Throw keyword in Java

Introduction

The throw keyword is used to send an error or exception when something goes wrong in your program. It helps you stop the normal flow and handle problems clearly.

When you want to stop the program because a wrong value is found.

When you want to tell the program to handle a special error case.

When you want to create your own error message for a problem.

When you want to pass an exception to another part of the program to handle.

When you want to make sure the program does not continue with bad data.

Syntax

Java

throw new ExceptionType("Error message");

You must create a new exception object with new before throwing it.

The throw keyword only sends one exception at a time.

Examples

This throws an exception when an invalid age is found.

Java

throw new IllegalArgumentException("Invalid age");

This throws an exception if an object is unexpectedly null.

Java

throw new NullPointerException("Object is null");

This throws a general runtime exception with a message.

Java

throw new RuntimeException("Something went wrong");

Sample Program

This program checks if a number is positive. If the number is negative, it throws an exception with a message. Otherwise, it prints the number.

Java

public class ThrowExample {
    public static void checkNumber(int number) {
        if (number < 0) {
            throw new IllegalArgumentException("Number must be positive");
        } else {
            System.out.println("Number is " + number);
        }
    }

    public static void main(String[] args) {
        checkNumber(10);
        checkNumber(-5);
    }
}

Important Notes

When you use throw, the program stops running the current method unless the exception is caught.

You can only throw objects that are subclasses of Throwable, like Exception or Error.

Use throw to create clear and meaningful error messages for easier debugging.

Summary

throw sends an exception to stop normal program flow when an error happens.

You create a new exception object and use throw to send it.

It helps make your program safer by handling problems clearly.

Practice

1.

What does the throw keyword do in Java?

easy

A. It catches an exception and handles it.

B. It sends an exception to stop normal program flow when an error occurs.

C. It declares a method can throw exceptions.

D. It creates a new thread for parallel execution.

Solution

1. Step 1: Understand the role of throw

   The throw keyword is used to send an exception object explicitly when an error happens.

2. Step 2: Differentiate from other keywords

   throw does not catch exceptions (that's catch), nor declare exceptions (that's throws), nor create threads.

3. Final Answer:

   It sends an exception to stop normal program flow when an error occurs. -> Option B

4. Quick Check:

   throw sends exception = B [OK]

Hint: Remember: throw sends, catch handles exceptions [OK]

Common Mistakes:

• Confusing throw with throws keyword
• Thinking throw catches exceptions
• Mixing throw with thread creation

2.

Which of the following is the correct way to throw a new IllegalArgumentException in Java?

?

easy

A. throws new IllegalArgumentException("Invalid argument");

B. throw IllegalArgumentException("Invalid argument");

C. throw new IllegalArgumentException("Invalid argument");

D. throw new IllegalArgumentException;

Solution

1. Step 1: Check syntax for throwing exceptions

   To throw an exception, use throw new ExceptionType("message") with parentheses and semicolon.

2. Step 2: Identify correct option

   throw new IllegalArgumentException("Invalid argument"); uses correct syntax with new, parentheses, and semicolon. Options B and D miss parentheses or new. throws new IllegalArgumentException("Invalid argument"); uses throws which is for method declarations, not throwing.

3. Final Answer:

   throw new IllegalArgumentException("Invalid argument"); -> Option C

4. Quick Check:

   throw + new + parentheses = A [OK]

Hint: Throw exceptions with 'throw new ExceptionType()' syntax [OK]

Common Mistakes:

• Omitting 'new' keyword
• Using 'throws' instead of 'throw'
• Missing parentheses after exception class

3.

What will be the output of the following Java code?

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

medium

A. Error happened

B. RuntimeException

C. Compilation error

D. No output

Solution

1. Step 1: Analyze the try block

   The code throws a new RuntimeException with message "Error happened".

2. Step 2: Analyze the catch block

   The catch block catches the exception and prints its message using e.getMessage(), which is "Error happened".

3. Final Answer:

   Error happened -> Option A

4. Quick Check:

   Exception message printed = C [OK]

Hint: Catch prints exception message with getMessage() [OK]

Common Mistakes:

• Expecting exception type name instead of message
• Thinking code causes compilation error
• Assuming no output without catch

4.

Identify the error in the following code snippet:

public class Example {
    public static void main(String[] args) {
        throw new Exception("Problem");
    }
}

medium

A. Missing try-catch block or throws declaration for checked exception.

B. Incorrect exception message format.

C. Cannot throw exceptions in main method.

D. Exception class does not exist.

Solution

1. Step 1: Identify exception type

   The code throws Exception, which is a checked exception in Java.

2. Step 2: Check handling of checked exceptions

   Checked exceptions must be either caught in a try-catch block or declared with throws in the method signature. This code does neither, causing a compile error.

3. Final Answer:

   Missing try-catch block or throws declaration for checked exception. -> Option A

4. Quick Check:

   Checked exceptions need handling = D [OK]

Hint: Checked exceptions require try-catch or throws declaration [OK]

Common Mistakes:

• Ignoring checked exception rules
• Thinking main cannot throw exceptions
• Confusing checked and unchecked exceptions

5.

Consider this method that throws an exception if the input is negative:

public void checkNumber(int num) {
    if (num < 0) {
        throw new IllegalArgumentException("Negative number not allowed");
    }
    System.out.println("Number is " + num);
}

How should you call this method safely in your code?

hard

A. Use throw keyword again when calling checkNumber.

B. Call checkNumber without any try-catch because IllegalArgumentException is checked.

C. Declare throws IllegalArgumentException in the calling method and do not catch.

D. Call checkNumber inside a try-catch block catching IllegalArgumentException.

Solution

1. Step 1: Identify exception type thrown

   The method throws IllegalArgumentException, which is an unchecked exception.

2. Step 2: Decide safe calling practice

   Although unchecked exceptions do not require declaration, to handle errors safely, call the method inside a try-catch block catching IllegalArgumentException.

3. Final Answer:

   Call checkNumber inside a try-catch block catching IllegalArgumentException. -> Option D

4. Quick Check:

   Catch unchecked exceptions to handle errors safely = A [OK]

Hint: Catch exceptions even if unchecked for safer code [OK]

Common Mistakes:

• Thinking unchecked exceptions must be declared
• Not catching exceptions leading to crashes
• Misusing throw keyword when calling methods