Java类：将实例变量限制为几个可能的值之一，具体取决于其他实例变量

Question

I am sorry for the vague question. I am not sure what I'm looking for here.

I have a Java class, let's call it Bar . In that class is an instance variable, let's call it foo . foo is a String.

foo cannot just have any value. There is a long list of strings, and foo must be one of them.

Then, for each of those strings in the list I would like the possibility to set some extra conditions as to whether that specific foo can belong in that specific type of Bar (depending on other instance variables in that same Bar ).

What approach should I take here? Obviously, I could put the list of strings in a static class somewhere and upon calling setFoo(String s) check whether s is in that list. But that would not allow me to check for extra conditions - or I would need to put all that logic for every value of foo in the same method, which would get ugly quickly.

Is the solution to make several hundred classes for every possible value of foo and insert in each the respective (often trivial) logic to determine what types of Bar it fits? That doesn't sound right either.

What approach should I take here?

Here's a more concrete example, to make it more clear what I am looking for. Say there is a Furniture class, with a variable material , which can be lots of things, anything from mahogany to plywood. But there is another variable, upholstery , and you can make furniture containing cotton of plywood but not oak; satin furniture of oak but not walnut; other types of fabric go well with any material; et cetera.

Answer 1

I wouldn't suggest creating multiple classes/templates for such a big use case. This is very opinion based but I'll take a shot at answering as best as I can.

In such a case where your options can be numerous and you want to keep a maintainable code base, the best solution is to separate the values and the logic. I recommend that you store your foo values in a database. At the same time, keep your client code as clean and small as possible. So that it doesn't need to filter through the data to figure out which data is valid. You want to minimize dependency to data in your code . Think of it this way: tomorrow you might need to add a new material to your material list. Do you want to modify all your code for that? Or do you want to just add it to your database and everything magically works? Obviously the latter is a better option. Here is an example on how to design such a system. Of course, this can vary based on your use case or variables but it is a good guideline. The basic rule of thumb is: your code should have as little dependency to data as possible.

Let's say you want to create a Bar which has to have a certain foo . In this case, I would create a database for BARS which contains all the possible Bar s. Example:

ID NAME FOO 1 Door 1,4,10

I will also create a database FOOS which contains the details of each foo . For example:

ID NAME PROPERTY1 PROPERTY2 ... 1 Oak Brown Soft

When you create a Bar:

Bar door = new Bar(Bar.DOOR);

in the constructor you would go to the BARS table and query the foo s. Then you would query the FOOS table and load all the material and assign them to the field inside your new object.

This way whenever you create a Bar the material can be changed and loaded from DB without changing any code. You can add as many types of Bar as you can and change material properties as you goo. Your client code however doesn't change much.

You might ask why do we create a database for FOOS and refer to it's ids in the BARS table? This way, you can modify the properties of each foo as much as you want. Also you can share foo s between Bar s and vice versa but you only need to change the db once. cross referencing becomes a breeze. I hope this example explains the idea clearly.

Answer 2

You say:

Is the solution to make several hundred classes for every possible value of foo and insert in each the respective (often trivial) logic to determine what types of Bar it fits? That doesn't sound right either.

Why not have separate classes for each type of Foo? Unless you need to define new types of Foo without changing the code you can model them as plain Java classes. You can go with enums as well but it does not really give you any advantage since you still need to update the enum when adding a new type of Foo.

In any case here is type safe approach that guarantees compile time checking of your rules:

public static interface Material{}

public static interface Upholstery{}

public static class Oak implements Material{}

public static class Plywood implements Material{}

public static class Cotton implements Upholstery{}

public static class Satin implements Upholstery{}


public static class Furniture<M extends Material, U extends Upholstery>{

    private M matrerial = null;
    private U upholstery = null;

    public Furniture(M matrerial, U upholstery){
        this.matrerial = matrerial;
        this.upholstery = upholstery;
    }

    public M getMatrerial() {
        return matrerial;
    }

    public U getUpholstery() {
        return upholstery;
    }  
}


public static Furniture<Plywood, Cotton> cottonFurnitureWithPlywood(Plywood plywood, Cotton cotton){
    return new Furniture<>(plywood, cotton);
}

public static Furniture<Oak, Satin> satinFurnitureWithOak(Oak oak, Satin satin){
    return new Furniture<>(oak, satin);
} 

Answer 3

It depends on what you really want to achieve. Creating objects and passing them around will not magically solve your domain-specific problems.

If you cannot think of any real behavior to add to your objects (except the validation), then it might make more sense to just store your data and read them into memory whenever you want. Even treat rules as data.

Here is an example:

public class Furniture {
     String name;
     Material material;
     Upholstery upholstery;

     //getters, setters, other behavior

     public Furniture(String name, Material m, Upholstery u) {
         //Read rule files from memory or disk and do all the checks
         //Do not instantiate if validation does not pass
         this.name = name;
         material = m;
         upholstery = u;
     }
}

To specify rules, you will then create three plain text files (eg using csv format). File 1 will contain valid values for material, file 2 will contain valid values for upholstery, and file 3 will have a matrix format like the following:

upholstery\material  plywood mahogany oak
cotton                  1       0      1
satin                   0       1      0

to check if a material goes with an upholstery or not, just check the corresponding row and column.

Alternatively, if you have lots of data, you can opt for a database system along with an ORM. Rule tables then can be join tables and come with extra nice features a DBMS may provide (like easy checking for duplicate values). The validation table could look something like:

MaterialID UpholsteryID Compatability_Score
plywood    cotton          1
oak        satin           0

The advantage of using this approach is that you quickly get a working application and you can decide what to do as you add new behavior to your application. And even if it gets way more complex in the future (new rules, new data types, etc) you can use something like the repository pattern to keep your data and business logic decoupled.

Notes about Enums:

Although the solution suggested by @Igwe Kalu solves the specific case described in the question, it is not scalable. What if you want to find what material goes with a given upholstery (the reverse case)? You will need to create another enum which does not add anything meaningful to the program, or add complex logic to your application.

Answer 4

This is a more detailed description of the idea I threw out there in the comment:

Keep Furniture a POJO, ie, just hold the data, no behavior or rules implemented in it.

Implement the rules in separate classes, something along the lines of:

interface FurnitureRule {
    void validate(Furniture furniture) throws FurnitureRuleException;
}

class ValidMaterialRule implements FurnitureRule {
    // this you can load in whatever way suitable in your architecture -
    // from enums, DB, an XML file, a JSON file, or inject via Spring, etc.
    private Set<String> validMaterialNames;

    @Overload
    void validate(Furniture furniture) throws FurnitureRuleException {
        if (!validMaterialNames.contains(furniture.getMaterial()))
            throws new FurnitureRuleException("Invalid material " + furniture.getMaterial());
    }
}

class UpholsteryRule implements FurnitureRule {
    // Again however suitable to implement/config this
    private Map<String, Set<String>> validMaterialsPerUpholstery;

    @Overload
    void validate(Furniture furniture) throws FurnitureRuleException {
        Set<String> validMaterialNames = validMaterialsPerUpholstery.get(furniture.getUpholstery();
        if (validMaterialNames != null && !validMaterialNames.contains(furniture.getMaterial()))
            throws new FurnitureRuleException("Invalid material " + furniture.getMaterial() + " for upholstery " + furniture.getUpholstery());
    }
}

// and more complex rules if you need to

Then have some service along the lines of FurnitureManager . It's the "gatekeeper" for all Furniture creation/updates:

class FurnitureManager {
    // configure these via e.g. Spring.
    private List<FurnitureRule> rules;

    public void updateFurniture(Furniture furniture) throws FurnitureRuleException {
        rules.forEach(rule -> rule.validate(furniture))
        // proceed to persist `furniture` in the database or whatever else you do with a valid piece of furniture.
    }
}

Answer 5

material should be of type Enum.

public enum Material {
   MAHOGANY,
   TEAK,
   OAK,
   ...

}

Furthermore you can have a validator for Furniture that contains the logic which types of Furniture make sense, and then call that validator in every method that can change the material or upholstery variable (typically only your setters).

public class Furniture {

  private Material material;
  private Upholstery upholstery; //Could also be String depending on your needs of course

  public void setMaterial(Material material) {
     if (FurnitureValidator.isValidCombination(material, this.upholstery)) {
        this.material = material;
     }
  }

  ...

  private static class FurnitureValidator {
     private static boolean isValidCombination(Material material, Upholstery upholstery) {
        switch(material) {
           case MAHOGANY: return upholstery != Upholstery.COTTON;
                break;
           //and so on
        }

     }   
  }
}

Answer 6

Probably the approach I'd use (because it involves the least amount of code and it's reasonably fast) is to "flatten" the hierarchical logic into a one-dimensional Set of allowed value combinations. Then when setting one of the fields, validate that the proposed new combination is valid. I'd probably just use a Set of concatenated Strings for simplicity. For the example you give above, something like this:

class Furniture {
  private String wood;
  private String upholstery;

  /**
   * Set of all acceptable values, with each combination as a String.
   * Example value: "plywood:cotton"
   */
  private static final Set<String> allowed = new HashSet<>();

  /**
   * Load allowed values in initializer.
   *
   * TODO: load allowed values from DB or config file
   * instead of hard-wiring.
   */
  static {
    allowed.add("plywood:cotton");
    ...
  }

  public void setWood(String wood) {
    if (!allowed.contains(wood + ":" + this.upholstery)) {
      throw new IllegalArgumentException("bad combination of materials!");
    }
    this.wood = wood;
  }

  public void setUpholstery(String upholstery) {
    if (!allowed.contains(this.wood + ":" + upholstery)) {
      throw new IllegalArgumentException("bad combination of materials!");
    }
    this.upholstery = upholstery;
  }

  public void setMaterials(String wood, String upholstery) {
    if (!allowed.contains(wood + ":" + upholstery)) {
      throw new IllegalArgumentException("bad combination of materials!");
    }
    this.wood = wood;
    this.upholstery = upholstery;
  }

  // getters
  ...
}

The disadvantage of this approach compared to other answers is that there is no compile-time type checking. For example, if you try to set the wood to plywoo instead of plywood you won't know about your error until runtime. In practice this disadvantage is negligible since presumably the options will be chosen by a user through a UI (or through some other means), so you won't know what they are until runtime anyway. Plus the big advantage is that the code will never have to be changed so long as you're willing to maintain a list of allowed combinations externally. As someone with 30 years of development experience, take my word for it that this approach is far more maintainable.

With the above code, you'll need to use setMaterials before using setWood or setUpholstery , since the other field will still be null and therefore not an allowed combination. You can initialize the class's fields with default materials to avoid this if you want.

Answer 7

We often are oblivious of the power inherent in enum types. The Java™ Tutorials clearly states "you should use enum types any time you need to represent a fixed set of constants."

How do you simply make the best of enum in resolving the challenge you presented? - Here goes:

public enum Material {
    MAHOGANY( "satin", "velvet" ),
    PLYWOOD( "leather" ),
    // possibly many other materials and their matching fabrics...
    OAK( "some other fabric - 0" ),
    WALNUT( "some other fabric - 0", "some other fabric - 1" );

    private final String[] listOfSuitingFabrics;

    Material( String... fabrics ) {
        this.listOfSuitingFabrics = fabrics;
    }

    String[] getListOfSuitingFabrics() {
        return Arrays.copyOf( listOfSuitingFabrics );
    }

    public String toString() {
        return name().substring( 0, 1 ) + name().substring( 1 );
    }
}

Let's test it:

public class TestMaterial {

    for ( Material material : Material.values() ) {
        System.out.println( material.toString() + " go well with " + material.getListOfSuitingFabrics() );
    }
}