Why do static classes crash my robot?

Question

I have made the static class below so any class may access any of my lejos robot's sensor methods without me having to make an instance for each class.

However whenever I call a method such as StandardRobot.motorA.setPower(100) my robot crashes. When I use exactly the same class and make a local instance of it this works fine. Why is this? Both times my code compiles fine and fails at runtime.

import lejos.nxt.*;

public class StandardRobot {

    public static ColorSensor colourSensor;
    public static TouchSensor touchSensor;
    public static UltrasonicSensor ultrasonicSensor;
    public static NXTMotor motorA, motorB;

    public StandardRobot() {
        // instantiate sensors
        ultrasonicSensor = new UltrasonicSensor(SensorPort.S1);
        colourSensor = new ColorSensor(SensorPort.S2);
        touchSensor = new TouchSensor(SensorPort.S4);

        //instantiate motors
        motorA = new NXTMotor(MotorPort.A);
        motorB = new NXTMotor(MotorPort.B);
    }
}

Answer 1

You're trying to create a utility class, but your variable initialization takes place in a constructor.

Constructors are only called when an instance is... constructed (via new).

You need to initialize the static properties statically, either in a static initialization block, or as they're declared.

// Initialize static properties as they're declared.
public static ColorSensor colourSensor = new ColorSensor(SensorPort.S2);

// Or initialize in a static initialization block to do them all at once.
public static TouchSensor touchSensor;
// ... and the others.
static {
    touchSensor = new TouchSensor(SensorPort.S4);
    // ... and the others.
}

Answer 2

Because when you don't call the constructor StandardRobot you're not instantiating motorA, motorB, ultrasonicSensor, etc so they default so null, leading to NullPointerExceptions during runtime. You could either make all those fields instance variables or consider using a static initialization block, i.e

   static {
 // instantiate sensors
    ultrasonicSensor = new UltrasonicSensor(SensorPort.S1);
    colourSensor = new ColorSensor(SensorPort.S2);
    touchSensor = new TouchSensor(SensorPort.S4);

    //instantiate motors
    motorA = new NXTMotor(MotorPort.A);
    motorB = new NXTMotor(MotorPort.B);
}

Answer 3

Static variables are defined for an class, not for an instance. The constructor you define is called for an instance, not the class. In result your variables might not be initialized.

On a related node: the idea to make the variables static is not very good. You limit yourself to having one robot only, as all robots would share state.

Answer 4

replace your constructor by a static section:

static {
        // instantiate sensors
        ultrasonicSensor = new UltrasonicSensor(SensorPort.S1);
        colourSensor = new ColorSensor(SensorPort.S2);
        touchSensor = new TouchSensor(SensorPort.S4);

        //instantiate motors
        motorA = new NXTMotor(MotorPort.A);
        motorB = new NXTMotor(MotorPort.B);
}