Creating custom exception class in Java - Performance & Efficiency

Time Complexity: Creating custom exception class

O(n)

Understanding Time Complexity

When creating a custom exception class in Java, it's important to understand how the time cost grows when throwing or handling exceptions.

We want to see how the program's running time changes as exceptions are created or thrown.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

public class MyException extends Exception {
    public MyException(String message) {
        super(message);
    }
}

public void checkValue(int value) throws MyException {
    if (value < 0) {
        throw new MyException("Negative value not allowed");
    }
}
    

This code defines a custom exception and throws it when a value is negative.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Creating and throwing the custom exception object.
• How many times: Each time the method is called with a negative value, the exception is created and thrown once.

How Execution Grows With Input

Each call to checkValue either throws an exception or not, so the time depends on how many calls happen and how many throw exceptions.

Input Size (n)	Approx. Operations
10	Up to 10 exception creations if all values are negative
100	Up to 100 exception creations if all values are negative
1000	Up to 1000 exception creations if all values are negative

Pattern observation: The time grows linearly with the number of exceptions created and thrown.

Final Time Complexity

Time Complexity: O(n)

This means the time grows in a straight line with the number of times the exception is thrown.

Common Mistake

[X] Wrong: "Creating a custom exception class makes the program slower for all code, even when exceptions are not thrown."

[OK] Correct: Defining the class itself does not slow down normal code; only throwing exceptions takes extra time.

Interview Connect

Understanding how exceptions affect time helps you write better error handling and shows you know how code performance changes in real situations.

Self-Check

"What if we changed the exception to be thrown inside a loop that runs n times? How would the time complexity change?"

Practice

1. What is the correct way to start creating a custom exception class in Java?

easy

A. Extend the Exception or RuntimeException class

B. Implement the Exception interface

C. Create a class with the same name as Exception

D. Use the throw keyword in the class declaration

Solution

1. Step 1: Understand Java exception hierarchy

   Custom exceptions must extend either Exception or RuntimeException to behave like exceptions.

2. Step 2: Recognize correct inheritance

   Implementing an interface or naming a class Exception does not create a proper exception class.

3. Final Answer:

   Extend the Exception or RuntimeException class -> Option A

4. Quick Check:

   Custom exception = extends Exception [OK]

Hint: Always extend Exception or RuntimeException for custom exceptions [OK]

Common Mistakes:

• Trying to implement Exception as an interface
• Naming class Exception instead of extending it
• Using throw keyword in class declaration

2. Which of the following is the correct constructor for a custom exception class named MyException?

easy

A. public MyException() { this.message = message; }

B. public void MyException(String message) { super(message); }

C. public MyException(String message) { super(message); }

D. public MyException(String message) { print(message); }

Solution

1. Step 1: Identify correct constructor syntax

   Constructors have no return type and call super(message) to pass the message to the parent Exception class.

2. Step 2: Check each option

   public MyException(String message) { super(message); } correctly defines a constructor calling super(message). public MyException() { this.message = message; } incorrectly assigns message without declaration. public void MyException(String message) { super(message); } has a void return type, so it's not a constructor. public MyException(String message) { print(message); } calls a non-existent method print.

3. Final Answer:

   public MyException(String message) { super(message); } -> Option C

4. Quick Check:

   Constructor calls super(message) = public MyException(String message) { super(message); } [OK]

Hint: Constructor must call super(message) without return type [OK]

Common Mistakes:

• Adding void return type to constructor
• Not calling super(message) in constructor
• Trying to assign message directly without declaration

3. What will be the output of this code?

class MyException extends Exception {
    public MyException(String message) {
        super(message);
    }
}

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

medium

A. Error occurred

B. MyException

C. Compilation error

D. No output

Solution

1. Step 1: Understand exception throwing and catching

   The code throws a MyException with message "Error occurred" and catches it immediately.

2. Step 2: Check output from getMessage()

   The catch block prints e.getMessage(), which returns the message passed to the exception.

3. Final Answer:

   Error occurred -> Option A

4. Quick Check:

   Exception message printed = Error occurred [OK]

Hint: getMessage() prints the exception message passed in constructor [OK]

Common Mistakes:

• Expecting class name instead of message
• Thinking code won't compile due to custom exception
• Missing try-catch block causing runtime error

4. Identify the error in this custom exception class:

public class MyException extends Exception {
    public void MyException(String message) {
        super(message);
    }
}

medium

A. super(message) cannot be called in this class

B. Missing import statement for Exception

C. Class must implement Serializable interface

D. Constructor has a void return type, so it's a method, not a constructor

Solution

1. Step 1: Check constructor syntax

   Constructors must not have a return type. Here, void makes it a method, not a constructor.

2. Step 2: Understand consequences

   Without a proper constructor, the class uses default constructor which does not call super(message), causing errors when throwing with message.

3. Final Answer:

   Constructor has a void return type, so it's a method, not a constructor -> Option D

4. Quick Check:

   Constructor no return type = Constructor has a void return type, so it's a method, not a constructor [OK]

Hint: Constructors never have a return type, not even void [OK]

Common Mistakes:

• Adding void return type to constructor
• Assuming import Exception is needed
• Thinking super() cannot be called in subclass

5. You want to create a custom unchecked exception named InvalidDataException. Which is the correct way to define it?

hard

A. public class InvalidDataException implements RuntimeException {
public InvalidDataException(String message) {
super(message);
}
}

B. public class InvalidDataException extends RuntimeException {
public InvalidDataException(String message) {
super(message);
}
}

C. public class InvalidDataException extends Exception {
public InvalidDataException(String message) {
super(message);
}
}

D. public class InvalidDataException extends Throwable {
public InvalidDataException(String message) {
super(message);
}
}

Solution

1. Step 1: Understand checked vs unchecked exceptions

   Unchecked exceptions extend RuntimeException, checked exceptions extend Exception.

2. Step 2: Analyze each option

   public class InvalidDataException extends RuntimeException { public InvalidDataException(String message) { super(message); } } correctly extends RuntimeException with proper constructor. public class InvalidDataException extends Exception { public InvalidDataException(String message) { super(message); } } creates a checked exception. public class InvalidDataException implements RuntimeException { public InvalidDataException(String message) { super(message); } } tries to implement an exception class, which is invalid. public class InvalidDataException extends Throwable { public InvalidDataException(String message) { super(message); } } extends Throwable directly, which is not recommended for custom exceptions.

3. Final Answer:

   public class InvalidDataException extends RuntimeException { public InvalidDataException(String message) { super(message); } } -> Option B

4. Quick Check:

   Unchecked exception = extends RuntimeException [OK]

Hint: Unchecked exceptions extend RuntimeException, not Exception [OK]

Common Mistakes:

• Extending Exception for unchecked exceptions
• Trying to implement exception classes
• Extending Throwable directly