JarCastingA tiny framework that makes it easy to write Test Data Builders in Java
Test Data Builders are described in the book Growing Object-Oriented Software, Guided by Tests by Steve Freeman and Nat Pryce. This library lets you write Test Data Builders with much less duplication and boilerplate code than the approach described in the book.
Download
You can download from Maven Central with the artifact coordinates:
com.natpryce:make-it-easy:4.0.0
Example
Consider the following class hierarchy. This hierarchy illustrates a couple of complicating factors: there is an abstract base class and there is a property (Fruit.ripeness) that is not set via the constructor but by an operation of the Fruit class.
public abstract class Fruit {
private double ripeness = 0.0;
public void ripen(double amount) {
ripeness = Math.min(1.0, ripeness+amount);
}
public boolean isRipe() {
return ripeness == 1.0;
}
}
public class Apple extends Fruit {
private int leaves;
public Apple(int leaves) {
this.leaves = leaves;
}
public int numberOfLeaves() {
return leaves;
}
}
public class Banana extends Fruit {
public final double curve;
public Banana(double curve) {
this.curve = curve;
}
public double curve() {
return curve;
}
}
You can define Test Data Builders for Apples and Bananas with Make It Easy as follows:
public class FruitMakers {
public static final Property<Fruit,Double> ripeness = newProperty();
public static final Property<Apple, Integer> leaves = newProperty();
public static final Property<Banana,Double> curve = newProperty();
public static final Instantiator<Apple> Apple = new Instantiator<Apple>() {
@Override public Apple instantiate(PropertyLookup<Apple> lookup) {
Apple apple = new Apple(lookup.valueOf(leaves, 2));
apple.ripen(lookup.valueOf(ripeness, 0.0));
return apple;
}
};
public static final Instantiator<Banana> Banana = new Instantiator<Banana>() {
@Override public Banana instantiate(PropertyLookup<Banana> lookup) {
Banana banana = new Banana(lookup.valueOf(curve, 0.5));
banana.ripen(lookup.valueOf(ripeness, 0.0));
return banana;
}
};
}
And use them like this:
Maker<Apple> appleWith2Leaves = an(Apple, with(2, leaves));
Maker<Apple> ripeApple = appleWith2Leaves.but(with(ripeness, 0.9));
Maker<Apple> unripeApple = appleWith2Leaves.but(with(ripeness, 0.125));
Apple apple1 = make(ripeApple);
Apple apple2 = make(unripeApple);
Banana defaultBanana = make(a(Banana));
Banana straightBanana = make(a(Banana, with(curve, 0.0)));
Banana squishyBanana = make(a(Banana, with(ripeness, 1.0)));
In contrast, doing so in the style documented in Growing Object-Oriented Software, Guided by Tests would look like this:
public interface Builder<T> {
T build();
}
public class AppleBuilder implements Builder<Apple> {
private double ripeness = 0.0;
private int leaves = 1;
private AppleBuilder() {}
public static AppleBuilder anApple() {
return new AppleBuilder();
}
public Apple build() {
Apple apple = new Apple(leaves);
apple.ripen(ripeness);
return apple;
}
public AppleBuilder withRipeness(double ripeness){
this.ripeness = ripeness;
return this;
}
public AppleBuilder withLeaves(int leaves) {
this.leaves = leaves;
return this;
}
public AppleBuilder but() {
return new AppleBuilder()
.withRipeness(ripeness)
.withLeaves(leaves);
}
}
public class BananaBuilder implements Builder<Banana> {
private double ripeness = 0.0;
private double curve = 0.5;
private BananaBuilder() {}
public static BananaBuilder aBanana() {
return new BananaBuilder();
}
public Banana build() {
Banana apple = new Banana(curve);
apple.ripen(ripeness);
return apple;
}
public BananaBuilder withRipeness(double ripeness){
this.ripeness = ripeness;
return this;
}
public BananaBuilder withCurve(double curve) {
this.curve = curve;
return this;
}
public BananaBuilder but() {
return new BananaBuilder()
.withRipeness(ripeness)
.withCurve(curve);
}
}
And be used like this:
AppleBuilder appleWith2Leaves = anApple().withLeaves(2);
AppleBuilder ripeApple = appleWith2Leaves.but().withRipeness(0.9);
AppleBuilder unripeApple = appleWith2Leaves.but().withRipeness(0.125);
Apple apple1 = ripeApple.build();
Apple apple2 = unripeApple.build();
Banana defaultBanana = aBanana().build();
Banana straightBanana = aBanana().withCurve(0.0).build();
Banana squishyBanana = aBanana().withRipeness(1.0).build();
As you can see, with Make It Easy you have to write a lot less duplicated and boilerplate code. What duplication there is - in the declaration of anonymous Instantiator classes, for example - can be automatically inserted and refactored by modern IDEs. (You could also factor out calls to Fruit.ripen to a private helper method, but I left them duplicated for clarity.)
The full code for this example is in the Make It Easy repository.