Welcome back to your next lesson in FRC programming training. Next up, you’re learning about methods. Methods, in one way or another, make up over half of our robot code.

Before moving on, learn about methods in the W3Schools Java Methods section , down through Java method parameters. Then, come back to complete the rest of this lesson.

Code in Practice

public void setRackState(IntakeState targetState) {
    if (!currentState.equals(targetState)) {
        timer.reset();
        timer.stop();
        timer.start();
    }
    this.currentState = targetState;
    switch (targetState) {
        case DEFAULT -> {
            setPosition(IntakeSubsystemConstants.retractedRotations);
        }
        case EXTENDED -> {
            rackIO.limitSwitchExtensionControl();
            setRollerState(RollerState.OFF);
        }
        case INTAKING -> {
            rackIO.limitSwitchExtensionControl();
            setRollerState(RollerState.ON);
        }
        case AGITATE_IN -> {
            setPosition(IntakeSubsystemConstants.retractedRotations);
        }
        case AGITATE_OUT -> {
            setPosition(IntakeSubsystemConstants.extendedRotations);
        }
    }
}

public void setRollerState(RollerState desiredState) {
    this.currentRollerState = desiredState;
}

public RollerState getRollerState() {
    return this.currentRollerState;
}

public Angle getRollerPosition() {
    return rollerInputs.rollerMotorPosition;
}

public Angle getRackPosition() {
    return rackInputs.rackMotorPosition;
}

public boolean getLimitSwitchValue() {
    return rackInputs.rackLimitSwitch;
}

public Voltage getRackAppliedVoltage() {
    return rackInputs.rackMotorAppliedVoltage;
}

How many methods can you count in this intake subsystem? What have you seen before? Do you recognize a switch statement, or perhaps a return type?

For your next challenge, you will be creating methods for a simple “subsystem.”

This is the most difficult challenge so far, and it has a lot of parts. Expect to take a while on it! Also, make sure you copy all of your code into something like a Google Doc periodically so you don’t lose it all while working in Programiz.

Exercise: Your First Robot Subsystem (Elevator)

Now that you’ve learned about variables, methods, conditionals, loops, and method parameters, it’s time to build your first very simple robot subsystem.

In every subsystem you make, there are going to be values called inputs.

Inputs are things like:

• Motor rotations
• Sensor values
• Device temperatures

The subsystem processes these inputs and determines the correct outputs. Outputs are the commands and values going out of the system. This could be a value on a dashboard or a command telling a motor to spin at a certain speed.

In this exercise, you’ll simulate an elevator subsystem entirely inside your main() method.

Step 1 - Create the Inputs

Create the following variables.

double motorRotations;
double motorVoltage;
boolean topLimitSwitch;
boolean bottomLimitSwitch;

These are your subsystem’s inputs.

Step 2 - Create Constants

Remember that constants use the final keyword.

Create these constants:

Example:

final double MAX_HEIGHT = 50;

Step 3 - Create Internal Variables

These variables aren’t inputs or outputs. They’re simply used to keep track of what the subsystem is doing.

Create:

String elevatorState;
int loopCount = 0;

The elevator state should only ever contain one of these three values:

• "Stopped"
• "Moving Up"
• "Moving Down"

Step 4 - Create the Program Loop

Inside main(), create an infinite loop.

while (true) {
    // Elevator code goes here
}

This loop represents the robot running continuously.

Step 5 - Update the Limit Switches

At the beginning of every loop, determine whether the elevator is touching either end of its travel.

If the elevator is at the bottom:

motorRotations <= 0

Then:

bottomLimitSwitch = true;

Otherwise:

bottomLimitSwitch = false;

If the elevator is at the top:

motorRotations >= MAX_HEIGHT

Then:

topLimitSwitch = true;

Otherwise:

topLimitSwitch = false;

These two switches represent the sensors mounted at the bottom and top of the elevator.

Step 6 - Move the Elevator

One volt of power moves the motor 0.02 rotations every loop.

For example:

• 1 volt → 0.02 rotations
• 3 volts → 0.06 rotations
• 6 volts → 0.12 rotations

Whenever the elevator is moving, it should always use the maximum allowed voltage so it travels as fast as possible.

Step 7 - Prevent Illegal Movement

The elevator should never move past its limits.

If:

bottomLimitSwitch == true

The elevator cannot move downward.

If:

topLimitSwitch == true

The elevator cannot move upward.

If movement would go past one of these limits, simply keep the position where it is.

Step 8 - Change States

Your elevator should continuously cycle through these states:

1. Moving Up
2. Stopped (for 100 loops)
3. Moving Down
4. Stopped (for 100 loops)
5. Moving Up

…and repeat forever.

While the elevator is moving upward:

• elevatorState should be "Moving Up".
• motorVoltage should be positive.

While moving downward:

• elevatorState should be "Moving Down".
• motorVoltage should be negative.

While stopped:

• elevatorState should be "Stopped".
• motorVoltage should be 0.

You’ll probably find that you need another variable to keep track of how many loops the elevator has been stopped.

Step 9 - Count Loops

Every iteration of the while loop should increase loopCount by one.

Step 10 - Print Diagnostic Information

Every 10th loop, print every non-constant variable to the terminal.

Your output should include:

• motorRotations
• motorVoltage
• topLimitSwitch
• bottomLimitSwitch
• elevatorState
• loopCount

A sample output might look something like this:

Loop: 120
State: Moving Up
Motor Rotations: 14.40
Motor Voltage: 6.0
Top Limit Switch: false
Bottom Limit Switch: false

Challenge Questions

1. How many loops should it take for the elevator to travel from the bottom to the top?

2. How long (in loops) does one complete cycle take?

   Bottom → Up → Stop → Down → Stop → Bottom

3. Why are the limit switches considered inputs, even though your program is setting their values?

4. In a real robot, where would the value of motorRotations come from instead of your simulation?

5. Suppose the maximum motor voltage changed from 6 V to 4 V. What would happen to the speed of the elevator without changing any other code?