如何避免if语句并调用多个构造函数

Question

I would like to ask how to handle with multiple constructors.

if(a != null && b != null)
    return new QueryProducer(query, a, b);
else if(a != null)
    return new QueryProducer(query, a);
else if(b != null)
    return new QueryProducer(query, b);
else return new QueryProducer(query);

I would like to avoid complex if else blocks. Scalability is also not very good in this case.

Answer 1

How about using builder pattern here? See this link from Javacodegeeks for code example

QueryProducer.withQuery(yourQuery).withA(a).withB(b).build();

Answer 2

Create a builder that builds a QueryProducer based on the parameters passed. You can put all the logic for creating the object in one place. The other thing that comes to my mind is passing all three parameters to the constructor, and handling it there, but I almost always avoid putting any logic in the constructor because of the exceptions that can occur.

Answer 3

As others have answered, if you have a situation in which any of the following are true, then you should strongly consider using the Builder Pattern (using a helper object -- usually defined by a static nested class in your object's class -- to construct the object you want):

• Do I have 4 or more parameters for my constructor?
• Am I overloading constructors?
• Does my constructor have parameters that specify data that is optional?
• Do I expect to add more parameters to my constructor at a future date?
• Do I want the readability of the JavaBean pattern while still being able to construct immutable objects?