A library providing an expressive syntax for defining Java variable values from multiple potential inputs, some of which may be null. It offers a succinct replacement for the following difficult-to-read boilerplate patterns:

void init(@Nullable String prop) {
  this.prop = prop;
  if (this.prop == null) {
    this.prop = lookupPropFromDb(); // slow
  }
  if (this.prop == null) {
    this.prop = DEFAULT_PROP_VALUE;
  }
  if (this.prop == null) {
    throw new IllegalStateException();
  }
}

...or the "simpler" variant:

String str = firstVal != null ? firstVal : secondVal != null ? secondVal : null;

(the latter example is especially problematic when, for example, firstVal is actually myObj.getFirstVal(): one must either assign it to a variable beforehand, or trust that getFirstVal() is a fast method that will return the same value when called multiple times)

In the absence of first-class support for this pattern in the language, this class offers an awkward but relatively elegant solution:

void init(@Nullable String prop) {
  Var<String> var = Var.var();
  this.prop = var.def(
    var.as(prop)
    || var.as(lookupPropFromDb())
    || var.as(System.getenv("PROP")));
}

In the above example, lookupPropFromDb() is only called if prop is null. Also, if all three potential values are null, it will throw an IllegalStateException to indicate that the variable is undefined.

Var<String> var = Var.var();
String str = var.def(var.as(firstVal) || var.as(secondVal) || var.asNull());

The call to var.asNull() allows the variable to contain an explicitly null value, without triggering an IllegalStateException.