Multiple optional arguments in constructor in Java - exponential number of constructors

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This is something I've encountered a few times and haven't been able to find a satisfying answer yet. This seems pretty stupid, but after googling this for a while I couldn't come up with something good.

Let's say I have a class with 20 instance variables, each of which is optional (will be initialized or not).

Now I want my constructor(s) to handle all the cases, in case of a few instance variables it's fine and I can just create constructors with different signatures, but here I have 20, so I would need 2^20=1,048,576 constructors to handle all the cases ! That seems ... not very optimal, don't you agree?

So since with this brute force approach I basically have to construct 2^n constructors, where n is the number of instance variables, I want to find a better way to do it.

I've seen a couple solutions for this problem, but I believe they all on assumptions on the data, but in my case each of these variables can be initialized or not at random, I have no way of knowing that before initialization.

I'm looking for some design patterns or ideas that I could apply to make my code a bit more ... maintainable (no don't worry I didn't create 1M+ constructors :)

Thanks.

Try the Builder pattern:

http://rwhansen.blogspot.com/2007/07/theres-builder-pattern-that-joshua.html

Optional Parameters in Java: Common Strategies and Approaches, When working with optional parameters, method overloading is one of the more make it a good choice for use cases with a small number of optional parameters. We start by defining our class with a private constructor but then In a sense, we're splitting the construction of the object over multiple calls. Consider using this if the number of optional parameters is small and if the risk of callers supplying parameters in the wrong order is minimal. Static factory methods Joshua Bloch, in his book – Effective Java, recommends in Item 1, to “…consider static factory methods instead of constructors.”

Combine the Bean pattern with a Builder. YourObject will not have inconsistent state halfway through its construction, but it will still take 20+ set operations and you can make YourObject immutable (if you want) - but not YourObjectBuilder.

public class YourObject {
    private Type field1;
    private Type field2;
    private Type field3;
    ...
    YourObject ( Type field1 , Type field2 , Type field3 ... ) { }
}

public class YourObjectBuilder {
    private Type field1;
    private Type field2;
    private Type field3;
    ...

    public YourObjectBuilder() {
    }

    public YourObject make ( ) {
         return new YourObject ( field1 , field2 , field3 ... ) ;
    }

    public void setField1(Type t) {
        field1 = t;
    }

   public void setField2(Type t) {
        field2 = t;
   }

    public void setField3(Type t) {
        field3 = t;
    }
    ...
}

ECOOP 2012 -- Object-Oriented Programming: 26th European , In most languages (for instance, C++ [48], C♯ [28], Delphi [2], Java [27], and Visual Basic [1]), the initialization Each constructor consists of: (i) a list of parameters; (ii) a body. an exponential number of constructors with respect to the number of optional parameters. Multiple Initialization Options and Code Duplication. Constructor with one argument - String : Shikhar Constructor with two arguments - String and Integer : Dharmesh 26 Constructor with one argument - Long : 325614567 How constructors are different from methods in Java? Constructor(s) must have the same name as the class within which it defined while it is not necessary for the method in java.

public class YourObject {
    private Type field1;
    private Type field2;
    private Type field3;
    ...

    public YourObject() {
    }

    public void setField1(Type t) {
        field1 = t;
    }

   public void setField2(Type t) {
        field2 = t;
   }

    public void setField3(Type t) {
        field3 = t;
    }
    ...
}

Java optional parameters in practice, We'll focus on regular method, class constructors with optional fields, and You can tackle Java optional parameters in method in several different ways. and many of them are optional parameters, applying constructor  If you have many optional arguments, you can avoid having a very confusing set of combinations of constructors. Heck, even with just 2 optional arguments (unrelated to each other), if you wanted separate, overloaded constructors, you'd have to have 4 constructors (version without any, version with each, and version with both).

Groovy Language Documentation, Method definition; Named arguments; Default arguments; Varargs 2.5. groovydoc, the Groovy & Java documentation generator Export all Constructors with "*"; Export Constructors using Parameter Descriptor; Export Constructor with exponent letter, followed by an optional sign, and a integral number  The Java class above contains two constructors. The first constructor is a no-arg constructor, meaning it takes no parameters (no arguments). The second constructor takes an int parameter. Inside the constructor body the int parameter value is assigned to a field,

Functions: infix, vararg, tailrec, fun powerOf(number: Int, exponent: Int) { /**/ } When overriding a method with default parameter values, the default parameter On the JVM: the named argument syntax cannot be used when calling Java functions because Java bytecode  Multiple constructors A class can have multiple constructors that assign the fields in different ways. Sometimes it's beneficial to specify every aspect of an object's data by assigning parameters to the fields, but other times it might be appropriate to define only one or a few.

Java Constructor: Overloaded, Default, This Constructors, And with multiple constructors, we vary the arguments passed to create classes. Two files. Test: In the Test constructor, we receive one integer parameter. When a Java class contains multiple constructors, we say that the constructor is overloaded (comes in multiple versions). Java constructor overloading example: public class MyClass {private int number = 0; public MyClass() {} public MyClass(int theNumber) {this.number = theNumber;}} The Java class above contains two constructors. The first constructor is a no-arg constructor, meaning it takes no parameters (no arguments). The second constructor takes an int parameter

Comments
  • bean pattern. Constructor has no args, all params go in through setters.
  • that seems silly, I said I have 20 instance variables but in reality I have more, I don't want to call 20 different setters ! It could be in inconsistent state partway through its execution, this seems like a very bad solution.
  • And this also forbids me to make my class immutable.
  • That sounds like a nice solution and way better than the Bean pattern, thanks !
  • As I said in comment above, the problem with this kind or approach is that my object can be in inconsistent state partway through its construction ! That's very dangerous in my case and I can't really afford to do that. And you have to make you class mutable, which is something I don't want to.
  • @linker The object will be in an inconsistent state partway through its construction only if you pass it to some other method before you finish creating it. If you need to worry about mutability, you could create a flag to determine if the fields are settable, and toggle it when you're done creating your object.
  • I was more talking in a multi-threaded context. You're right I could ensure the object is not accessed before it is done building, but that requires some extra effort to ensure thread-safety.