A basic tank drive OpMode

MyFirstOpMode.java

package org.firstinspires.ftc.teamcode;

import com.qualcomm.robotcore.eventloop.opmode.OpMode;
import com.qualcomm.robotcore.eventloop.opmode.TeleOp;
import com.qualcomm.robotcore.hardware.DcMotor;
import com.qualcomm.robotcore.hardware.DcMotorSimple;

@TeleOp
public class MyFirstOpMode extends OpMode {

    /*
     * Declare the four motor objects that we will be controlling throughout this OpMode. They are
     * instance variables (a.k.a. data fields) so that _all_ methods can access them and they will
     * retain their values between method calls.
     */
    private DcMotor
            leftFront,
            leftRear,
            rightFront,
            rightRear;

    /*
     * Initialize the motor objects to the values already stored in the Robot Controller app
     * configuration on the phone, so that our DcMotor objects refer to the actual motors connected
     * physically to the robot controller phone.
     */
    @Override
    public void init() {
        leftFront = hardwareMap.get(DcMotor.class, "Left Front");
        leftRear = hardwareMap.get(DcMotor.class, "Left Rear");
        rightFront = hardwareMap.get(DcMotor.class, "Right Front");
        rightRear = hardwareMap.get(DcMotor.class, "Right Rear");

        /*
         * Remember that forward and reverse refer to the motor's rotational direction (generally
         * a motor turning forwards will rotate clockwise as you stare end-on at the output shaft,
         * but it's _always_ a good idea to test that the wiring polarity didn't get reversed!)
         *
         * This means that for the motors to move the _robot_ forward, the motors on the left side
         * will need to rotate in _their_ reverse direction to be pushing in the same direction as
         * the motors on the right side of the robot.
         */
        leftFront.setDirection(DcMotorSimple.Direction.REVERSE);
        leftRear.setDirection(DcMotorSimple.Direction.REVERSE);
    }

    @Override
    public void loop() {
        /*
         * We read the current position of the left and right joysticks' Y-axis as a value -1 to 1.
         * We think of 1 as being "100% pushed forward" and -1 as "100% pushed backwards" -- 0 is
         * when the joystick is centered.
         */
        double
                leftPower = gamepad1.left_stick_y,
                rightPower = gamepad1.right_stick_y;

        /*
         * We translate the joystick positions directly into motor power, since the motor is
         * expecting a power value between -1 and 1, where 1 is "100% power forward" and -1 is
         * "100% power in reverse".
         *
         * Note that, because we reversed the motor directions for the left-side motors in our
         * init() method above, we don't have to worry about that here: the left-side motors are
         * treating all of our instructions as "opposite day" already.
         */
        leftFront.setPower(leftPower);
        leftRear.setPower(leftPower);
        rightFront.setPower(rightPower);
        rightRear.setPower(rightPower);
    }
}