throws keyword in Java

The “throws” keyword is an important aspect of Java programming used for handling and propagating exceptions. It allows methods to declare the exceptions that they might throw during runtime. These exceptions can be handled by the caller method or propagated up the call stack. Understanding how to use “throws” is essential for writing robust and reliable Java code.
In this article, we will discuss

• Syntax and Usage of “Throws”
• Checked and Unchecked Exceptions
• Propagating Exceptions with “Throws”

Syntax and Usage of “Throws”

The syntax of the “throws” keyword is quite simple. It is followed by the name of the exception or exceptions that the method may throw, separated by commas. For example, the following code snippet declares that the method “divide” may throw an ArithmeticException:

public double divide(int a, int b) throws ArithmeticException {
    return a/b;
}

When a method throws an exception, the caller method can handle it using a “try-catch” block. The “try” block contains the code that may throw an exception, while the “catch” block catches the exception and handles it accordingly. The following code snippet demonstrates the use of “try-catch” to handle an exception thrown by the “divide” method:

public void calculate(int a, int b) {
    try {
        double result = divide(a, b);
        System.out.println("Result: " + result);
    } catch (ArithmeticException e) {
        System.out.println("Cannot divide by zero!");
    }
}

In this example, if the division of two numbers throws an ArithmeticException, the “catch” block catches the exception and prints a message to the console.

In the next section, we will explore the difference between checked and unchecked exceptions in Java and how to handle them using “throws.”

Output

Cannot divide by zero!

Checked and Unchecked Exceptions:

In Java, there are two types of exceptions: checked exceptions and unchecked exceptions. Checked exceptions are exceptions that are checked at compile time. This means that the compiler will verify whether the method handles or declares the exception that can be thrown by a method call. On the other hand, unchecked exceptions are exceptions that are not checked at compile time and occur at runtime.
Checked exceptions are the exceptions that need to be handled or declared using the “throws” keyword in the method signature.

When a method throws a checked exception, there are two options for the calling method. It can handle the exception using the “try-catch” block. Or it can declare the exception in its own method signature using the “throws” keyword.

For example, suppose we have a method that reads data from a file. The FileReader constructor throws a checked FileNotFoundException, so we need to handle it or declare it in our method signature using the “throws” keyword.

public void readFile(String fileName) throws FileNotFoundException {
    FileReader fr = new FileReader(fileName);
    // code to read the file
}

On the other hand, unchecked exceptions are the exceptions that occur at runtime, and they don’t need to be declared using the “throws” keyword. Unchecked exceptions are usually caused by programming errors such as null pointer exceptions, array index out-of-bounds exceptions, etc.

To handle unchecked exceptions, we can use the “try-catch” block. When a method throws a checked exception, there are two options for the calling method. It can handle the exception using the “try-catch” block. Or it can declare the exception in its own method signature using the “throws” keyword.

For example, let’s consider a method that divides two numbers. If the denominator is zero, it will throw an ArithmeticException.

public void divide(int num1, int num2) {
    try {
        int result = num1 / num2;
    } catch (ArithmeticException e) {
        System.out.println("Denominator cannot be zero!");
    }
}

In the above example, we catch the ArithmeticException in the “catch” block and print a message to the console.

It is important to note that we should only catch the exceptions that we can handle. When we catch an exception that we cannot handle, we have two options. First, we can rethrow it using the “throws” keyword. Alternatively, we can wrap it in another exception and throw it.

Propagating Exceptions with “Throws”

In Java, exceptions can be propagated to the calling method or higher in the call stack using the “throws” keyword. An exception that occurs in a method can be thrown to the calling method. This is done by declaring it in the method signature using the “throws” keyword. For example, consider a scenario where a method “methodA” calls another method “methodB”. If an exception occurs in “methodB” that cannot be handled within the method, it can be propagated to “methodA” using “throws”. In “methodA”, the exception can then be handled using a “try-catch” block or propagated further up the call stack.

In addition to propagating exceptions between methods, “throws” can also be used to propagate exceptions within a method chain. This involves declaring the exceptions that can be thrown by the methods in the chain in the method signature.

When using “throws” to propagate exceptions, it is important to follow some best practices to ensure clean and effective code. These include:

1. Declaring only the exceptions that can be thrown: Only declare the exceptions that can be thrown by a method in its signature. Declaring unnecessary exceptions can clutter the code and make it difficult to read.

2. Documenting the exceptions: document the exceptions that can be thrown by a method in its Javadoc comments. This can help other developers understand the behaviour of the method and handle exceptions appropriately.

3. Handling exceptions at the appropriate level: Handle exceptions at the appropriate level in the call stack. If an exception can be handled within a method, it should be handled there instead of being propagated to a higher level.
Overall, the “throws” keyword is a powerful tool for handling and propagating exceptions in Java. Once understood, developers can write clean and effective code that handles exceptions gracefully.

Conclusion

In conclusion, the “throws” keyword is an essential aspect of Java programming that allows developers to handle and propagate exceptions effectively. We have discussed the usage and syntax of the “throws” keyword to handle and declare exceptions in this article. Additionally, we have explored the contrast between checked and unchecked exceptions. We also explained how to use “throws” to propagate exceptions in a method chain and provided some best practices for using the keyword.

It is crucial to understand the use of the “throws” keyword as exceptions are a common occurrence in Java programming. Properly handling and propagating exceptions can help ensure that your code is reliable and robust. By following the best practices and techniques discussed in this article, you can effectively use the “throws” keyword. This can help you write efficient and maintainable Java code.