Exceptions

An exception is an error that occurs while your program is running. When an exception happens, Java stops the current operation and prints information about what went wrong.

Learning to read exceptions is an important debugging skill.

Stack Trace

When an exception occurs, Java prints a stack trace.

Exception in thread "main" java.lang.NullPointerException
    at org.firstinspires.ftc.teamcode.Robot.drive(Robot.java:42)

The stack trace tells you:

• What type of exception occurred
• Which file it happened in
• Which line caused the problem

Always start by looking at the first line in your own code.

Common Exceptions

NullPointerException

This happens when you try to use an object that hasn't been created.

DcMotor motor = null;

motor.setPower(1.0); // Error

This is one of the most common exceptions in FTC.

ArrayIndexOutOfBoundsException

This happens when you access an index that doesn't exist.

int[] numbers = {1, 2, 3};

System.out.println(numbers[3]); // Error

Remember that arrays start at index 0.

IllegalArgumentException

Some methods only accept certain values.

servo.setPosition(2.0); // Error

Always check the documentation to see what values are allowed.

try-catch

Sometimes you can handle exceptions using a try-catch block.

try {
    // Code that might fail
} catch (Exception e) {
    e.printStackTrace();
    // Fallback
}

Most FTC code doesn't use try-catch very often. It's usually better to fix the root cause of the exception.

Tips for Debugging

• Read the exception message.
• Find the line where the exception occurred.
• Check the values of your variables.
• Use telemetry to print information while debugging.

Key Takeaways

• Exceptions are runtime errors.
• The stack trace tells you where the error occurred.
• NullPointerException is one of the most common FTC errors.
• Fix the cause instead of hiding exceptions.