javaimmutability

Java: pseudo-setter method for immutable classes

Let's say I have a class Foo in Java that has immutable data:

class Foo {
    final private int x;
    public int getX() { return this.x; }
    final private OtherStuff otherstuff;
    public Foo(int x, OtherStuff otherstuff) { 
       this.x = x;
       this.otherstuff = otherstuff;
    }   
    // lots of other stuff...
}

Now I'd like to add a utility method that creates a "sibling" value with identical state but with a new value of x. I could call it setX():

class Foo
{
    ...
    Foo setX(int newX) { return new Foo(newX, this.otherstuff); }
    ...
}

but the semantics of setX() are different than the standard setter convention for mutable bean objects, so somehow this doesn't feel right.

What's the best name for this method?

Should I call it withX() or newX() or something else?

edit: additional priority in my case: I have scripting clients (through JSR-223 and an object model I export) that can easily obtain a Foo object. It's cumbersome, however, to call constructors or create builders or whatever. So it's desirable for me to provide this method as a convenience for scripting clients.

Solution

  • withX(...)

    This is the de facto standard naming convention for immutable setters. This is for example the default name for setters generated by the Immutables framework. Here's an example:

    Foo newFoo = foo.withX(1047);

    There is a @Value.Style option to change this pattern, but the option itself is called with="...", which emphasizes what the default convention is.

    Being the most widespread convention, it's easy to find examples of this. Guava and the Java time package being two.

    Just x(...)

    Another approach is to not have a prefix at all. You see this in for example builders generated by the Immutables framework:

    Foo foo = ImmutableFoo.builder()
                      .x(1047)
                      .y("Hello World")
                      .build();

    If you use this approach directly on the immutable class (that is, no builder involved) you'd typically have it as an overload to the getter:

    Foo newFoo = foo.x(5);  // setter - one argument
int x = newFoo.x();     // getter - no arguments

    This convention is used in for example the Java Spark framework.

    setX(...)

    Some APIs use the same naming convention as for setters in mutable classes. This has the obvious drawback that it can be surprising when you're new to a code base. Working with BigInteger and writing…

    bigInt.setBit(2);

    …would for example be a mistake, since the returned object is discarded. With this naming pattern you have to get used to writing

    BigInteger newBigInt = bigInt.setBit(2);

    deriveX(...)

    To highlight the fact that the new value is derived from the existing object, you could use deriveX(...). The immutable Font class in the Java API follows this pattern. If you want to create a new font with, for example, a specific size you use

    Font newFont = font.deriveFont(newSize);

    The Font class has been around since the beginning of time. This convention is not very common as of today.

    Immutable object being an operand

    When the immutable object is itself an operand to the transformation it's not really a setter in the traditional sense, and there's no need to have a prefix for the method. For example…

    BigDecimal newBigDec = bigDec.multiply(BigDecimal.TEN);

    …has the same signature as a setter, but multiply is clearly a better method name than any other alternative.

    Same with String.substring, Path.resolve, etc.