Throws keyword in Java - Mini Project: Build & Apply

Handling Exceptions with the Throws Keyword in Java

📖 Scenario: Imagine you are writing a simple Java program that reads a number from the user and divides 100 by that number. Sometimes, the user might enter zero, which causes an error called ArithmeticException. To handle this, you will use the throws keyword to declare that your method might throw an exception.

🎯 Goal: You will create a method that divides 100 by a number given as input. You will declare that this method throws an ArithmeticException. Then, you will call this method from main and print the result.

📋 What You'll Learn

Create a method called divide that takes an int parameter called number.

Declare that divide throws ArithmeticException.

Inside divide, return the result of dividing 100 by number.

In the main method, call divide with the value 0.

Print the result of the divide method call.

💡 Why This Matters

🌍 Real World

In real programs, methods often need to declare exceptions they might throw so callers know to handle errors like dividing by zero or file not found.

💼 Career

Understanding the <code>throws</code> keyword is essential for writing robust Java applications that handle errors properly, a key skill for Java developers.

1

Create the divide method signature

Write a method called divide that takes an int number parameter and declares it throws ArithmeticException. Do not write the method body yet.

Java

public class ThrowsExample {

    public static void main(String[] args) {

    }

    // Write the divide method signature here
    // Your code here
}

Hint

Remember to include throws ArithmeticException after the method parameters.

2

Implement the divide method body

Inside the divide method, return the result of dividing 100 by the parameter number.

Java

public class ThrowsExample {

    public static void main(String[] args) {

    }

    public static int divide(int number) throws ArithmeticException {
        // Return 100 divided by number
        // Your code here
    }
}

Hint

Use the division operator / to divide 100 by number.

3

Call divide method from main

In the main method, call the divide method with the argument 0 and store the result in an int variable called result.

Java

public class ThrowsExample {

    public static void main(String[] args) {
        // Call divide with 0 and store in result
        // Your code here
    }

    public static int divide(int number) throws ArithmeticException {
        return 100 / number;
    }
}

Hint

Remember to assign the call to divide(0) to a variable named result.

4

Print the result

Add a System.out.println statement in the main method to print the value of result.

Java

public class ThrowsExample {

    public static void main(String[] args) {
        int result = divide(0);
        // Print the result
        // Your code here
    }

    public static int divide(int number) throws ArithmeticException {
        return 100 / number;
    }
}

Hint

When you run this code, it will throw an exception because dividing by zero is not allowed.

Practice

1.

What is the main purpose of the throws keyword in Java?

easy

A. To declare that a method might throw certain checked exceptions

B. To catch exceptions inside a method

C. To create a new exception object

D. To stop the program immediately when an error occurs

Solution

1. Step 1: Understand the role of throws

   The throws keyword is used in a method signature to declare that the method might throw certain checked exceptions.

2. Step 2: Differentiate from other keywords

   It does not catch exceptions (that's try-catch), nor create exceptions or stop the program immediately.

3. Final Answer:

   To declare that a method might throw certain checked exceptions -> Option A

4. Quick Check:

   throws declares exceptions [OK]

Hint: Remember: throws declares, catch handles exceptions [OK]

Common Mistakes:

• Confusing throws with catch
• Thinking throws creates exceptions
• Believing throws stops program immediately

2.

Which of the following is the correct way to declare a method that might throw an IOException?

public void readFile() _____ IOException { }

easy

A. thrown

B. throw

C. throws

D. throws new

Solution

1. Step 1: Recall correct syntax for exception declaration

   In Java, the keyword to declare exceptions a method might throw is throws.

2. Step 2: Check options for syntax correctness

   throw is used to actually throw an exception inside method body, not in declaration. thrown and throws new are invalid.

3. Final Answer:

   throws -> Option C

4. Quick Check:

   Method declaration uses throws [OK]

Hint: Method declarations use 'throws', not 'throw' [OK]

Common Mistakes:

• Using 'throw' instead of 'throws' in method signature
• Adding 'new' after throws
• Using non-existent keywords like 'thrown'

3.

What will be the output of the following code?

import java.io.*;

public class Test {
    public static void risky() throws IOException {
        throw new IOException("Error happened");
    }
    public static void main(String[] args) {
        try {
            risky();
        } catch (IOException e) {
            System.out.println(e.getMessage());
        }
    }
}

medium

A. Error happened

B. Compilation error due to missing throws

C. No output

D. Runtime error without message

Solution

1. Step 1: Analyze method throwing exception

   The method risky() declares it throws IOException and actually throws it with message "Error happened".

2. Step 2: Check exception handling in main

   The main method calls risky() inside a try block and catches IOException, printing the exception message.

3. Final Answer:

   Error happened -> Option A

4. Quick Check:

   Exception caught and message printed [OK]

Hint: Thrown exceptions must be caught or declared [OK]

Common Mistakes:

• Thinking throws causes compile error if caught
• Expecting no output because exception thrown
• Confusing throws with throw inside method body

4.

Identify the error in the following code snippet:

public void process() {
    riskyMethod() throws IOException;
}

medium

A. Incorrect use of throws keyword inside method body

B. Missing try-catch block around riskyMethod() call

C. Method process() should declare throws IOException

D. All of the above

Solution

1. Step 1: Check syntax of throws usage

   The throws keyword cannot be used inside a method body; it belongs in the method signature.

2. Step 2: Analyze exception handling requirements

   Calling riskyMethod() which throws IOException requires either a try-catch block or declaring throws IOException in process().

3. Step 3: Combine all errors

   All these issues are present: wrong throws usage, missing try-catch, and missing throws declaration.

4. Final Answer:

   All of the above -> Option D

5. Quick Check:

   Throws only in signature + handle exceptions [OK]

Hint: Throws keyword only in method signature, not inside body [OK]

Common Mistakes:

• Using throws inside method body
• Not handling checked exceptions properly
• Forgetting to declare throws in method signature

5.

You have a method readData() that calls two other methods: openFile() and parseFile(). Both can throw IOException. How should you declare readData() to properly handle exceptions?

hard

A. Do nothing, exceptions will be handled automatically

B. Declare readData() with throws IOException and let caller handle it

C. Declare readData() with throws Exception to cover all exceptions

D. Use try-catch inside readData() to catch and ignore exceptions

Solution

1. Step 1: Understand exception propagation

   If openFile() and parseFile() throw IOException, readData() must either handle or declare these exceptions.

2. Step 2: Choose proper declaration

   Declaring throws IOException in readData() lets the caller decide how to handle exceptions, keeping code clean and clear.

3. Step 3: Evaluate other options

   Ignoring exceptions is bad practice. Declaring throws Exception is too broad. Exceptions are not handled automatically.

4. Final Answer:

   Declare readData() with throws IOException and let caller handle it -> Option B

5. Quick Check:

   Declare checked exceptions to propagate [OK]

Hint: Declare throws for checked exceptions to pass responsibility [OK]

Common Mistakes:

• Ignoring exceptions instead of declaring or catching
• Declaring too broad exceptions like Exception
• Assuming exceptions are handled automatically