Arrays

What are Arrays?

An array contains a limited amount of numbered spots (indices) for values. The length (or size) of an array is the amount of these spots - how many values you can place in the array. The values in an array are called elements. Arrays can store primitive types (int, double, boolean) or objects (String, custom classes).

Creating Arrays - Basic Types:

// Arrays of primitive types
int[] sensorReadings = new int[5];     // Array for 5 integers
double[] coordinates = new double[6];  // Array for 6 coordinates
boolean[] motorStatus = new boolean[4]; // Array for 4 boolean values

// Initialize with values
int[] powerLevels = {25, 50, 75, 100};
double[] targetPositions = {12.5, 24.0, 36.5};

// Access elements using index (starts at 0)
sensorReadings[0] = 100;  // First element
sensorReadings[4] = 95;   // Last element (index 4 for array of size 5)

System.out.println("Array length: " + sensorReadings.length);
System.out.println("First reading: " + sensorReadings[0]);

Arrays of Objects

Just like primitive types, arrays can store objects. String arrays are a common example, but you can create arrays of any object type, including custom classes. This is where arrays become really powerful for robotics applications.

String Arrays Example:

// Create an array of strings
String[] teamMembers = new String[4];

// Add names to the array
teamMembers[0] = "Alice";
teamMembers[1] = "Bob";
teamMembers[2] = "Charlie";
teamMembers[3] = "Diana";

// Or initialize directly
String[] robotSystems = {"Drive", "Arm", "Intake", "Launcher"};

// Iterate through the array
System.out.println("Team Members:");
for (int i = 0; i < teamMembers.length; i++) {
    System.out.println((i + 1) + ". " + teamMembers[i]);
}

// Enhanced for loop (for-each)
System.out.println("\nRobot Systems:");
for (String system : robotSystems) {
    System.out.println("- " + system);
}

Adding Objects to Arrays

Objects can be added to arrays just like primitive values. You can create objects first and then add them, or create objects directly when assigning to array positions. Let's use a Robot class example.

Robot Objects in Arrays:

// Assume we have a Robot class
public class Robot {
    private String name;
    private int teamNumber;
    private boolean isActive;
    
    public Robot(String name, int teamNumber) {
        this.name = name;
        this.teamNumber = teamNumber;
        this.isActive = false;
    }
    
    public String getName() { return this.name; }
    public int getTeamNumber() { return this.teamNumber; }
    public boolean isActive() { return this.isActive; }
    public void activate() { this.isActive = true; }
    
    public String toString() {
        return name + " (Team " + teamNumber + ") - " + (isActive ? "Active" : "Inactive");
    }
}

// Create an array for robot objects
Robot[] competition = new Robot[4];

// Method 1: Create robot first, then add to array
Robot robot1 = new Robot("SpeedBot", 12345);
competition[0] = robot1;

// Method 2: Create robot directly in array assignment
competition[1] = new Robot("PowerBot", 67890);
competition[2] = new Robot("PrecisionBot", 11111);
competition[3] = new Robot("AllStarBot", 22222);

// Activate all robots and display
for (int i = 0; i < competition.length; i++) {
    competition[i].activate();
    System.out.println(competition[i]);
}

User Input for Object Arrays

You can collect user input to populate arrays with objects. This is useful for creating team rosters, robot configurations, or competition data from user input.

Building Object Arrays from User Input:

import java.util.Scanner;

public class RobotTeamBuilder {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.print("How many robots in your team? ");
        int teamSize = Integer.valueOf(scanner.nextLine());
        
        // Create array based on user input
        Robot[] robotTeam = new Robot[teamSize];
        
        // Collect robot information
        for (int i = 0; i < robotTeam.length; i++) {
            System.out.println("\nEntering Robot " + (i + 1) + ":");
            
            System.out.print("Robot name: ");
            String name = scanner.nextLine();
            
            if (name.isEmpty()) {
                System.out.println("Using default name: Robot" + (i + 1));
                name = "Robot" + (i + 1);
            }
            
            System.out.print("Team number: ");
            int teamNumber = Integer.valueOf(scanner.nextLine());
            
            // Create and add robot to array
            robotTeam[i] = new Robot(name, teamNumber);
        }
        
        // Display the complete team
        System.out.println("\n=== Robot Team Roster ===");
        System.out.println("Total robots: " + robotTeam.length);
        
        for (int i = 0; i < robotTeam.length; i++) {
            System.out.println((i + 1) + ". " + robotTeam[i]);
        }
    }
}

Object Array Best Practices:

Guidelines for Working with Object Arrays:

Null Checks: Always check if array elements are null before using them
Bounds Checking: Validate array indices before access
Initialization: Remember that object arrays start with null values
Enhanced Loops: Use for-each loops when you don't need the index
Array Length: Use .length property (not .length() method) for arrays
Meaningful Names: Use descriptive names for arrays and their elements

Try It Yourself:

Exercise 1: Complete Object Array System

Create a Competition class and build an array of competitions
Implement user input system for adding competitions with multiple parameters
Create filtering system to find competitions by criteria

Build a comprehensive FRC competition management system using object arrays:

Open full interactive app