创建采用布尔值isX并根据isY需要布尔值的构造函数的最佳实践是什么？

Question

In this hypothetical problem I'm creating a class to represent Sausage which can be: A. Fresh and Packaged B. Fresh and Not Packaged C. Not Fresh and Not Packaged

Note: It can't be Not Fresh and Packaged.

I'm looking for the best practice to create constructor with this in mind.

The way I tried is below, but I think there should be a better solutions.

public class Sausage {
    Meat meat;
    boolean isFresh;
    boolean isPackaged;

    public Sausage(Meat meat, Boolean isFresh, Boolean isPackaged) {
        this.meat = meat;
        if (!isFresh) {
            this.isFresh = false;
            this.isPackaged = false;
        }
        else if (isPackaged) {
            this.isFresh = true;
            this.isPackaged = true;
        }
        else {
            this.isFresh = true;
            this.isPackaged = false;
        }
    }
}

I'm looking for a cleaner way to provide this functionality.

Answer 1

You can represent the possible states as an enumeration type:

enum SausageType {
    FreshPackaged,
    FreshUnpackaged,
    NotFresh;
}

Then change your constructor to take a SausageType instead of two Boolean s.

The advantage of this method is that it is not possible to call the constructor with the wrong combination of states, it is obvious to users of your class what the possible values are, and you do not need to remember to keep brittle runtime checking of arguments up to date with changing requirements.

Answer 2

There are few problems with your current approach:

1. There is a way to create write new object which makes no sense eg new Sausage(..., false, true) where packaged will be silently converted to false . This is confusing and makes code harder to understand.

2. You are mixing Boolean object type and boolean primitive type which will result in unnecessary auto-boxing. If you don't need to support null use boolean .

3. boolean fields probably don't need is prefix.

Since there are two boolean flags and 4 possibilities they can be set ( true true , true false , false true , false false ) but only 3 valid options perhaps named factory methods would be more suitable:

public class Sausage {

    public static Sasuage newFreshPackaged(Meat meat) {
      return new Sasuage(meat, true, true);
    }

    public static Sasuage newFreshNotPackaged(Meat meat) {
      return new Sasuage(meat, true, false);
    }

    public static Sasuage newNotFreshNotPackaged(Meat meat) {
      return new Sasuage(meat, false, false);
    }

    private Sausage(Meat meat, boolean fresh, boolean packaged) {
        this.meat = meat;
        this.fresh = fresh;
        this.packaged = packaged;
    }
}

Answer 3

Silently changing isPackaged to false may be a surprise to calling code. Here, you may choose to throw an exception if the combination (not fresh, packaged) is chosen.

if (!isFresh && isPackaged) {
    throw new IllegalArgumentException("Can't be both not fresh and packaged!");
}
this.isFresh = isFresh;
this.isPackaged = isPackaged;

You may also decide to use factory methods (making your constructor private ) that enforce your requirements. This would avoid needing an exception to be thrown.

public static Sausage createFreshSausage(Meat meat, boolean isPackaged) {
    return new Sausage(meat, true, isPackaged);
}

public static Sausage createNotFreshSausage(Meat meat) {
    return new Sausage(meat, false, false);
}

Incidentally, normally the primitive boolean would be used in the constructor's parameters' types, not the object wrapper Boolean , like your instance variables. A null value here that would be allowed by Boolean doesn't make any sense.