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Q: I have a class with six instance variables, each of which could be initialize

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Q: I have a class with six instance variables, each of which could be initialized or not. Should I write 64 constructors?

Answer: Of course you don't need (26) constructors. Let's say you have a class C defined as follows:

public class C { int a,b,c,d,e,f; }

Here are some things you can do for constructors:

  1. Guess at what combinations of variables will likely be wanted, and provide constructors for those combinations. Pro: That's how it's usually done. Con: Difficult to guess correctly; lots of redundant code to write.

  2. Define setters that can be cascaded because they return this. That is, define a setter for each instance variable, then use them after a call to the default constructor:

    public C setA(int val) { a = val; return this; }
    ...
    new C().setA(1).setC(3).setE(5);

    Pro: This is a reasonably simple and efficient approach. A similar idea is discussed by Bjarne Stroustrop on page 156 of The Design and Evolution of C++. Con: You need to write all the little setters, they aren't JavaBean-compliant (since they return this, not void), they don't work if there are interactions between two values.

  3. Use the default constructor for an anonymous sub-class with a non-static initializer:

    new C() {{ a = 1; c = 3; e = 5; }}

    Pro: Very concise; no mess with setters. Con: The instance variables can't be private, you have the overhead of a sub-class, your object won't actually have C as its class (although it will still be an instanceof C), it only works if you have accessible instance variables, and many people, including experienced Java programmers, won't understand it. Actually, its quite simple: You are defining a new, unnamed (anonymous) subclass of C, with no new methods or variables, but with an initialization block that initializes a, c, and e. Along with defining this class, you are also making an instance. When I showed this to Guy Steele, he said "heh, heh! That's pretty cute, all right, but I'm not sure I would advocate widespread use..." As usual, Guy is right. (By the way, you can also use this to create and initialize a vector. You know how great it is to create and initialize, say, a String array with new String[] {"one", "two", "three"}. Now with inner classes you can do the same thing for a vector, where previously you thought you'd have to use assignement statements: new Vector(3) {{add("one"); add("two"); add("three")}}.)

  4. You can switch to a language that directly supports this idiom.. For example, C++ has optional arguments. So you can do this:

    class C {
    public: C(int a=1, int b=2, int c=3, int d=4, int e=5);
    }
    ...
    new C(10); // Construct an instance with defaults for b,c,d,e

    Common Lisp and Python have keyword arguments as well as optional arguments, so you can do this:

    C(a=10, c=30, e=50)            # Construct an instance; use defaults for b and d.


This tip is reprinted on JavaFAQ.nu by by courtesy of Peter Norvig I am thankful for his important contributions to my site - 21 Infrequently Answered Java Questions. Alexandre Patchine


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