Lesson 1 - Introduction to Java

Raider Robotix and Java

FRC Team 25, Raider Robotix, uses the Java programming language for programming both the robot and the scouting system. Java is also taught in NBTHS's computer science courses.

These lessons will provide you with the building blocks of Java that are necessary to program, maintain, and understand the scouting system. It currently has two components:

• Android scouting app - collects data from scouts during events and exports it in JSON (JavaScript Object Notation) files
  • Every year, this needs to be updated with the correct metrics, events, game manual, and other options, as necessary
• Desktop scouting client - compiles the JSON files from the app and outputs spreadsheets of each scouting entry and aggregated statistics for each team
  • Every year, this needs to be updated with the correct metrics, which includes averages, percentages, probabilities, and standard deviations, in addition to those in the scouting app

Starting from the 2017 season, Tableau is used to visualize and analyze the data from the spreadsheets generated by the client.

The goal is to have you understand how the scouting system works and be comfortable with changing its code by next season's kickoff. If you want to work on robot programming instead, additional knowledge of WPILib API will be needed.

Lesson Structure

Generally, several concepts will be explained and grouped together to form a lesson. As each concept is taught, several practice exercises are given for you to check your understanding. There are solutions provided with the exercises, but please don't check them until you've fully attempted each exercise.

At the end of the lesson, one or two problems will be assigned on HackerRank that test your understanding of all concepts in the lesson. The site allows you to check your program against several test cases and will "grade" it (pass or fail) instantly.

Java Code Structure

Java source files have a .java extension and each contain a class, which acts like an object (we'll get to this later). Each class/source file is located in a package, which acts as a folder to organize various files. Inside an executable program is a main method, in which the code inside it executes. In a desktop Java program, the method signature public static void main(String[] args) is used, followed by a set of curly braces. Below is an example inside a file called Example.java:

//Place import statements here
public class Example { 
    public static void main(String[] args) {
        //Write your code to execute here
    }
}

As seen above line comments can be written if they are proceeded with two forward slashes (//). Anything following the two slashes will be ignored by the compiler when your program executes. Block comments may also be written like this:

/* Hi!
 * I'm a block comment.
 */

Note the two slashes showing the beginning/end of the comment and the asterisk on each line.

Variables

Any data or value you use in Java is stored in a variable. A variable can have one of many different datatypes. The following are common datatypes:

• int: an integer with a value between $-2^{31} - 1$ and $2^{31} -1$ (123, 25, -900, 0, etc.)
• double: a positive or negative decimal number (1.23, -2.5, 1.0005, etc.)
• boolean: has either a true or false value
• String: a sequence of characters, enclosed in a set of quotation marks ("Hello World", "1+1=2", "Raider Robotix", etc.)

You can declare variables with its datatype, followed by a space and its identifier, then a semicolon. The identifier must start with a letter or underscore, then be followed by more letters, underscores, or numbers, with no spaces. Valid identifiers include:

• _t
• motor1
• MOTOR_SPEED

Generally, variables are named in a camelCase notation, where the first word is lowercase, then the remaining words are uppercase. Examples of variable declarations with camelCase naming include:

int teleOpGearsCollected;
double avgKpaPerGame;
boolean baselineCrossed;
//multiple variables can be declared with a comma separating them
String robotComment, pilotComment; 

On the other hand, variables that are constant throughout a program should be named with only uppercase letters and underscores. Examples of these variables include:

double TURN_ANGLE_TOLERANCE;
int LEFT_JOYSTICK_PORT;
String RULES_FILEPATH;
int VERSION_NUMBER;

Keep in mind that variables should not have generic identifiers like x or var1, but ones that are descriptive.

A variable can be assigned a value with its identifier followed by the assignment operator, =, and its value. This value can be literal or that of another variable. A variable may be assigned a value and declared at the same time as well. Some examples:

int teleOpGearsCollected; // declaration
double avgHighGoalsTeleOp = 12.555; // variable declared and value of 12.555 assigned
teleOpGearsCollected = 4; // assignment afterwards
boolean baselineCrossed = true, readyTakeoff = false; // two variables declared and assigned value
String robotComment = "This is a good robot";
double avgTeleOpKpa = avgHighGoalsTeleOp/3; //value dependent on another variable

Output

Output to the console in Java can be executed with the following statement:

System.out.println(<some value>);    

This calls the println method on a value, where <some value> can be a literal value or a variable. Remember to terminate the print statement with a semicolon. For example, executing

System.out.println(teleOpGearsCollected);
System.out.println("gears collected in tele-op in match");
System.out.println(25);

will result in the following output:

4
gears collected in tele-op in match
25

If you don't want a new line after the output, the System.out.print(<some value>) method may be used. Executing

System.out.print(teleOpGearsCollected);
System.out.println(" gears collected in tele-op.");
System.out.println("Robots are cool");

will result in the following output:

4 gears collected in tele-op.
Robots are cool

An IDE

An IDE (Integrated Development Environment) will be used to program in Java. It allows you to write code, execute, and debug it in one application. I recommend using Eclipse, as NBTHS and the WPILib API uses it, though alternatives like InteliJ and BlueJ exist. We will be using Android Studio as our IDE for Android development.

To get started with Eclipse:

1. Download and extract the ZIP file from the link above
2. Run the executable and choose your workspace (default should work)
3. Go to File > New > Java Project and name your project
4. Close the Welcome window
5. Right click your project and go to New > Class, and name your class (without the .java extension)
6. Write your main method (see above)
7. Try replacing the comment with a statement to print Hello World
8. Press the green "Play" button to execute your program

If there are any syntax errors in your program, Eclipse will underline them in red, and the program will not run. Try fixing them by reading the error message and reviewing your code or by using Google.

A Brief Exercise

Assign the values of true, "Raider Robotix", and 25 to variables. Use print statements with the variables and literal values to generate the output

Hello World!
It is true that I am on Team 25, Raider Robotix
25 is the Team Number.

Click here for solution

public class PrintingExercise {

  public static void main(String[] args) {

    boolean onTeam = true;
    String teamNickname = "Raider Robotix";
    int teamNum = 25;

    System.out.println("Hello World!");
    System.out.print("It is ");
    System.out.print(onTeam);
    System.out.print(" that I am on Team ");
    System.out.print(teamNum);
    System.out.print(", ");
    System.out.println(teamNickname);
    System.out.print(teamNum);
    System.out.println(" is the Team Number");
  }

}

Input

To take user input, you will need to use a Scanner object. First, you will need to import the class by placing the following import statement before the class starts (see example above):

import java.util.Scanner;

The Scanner object can be declared and constructed like this to read from the system console:

Scanner myScanner = new Scanner(System.in);

The datatype is Scanner, and myScanner can be any valid identifier. Note the use of the new keyword to create a new object and the parentheses following.

Several methods can be accessed through the Scanner object for different datatypes. A method essentially acts like a function and returns a value to the main program. These may be used to let users assign values to variables:

int anInteger = myScanner.nextInt();
String aLine = myScanner.nextLine(); // takes the next whole line

Essentially, the dot operator (.) shows that the methods nextInt() and nextLine() are written in the Scanner class, and you are accessing them from an outside class. More methods for input can be found by reading the documentation for the class. If the wrong datatype is used, there will be an error in your program.

By default, spaces and line breaks separating tokens in the input will result two different variables. For example, if the input is

Raider Robotix

executing

String teamName = myScanner.next();
System.out.println(teamName);

will output

Raider

while executing

String teamName = myScanner.nextLine();
System.out.println(teamName);

will output

Raider Robotix

because a whitespace separates Raider and Robotix in the input.

Exercise

Write a program that allows prompts a user to input a string, an integer, and a floating point number, then print all three once they are inputted.

Click here for a solution

import java.util.Scanner;

public class VariableInput{

  public system void public static void main(String[] args) {

    int anInt;
    String aString;
    double aDouble;

    Scanner scanner = new Scanner(System.in);
    System.out.print("Enter an integer: ");
    anInt = scanner.nextInt();
    System.out.print("Enter a string: ");
    aString = scanner.nextLine();
    System.out.print("Enter a floating point number: ");
    aDouble = scanner.nextDouble();

    System.out.println(anInt);
    System.out.println(aString);
    System.out.println(aDouble);
  }
}

Operators

Operators in Java allow you to interact between different datatypes.

For these examples, we will declare

int a = 5, b = 4;
double c = 2.5;
String s = "Hello";

Mathematical

Evaluating these operators will result in a number of type int if both are integers, or of type double if at least one is a double:

• +: adds the number (an int or double) to the left and right of it
  • a+b is equal to 9, a+c is equal to 7.5
• -: subtracts the number on the right from the left
  • b-a is equal to -1, b-c is equal to 1.5
• *: multiplies the number to the left and right of it
  • a*b is equal to 20, b*c is equal to 10.0
• /: divides the number on the left by the number on the right. However, if both numbers are ints, integer division will occur. That is, the remainder is discarded, and only the quotient remains.
  • a/c is equal to 2.0, b/c is equal to 1.6
  • a/b is equal to 1, b/a is equal to 0
  • A literal int value (1) may be converted into a double value by adding a decimal point and a zero (1.0)
• %: the modulus operator, a % b returns the remainder of a / b
  • a % b is equal to 1, b % a is equal to 4, a % c is equal to 0.0

Exercises:

• Write a program that allows the user to input (integer) dimensions of a rectangular prism and outputs its surface area and volume
• Write a program that prints the day of the week of a day, given the day of the week of the first day of the month (where 1=Monday, 7=Sunday)
• Write a program that calculates the score of a match in FIRST Steamworks, where the user inputs the number of rotors activated during tele-op/autonomous, high/low goals in tele-op/autonomous, the times the baseline was crossed, the times robots took off, and foul points that were incurred by the opposing alliance

Rectangular prism solution

import java.util.Scanner;

public class RectangularPrismExercise {

  public static void main(String[] args) {

    Scanner sc = new Scanner (System.in);

    int length, height, width;

    System.out.print("Enter height: ");
    height = sc.nextInt();

    System.out.print("Enter length: ");
    length = sc.nextInt();

    System.out.print("Enter width: ");
    width = sc.nextInt();

    System.out.println("Surface area: ");
    System.out.println(2*(width*height+width*length+length*height));

    System.out.println("Volume: " + (height*width*length));
  }

}

Day of the Week Solution

import java.util.Scanner;

public class DayWeekExercise {

  public static void main(String[] args) {

    Scanner sc = new Scanner (System.in);

    int desiredDay, firstDay;

    System.out.print("Day of the week of the first day of the month (1=Monday, 7=Sunday): ");
    firstDay = sc.nextInt();
    System.out.print("Day you want the day of the week for: ");
    desiredDay = sc.nextInt();

    System.out.print("The day of the week of Day ");
    System.out.print(desiredDay);
    System.out.print(" of the month is ");
    System.out.print((desiredDay+firstDay-2)%7+1);
  }

}

Steamworks Score Solution

import java.util.Scanner;

public class SteamworksExercise {

  public static void main(String[] args) {

    Scanner sc = new Scanner (System.in);

    int autoRotors, teleRotors, autoHigh, autoLow, 
      teleHigh, teleLow, baselineCrossCount, takeoffCount, foulPoints;

    int AUTO_ROTOR = 60, TELE_ROTOR = 40, BASELINE = 5, TAKEOFF=40;

    System.out.print("Autonomous rotors: ");
    autoRotors = sc.nextInt();

    System.out.print("Tele-op rotors: ");
    teleRotors = sc.nextInt();

    System.out.print("Autonomous high goals: ");
    autoHigh = sc.nextInt();

    System.out.print("Autonomous low goals: ");
    autoLow = sc.nextInt();

    System.out.print("Tele-op high goals: ");
    teleHigh = sc.nextInt();

    System.out.print("Tele-op low goals: ");
    teleLow = sc.nextInt();

    System.out.print("Baseline cross count: ");
    baselineCrossCount = sc.nextInt();

    System.out.print("Takeoff count: ");
    takeoffCount = sc.nextInt();

    System.out.print("Foul points: ");
    foulPoints = sc.nextInt();

    int score = AUTO_ROTOR*autoRotors+teleRotors*TELE_ROTOR
      +BASELINE*baselineCrossCount+TAKEOFF*takeoffCount+foulPoints
      +autoHigh+autoLow/3+teleHigh/3+teleLow/9;

    System.out.print("Score: " + score);

  }

}

Relational

Evaluating these operators will result in a boolean value (true or false):

• <: less than, <= - less than or equal to
  • b<a is true, 4.5 < 0 is false
  • -2 <= c is true, 5<=5 is true
• >: greater than, >= - greater than or equal to
• ==: checks for equality. This can only be used for primitive datatypes, and not objects
  • a==5 is true, 2.0==a/c is true
  • false == true is false
  • For strings, the method someString.equals(/*another string*/) is used for equality because a string is an object. We'll get into this more in the next lesson.
  • s.equals("Hello") is true, s.equals("World") is false
• !=: checks if the two objects are not equal
  • a!=5 is false, 1!=a/c is true
  • false != true is true

Exercise

Write a program to determine if it is true that a quadratic equation of the form $0=ax^2+bx+c$ has a real solution, where the user inputs $a$, $b$, and $c$.

(Hint: For now the best way to square an integer x is x*x, as x^2 will not evaluate correctly)

Click for a solution

import java.util.Scanner;

public class QuadraticSolutionExistsExercise {

  public static void main(String[] args) {

    Scanner sc = new Scanner (System.in);

    int a,b,c;

    System.out.print("Enter a: ");
    a = sc.nextInt();

    System.out.print("Enter b: ");
    b = sc.nextInt();

    System.out.print("Enter c: ");
    c = sc.nextInt();

    boolean hasSolution = b*b-4*a*c>=0

      System.out.print("It is " + hasSolution + "that the equation 0=");
      System.out.print(a + "*x^2 + " + b + "*x + " + c + " has a solution");
  }

}

Logical

These operate on boolean values and evaluating them will also result in a boolean value:

OR Operator

Written as a || b and evaluates to true only if either a or b are true

a	b	a || b
false	false	false
false	true	true
true	false	true
true	true	true

AND Operator

Written as a && b and evaluates to trueonly if both a and b are true

a	b	a && b
false	false	false
false	true	false
true	false	false
true	true	true

NOT Operator

Written as !a and evaluates to true only if a is false

a	!a
false	true
true	false

Exercises

Evaluate the following expressions (without actually writing any code) for when boolean a = true, b = false and for when boolean a = true, b = true:

• (a || b) && (!a || !b)
• !(a && !b) || (!a && b)
• !(a && b)

Click for the answers

Expression/Condition	a = true, b = false	a = true, b = true
(a || b) && (!a || !b)	true	false
!(a && !b) || (!a && b)	false	true
!(a && b)	true	false

Some Shortcuts

You already learned about the assignment operator. Here are some shortcuts used for assignment with operators, if x and y are numbers and both are already declared and assigned values:

x += y; // same as x = x+y
x -= y; // same as x = x-y
x /= y; // same as x = x/y
x %= y; // same as x = x % y
x++; // same as x+=1
x--; // same as x-=1

Exercise: What is the value of x after the following is executed?

int x = 60, y = 10;
y /= 5;
x %= x -y;
x++;
x *= 4;

Click for the answer

int x = 60, y = 10;
y /= 5; // x = 60, y = 2
x %= x -y; // x = 2, y = 2
x++; // x = 3, y = 2
x *= 4; // x = 12, y = 2

The final value of x is 12.

Boolean Expressions

A boolean expression is any expression that evaluates to true or false. It may contain literals, variables, and any of the above operators. The order of operations for evaluation are:

1. Parentheses
2. NOT
3. Multiplication, division, modulus
4. Addition, subtraction
5. Greater/less than (or equal to)
6. Equivalence/not equivalent
7. AND
8. OR

The following are valid boolean expressions if x and y are numbers and b is a boolean:

• x >= 5 + y && !(y + 5 == 7 || y < 0 || !b)
• takeoffPercentage / avgTakeoffPercentage + 0.1 >= 1.2 && (avgKpa >= 30 || avgGears >= 4) && teamNum !=25 && !droppedGearOften

Exercises

• The XOR (exclusive-or) operator evaluates to be true if either A or B are true, but not both. Write an expression that is logically equivalent to "A XOR B" using the AND, OR, and NOT operators
• Write a boolean expression to determine if at least 3 ranking points were earned by the red alliance in a match of FIRST Steamworks, given the scores of both the red and blue alliances, the number of gears delivered by the red alliance, whether or not the reserve gear was used, and the number of high/low goals into the red boiler in autonomous/tele-op. Use appropriate identifiers for variables and declare multiple, intermediate boolean expressions as necessary.

XOR Solution

Below is a table of the XOR operator:

a	b	a XOR b
false	false	false
false	true	true
true	false	true
true	true	false

Using this, we can write

boolean XOR = (!a && b) || (a && !b);

Steamworks ranking point solution

The following variables are given:

int redAllianceScore, blueAllianceScore;
int redGears;
int redHighGoalsAuto, redLowGoalsAuto;
int redHighGoalsTele, redLowGoalsTele;
boolean reserveGearUsed;

2 ranking points are earned for a win, 1 point for a tie, 1 point for scoring 40 kPa or more, and 1 point for the activation of four rotors. Thus, three or more points are achieved under the following conditions:

boolean threePlusRP = (win && (kpaReached || fourRotors)) || (tie && kpaReached && fourRotors);

These are the conditions for the ranking points:

boolean win = redAllianceScore > blueAllianceScore;
boolean tie = redAllianceScore == blueAllianceScore;
boolean fourRotors = (redGears >= 12 && reserveGearUsed) || redGears >= 13;
boolean kpaReached = (redHighGoalsAuto+(redHighGoalsTele+redLowGoalsAuto)/3.0+redLowGoalsTele/9.0)>=40;

Conditionals

A conditional statement is one that executes if a particular boolean expression is true.

The if, else, and else-if statements are written and used as follows:

boolean a = <some expression>;
if (a) {
    //Code here executes if expression a is true
}
else if (<some boolean expression b>) {
    // Code here executes if expression b is true and a is false
}
else{
   // Code here executes if a and b are both false
}

Note that an if statement may be written without any else statements, but else and else-if statements must follow an if statement or an else statement. The boolean expression may also be placed inside the parentheses, rather than assigning its value to a boolean variable.

These conditional statements may also be nested inside each other as follows:

boolean a = <some expression>;
boolean b = <some expression>;
boolean c = <some expression>;
if (a) {
    if (c) {
        // Code here executes if a and c are true
    }
    else{
      //Code here executes if a is true and c is false
    }
    // Code here executes as long as a is true, no matter what the value of c is
}
else if (b) {
    // Code here executes if expression b is true and a is false, no matter what the value of c is
    if (!c) {
      //Code here executes if a is false, b is true, and c is false
    }
}
else{
   // Code here executes if a and b are both false
}

Exercises

• Write a program to determine if a number is even or odd
• Write a program that prints the month, given its numerical value (1 to 12)
• Write a program that converts a day of the week to a number (Monday = 1, Sunday = 7)
• Write a program that converts feet to yards and vice-versa, depending on what the user wants

Odd or Even Solution

import java.util.Scanner;

public class OddEven {

  public static void public static void main(String[] args) {

    System.out.print("Enter a number: ");

    Scanner sc = new Scanner(System.in);

    int num;
    num = sc.nextInt();

    boolean isEven = num % 2 ==  0

    System.out.print(num + " is ");

    if (isEven) {
      System.out.println("even.");
    } 
    else System.out.println("odd.");
  }

}

Number Month Converter Solution

import java.util.Scanner;

public class NumberMonthxConverter {

  public static void public static void main(String[] args) {

    System.out.print("Enter a month: ");

    Scanner sc = new Scanner(System.in);

    String day;
    day = sc.next();

    int num;

    if (day.equals("Monday"))
      num = 1;
    else if (day.equals("Tuesday"))
      num = 2;
    else if (day.equals("Wednesday"))
      num = 3;
    else if (day.equals("Thursday"))
      num = 4;
    else if (day.equals("Friday"))
      num = 5;
    else if (day.equals("Saturday"))
      num = 6;
    else if (day.equals("Sunday"))
      num = 7;

    System.out.print(num);

  }

}

Day Number Converter Solution

import java.util.Scanner;

public class DayNumberConverter {

  public static void public static void main(String[] args) {

    System.out.print("Enter a month: ");

    Scanner sc = new Scanner(System.in);

    String day;
    day = sc.next();

    int num;

    if (day.equals("Monday"))
      num = 1;
    else if (day.equals("Tuesday"))
      num = 2;
    else if (day.equals("Wednesday"))
      num = 3;
    else if (day.equals("Thursday"))
      num = 4;
    else if (day.equals("Friday"))
      num = 5;
    else if (day.equals("Saturday"))
      num = 6;
    else if (day.equals("Sunday"))
      num = 7;

    System.out.print(num);

  }

}

Feet-Yard Converter Solution

import java.util.Scanner;

public class FeetYardConverter {

  public static void public static void main(String[] args) {

    System.out.println("1) Feet to Yards");
    System.out.println("2) Yards to Feet");
    System.out.println("Enter your choice (1 or 2): ");

    Scanner sc = new Scanner(System.in);

    int choice;
    choice = sc.nextInt();

    if (choice==1) {
      System.out.print("Enter the number of feet: ");
      int feet = sc.nextInt();
      System.out.println(feet + " feet is " + (feet/3.0) + " yards");
    }
    else{
      System.out.print("Enter the number of feet: ");
      int yards = sc.nextInt();
      System.out.println(yards + " yards is " + (yards*3) + " feet");
    }
  }

}

Assignment

Download Eclipse and complete the exercises above as necessary to check your understanding. Afterwards, sign up for a HackerRank account and complete the "A School Day" program here.

Click here to read the problem statement on this site.

I suggest that you write and test the program on Eclipse, then check if it works on HackerRank.