throw Statement in Java Exception Handling

It is possible for a Java program to explicitly raise an exception, that is done using the throw statement in Java.

When the throw statement is executed inside a method, the program’s flow of execution halts immediately; any code written after the throw statement will not run.

General Syntax of throw Statement

General form of throw statement is-

throw throwableObject;

Here, throwableObject must be an instance of Throwable class or one of its subclasses.

Ways to Obtain a Throwable Object

We can get this throwableObject in 2 ways-

By using the Exception parameter in a catch block- re‑throwing a caught exception.
Create a new exception object using the new operator. For example
```
throw new IllegalArgumentException("Invalid input provided");
```

Example Program Using throw Statement in Java

public class ThrowDemo {

 public static void main(String[] args) {
  ThrowDemo throwDemo = new ThrowDemo();
  try{
    throwDemo.displayValue();
  }catch(NullPointerException nExp){
     System.out.println("Exception caught in catch block of main");
     nExp.printStackTrace();;
  }
 }
 
 public void displayValue(){
  try{
    // Explicitly throwing a new exception object
    throw new NullPointerException();   
  }catch(NullPointerException nExp){
     System.out.println("Exception caught in catch block of displayValue");
     // Re‑throwing the caught exception
     throw nExp;
  }
 }
}

Note that in this program throw keyword is used at two places. First time, in the try block of displayValue() method, it uses the new operator to create an instance of type throwable.

Second time it uses the parameter of the catch block.

throw statement helps in preserving loose coupling

One of the best practices for the exception handling is to preserve loose coupling between layers of an application. This means that an implementation specific checked exception should not propagate to another layer.

As Example SQL exception from the DataAccessCode (DAO layer) should not propagate to the service (Business) layer. The common approach in that case is to wrap or convert database specific SQLException into an unchecked exception and then throw it.

catch(SQLException ex){
    throw new RuntimeException("Database error occurred", ex);
}

This ensures that the service layer deals with generalized exceptions rather than being tightly coupled to database‑specific details, making the application more maintainable and flexible.

That's all for this topic throw Statement in Java Exception Handling. If you have any doubt or any suggestions to make please drop a comment. Thanks!

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