Throws keyword in Java - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the throws keyword in Java?

The throws keyword is used in a method signature to declare that the method might throw certain checked exceptions. It tells the caller to handle or further declare these exceptions.

beginner

How does throws differ from throw in Java?

throw is used to actually throw an exception object inside a method, while throws declares that a method might throw exceptions, informing the caller to handle them.

intermediate

Can a method declare multiple exceptions with throws? How?

Yes, a method can declare multiple exceptions by listing them separated by commas after the throws keyword, for example: void myMethod() throws IOException, SQLException.

beginner

What happens if a method does not handle a checked exception and does not declare it with throws?

The Java compiler will produce an error because checked exceptions must be either caught with a try-catch block or declared with throws in the method signature.

intermediate

Is it mandatory to declare unchecked exceptions with throws?

No, unchecked exceptions (like RuntimeException) do not need to be declared with throws. It is optional because they can occur anywhere and are not checked at compile time.

What does the throws keyword do in a Java method?

A Creates a new exception object

B Declares exceptions that the method might throw

C Catches exceptions inside the method

D Prevents exceptions from occurring

The throws keyword declares exceptions that the method might throw, informing callers to handle them.

Which of these is a correct way to declare multiple exceptions using throws?

A void example() throws IOException, SQLException

B void example() throws IOException SQLException

C void example() throw IOException, SQLException

D void example() throws (IOException, SQLException)

Multiple exceptions are declared separated by commas after the throws keyword without parentheses.

If a method throws a checked exception but does not declare it with throws, what happens?

A The exception is ignored at runtime

B The program compiles and runs fine

C The compiler gives an error

D The method automatically catches the exception

The compiler requires checked exceptions to be declared or caught; otherwise, it produces an error.

Which keyword is used to actually throw an exception object inside a method?

A throw

B throws

C catch

D try

throw is used to throw an exception object, while throws declares exceptions.

Is it necessary to declare unchecked exceptions with throws?

A Only if the method catches them

B No, never

C Yes, always

D Optional, but usually not declared

Unchecked exceptions do not have to be declared with throws, but it is allowed.

Explain the role of the throws keyword in Java methods and how it helps with exception handling.

Think about how Java forces you to handle certain errors.

Describe the difference between throw and throws keywords in Java with examples.

One is for declaring, the other is for actually throwing.

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