()throws IOException{
throw new IOException("device error");
}
}
public class Main{
public static void main(String args[]){
try{
M m=new M();
m.method();
}catch(Exception e){System.out.println("exception handled");}
System.out.println("normal flow...");
}
}
Output:
exception handled
normal flow...
Case 2: Declare Exception
- If we declare an exception and it does not occur, the code will execute correctly.
- If we declare the exception and it does occur, it will be thrown at runtime, as the throws keyword does not manage the exception.
Let's see examples for both scenarios.
A) If an exception does not occur
Example
import java.io.*;
class M{
void method()throws IOException {
System.out.println("Device operation performed");
}
}
class Main {
public static void main(String args[])throws IOException{//declare exception
M m=new M();
m.method();
System.out.println("normal flow...");
}
}
Output:
Device operation performed
normal flow...
B) If an exception occurs
Example
import java.io.*;
class M{
void method()throws IOException{
throw new IOException("device error");
}
}
class Main{
public static void main(String args[])throws IOException{//declare exception
M m=new M();
m.method();
System.out.println("normal flow...");
}
}
Upon executing the aforementioned code, the following exception is generated.
Exception in thread "main" java.io.IOException: device error
at M.method(Main.java:4)
at Main.main(Main.java:10)
When to use the throws keyword?
The throws keyword is utilized when a method could potentially lead to a checked exception but is not tasked with directly addressing it. It is suitable for situations where you wish to notify the caller about possible exceptions, enabling them to determine how to manage such situations.
Functions that involve file manipulation, connecting to databases, or networking tasks commonly specify exceptions with the "throws" keyword. By doing so, the code structure remains more organized as it shifts the duty of managing the exception to upper tiers within the software's call hierarchy.
Common Mistakes While Using Throws
One frequent error that developers make is utilizing the throws keyword for unchecked exceptions like NullPointerException, which do not necessitate explicit declaration. It is also a misconception to believe that specifying an exception with throws implies its resolution - it simply alerts the calling code. Moreover, excessive use of throws can result in subpar exception handling, complicating code maintenance. It is essential for developers to guarantee that exceptions are adequately caught at a suitable level to avert unforeseen application failures.
Important Points to Remember
- Notice that the throws keyword is used to define exceptions in the header of a method in question, which notifies the caller of some exceptions that can be thrown.
- It is only appropriate to use throws with checked exceptions, such as IOException or SQLException, and not with unchecked exceptions like NullPointerException or ArithmeticException.
- A method that calls another method that declares a checked exception should either try to handle it within a try-catch block or re-declare it with a throws clause.
- Using throws to define an exception does not mean that the exception is being handled. It is just being handed over to the parent level.
- An exception that goes uncaught will be thrown at runtime, causing an unexpected and uncontrolled crash of the application.
- Classes of Error, like OutOfMemoryError and StackOverflowError, do not need to be caught with defined throws because such errors are not meant to be controlled by the program.
Conclusion
The throws functionality within Java is crucial for managing exceptions, especially checked exceptions. It allows the code to shift the responsibility of handling exceptions to the higher-level calling method. This contributes to better code organization and promotes modularity. Effective exception handling enhances the program's resilience and simplifies debugging by clearly indicating the error's location and type.
Java throws Keyword FAQs
- What type of exception needs to be specified for declaration?
Ans: Checked exception only, because:
- Unchecked exceptions are under our control so that we can correct our code.
- Errors are beyond our control. For example, we are unable to do anything if occurs VirtualMachineError or a StackOverflowError.
- Can we rethrow an exception?
Yes, it is possible to achieve this by rethrowing the same exception within the catch block.
What is the distinction between the keywords throw and throws?
What occurs if a method specifies an exception using throws but does not manage it?
Indeed! It is possible for a method to specify multiple exceptions by separating them with commas. For instance:
public void myMethod() throws IOException, SQLException {
// Code that might throw IOException or SQLException
}