Programming Concepts in Java

A program is a precise list of instructions. The robot does exactly what you write, in order, one line at a time, many times per second. It never guesses what you meant. That sounds strict, but it is also good news: when the robot does something strange, there is always a reason in the code, and you can find it. In FTC you will write programs in Java (with OnBot Java or Android Studio) or build the same ideas visually with Blocks. The concepts in this lesson are the same in all three.

Variables: named boxes for values

A variable is a named box that stores a value so you can use it later. In Java every variable has a type that says what kind of value it can hold. You create a variable by writing the type, a name, and usually a starting value. You can change the value later with =, which means 'store this value', not 'is equal to'.

1int loopCount = 0;              // whole number
2double drivePower = 0.5;        // decimal number
3boolean clawOpen = false;       // true or false
4String status = "Ready";        // text
5final double MAX_POWER = 0.8;   // final means it can never change
6
7loopCount = loopCount + 1;      // now 1
8drivePower = drivePower * 0.5;  // now 0.25

Conditionals: making decisions

A conditional lets the program choose what to do. An if statement checks a condition that is either true or false. If it is true, the code inside the braces runs; if not, Java checks the next else if, and finally runs the else part. You can combine conditions with && (and), || (or) and ! (not). Comparisons use >, <, >=, <=, == and !=.

1double trigger = gamepad1.right_trigger;   // 0.0 (released) to 1.0 (fully pressed)
2
3if (trigger > 0.5) {
4    status = "Fast";
5} else if (trigger > 0.1) {
6    status = "Slow";
7} else {
8    status = "Stopped";
9}
10
11if (gamepad1.a && !clawOpen) {   // A is pressed AND the claw is not open
12    clawOpen = true;
13}

Loops: doing things again and again

A loop repeats code. A for loop is best when you know how many times to repeat. A while loop repeats as long as a condition stays true. Every TeleOp program you write this week is built around one big while loop: read the gamepad, set the motors, show telemetry, then do it all again, dozens of times per second. That is why the robot reacts instantly when the driver moves a stick.

1// for loop: repeat a known number of times
2for (int i = 1; i <= 3; i++) {
3    telemetry.addData("Count", i);
4}
5
6// while loop: repeat while the condition is true
7while (opModeIsActive()) {
8    // read the gamepad, set motor powers, update telemetry
9}

Methods: naming a piece of work

A method is a named block of code you can run whenever you need it. It can take inputs called parameters and can send back a result with return. Methods keep programs short and readable: instead of copying the same five lines in three places, you write them once and call the method. You already use methods written by others: setPower, setPosition and telemetry.addData are all methods. The dot in leftFront.setPower(0.5) means 'call the setPower method of the leftFront object'. An object is a thing in code, such as a motor, that has its own data and methods.

1// A method that returns a slower power when slow mode is on
2double scalePower(double power, boolean slowMode) {
3    if (slowMode) {
4        return power * 0.4;
5    }
6    return power;
7}
8
9// Calling the method
10double leftPower = scalePower(-gamepad1.left_stick_y, gamepad1.left_bumper);

• Variables store values: pick the right type and a clear name.
• Conditionals choose between actions with if, else if and else.
• Loops repeat actions; your TeleOp runs inside while (opModeIsActive()).
• Methods give a name to a piece of work so you can reuse it.