Java software design [closed]

Question

Scenario:

Consider the following abstract class MeasurementType and its subtypes Acceleration and Density:

public abstract class MeasurementType {
  protected String name;
  protected String[] units;

  public MeasurementType() {}

  public MeasurementType(String name) {
    this.name = name;
  }

  protected void setName(String name) {
    this.name = name;
  }

  protected String getName() {
    return name;
  }

  protected void setUnits(String[] values) {
    this.units = values;
  }
}

public class Acceleration extends MeasurementType {
  private String name;

  public Acceleration () {
    super();
  }

  public Acceleration(String name) {
    super();
  }

}

public class Density extends MeasurementType {
  private String name;

  public Density() {
    super();
  }

  public Density(String name) {
    super();
  }

}

Question:

What is the need for the subtypes are since there isn't anything in those classes that differentiates them apart enough (at least in my implementations) to give them the right to exist as their own entities, but for the sake of the example, lets say they had their own arrays containing units for each distinct type, so density would have kilograms, grams etc.. and acceleration would have mph, kmph etc..

Finally I have this run class which I quickly made up:

public class run {

  public run() {}

  public static MeasurementType testType() {
    Acceleration a = new Acceleration("meters per second");
    return (MeasurementType) a;
  }

  public static void main(String[] args) {
    MeasurementType mt = testType();
    System.out.println(mt.getClass().toString());
    System.out.println(mt.getClass().getName());

    String type = "acceleration";
    String[] units = new String[8];

    Object mType;
    if(type.equals("acceleration")) {
      mType = new Acceleration();
    } else if(type.equals("density")) {
      mType = new Density();
    } else {
      mType = new Object();
    }

    System.out.println(mType.getName());
  }
}

This is where i encountered the problem that I was thinking about. I was using strings to determine the type that I would instantiate, but the problem is, by default right at the end of the conditional statements I instantiate Object because otherwise the compiler would complain that the mType variable wouldn't have been initialized. I can't create a MeasurementType object since it is an abstract class.

I guess this is more of a design question, but is there a better way of determining the class to instantiate? Please assume that the String type = "acceleration" came from a drop down or somewhere from some user interface, I have just hard coded it in because it isn't really necessary for the question.

Could someone shed some light on this design fault as I see it please.

Answer 1

Looking at the crux of your question, what to do when the string is neither "acceleration" or "density".

if(type.equals("acceleration")) {
  mType = new Acceleration();
} else if(type.equals("density")) {
  mType = new Density();
} else {
  mType = new Object();
}

Is this a sign that something in your program has broken, that it is irretrievable wrong? In which case; throw an exception.

if(type.equals("acceleration")) {
  mType = new Acceleration();
} else if(type.equals("density")) {
  mType = new Density();
} else {
  throw new RuntimeException("variable type is not a valid type, valid types are 'acceleration' and 'density'";
}

This is a realisation you will come to; exceptions are your friend. They mean when the program is wrong it screams where and why and then you fix it. On the other hand quietly ignoring the problem leads to subtle bugs and strange behaviour that can take days to diagnose.

On the other hand is this a sign of the user entering inappropriate input? In that case, validate the input and if its invalid tell the end user and ask for it again.

Using Strings and if statements like this is likely not to be what you want to do at all, but under the assumption it is what you want to do then this is the way forward (a switch statement would be slightly nicer).