Customizing Interface

The ArbitraryBuilder API is also valid within the interface. You can customize the interface properties regardless of the interface type as said in Generating Interface Type The interface type refers to interface, generic interface ,selaed interface.

public interface StringSupplier {
	String getValue();
}

FixtureMonkey fixture = FixtureMonkey.create();

String result = fixture.giveMeBuilder(StringSupplier.class)
	.set("value", "fix")
	.sample()
	.getValue();

The properties of the interface in ArbitraryBuilder differ in the implementation. Unfortunately, there is currently no ArbitraryBuilder API that resolves the implementation of the interface. Unless the interface has only one implementation, you can customize the properties of the interface, not the implementation.

You cannot generate the randomly populated intended implementation, but you can generate the fixed implementation by using the set API. There are two cases, with the interfaceImplements option and without the interfaceImplements option.

With the option #

The interface type is resolved to the implementation when you use the set API with the option.

public interface ObjectValueSupplier {
	Object getValue();
}

public class StringValueSupplier implements ObjectValueSupplier {
	private final String value;

	@ConstructorProperties("value") // It is not needed if you are using Lombok.
	public StringValueSupplier(String value) {
		this.value = value;
	}

	@Override
	public String getValue() {
		return value;
	}
}

public class IntegerValueSupplier implements ObjectValueSupplier {
	private final int value;
	private final int implementationValue;

	@ConstructorProperties({"value", "implementationValue"}) // It is not needed if you are using Lombok.
	public IntegerValueSupplier(int value, int implementationValue) {
		this.value = value;
		this.implementationValue = implementationValue;
	}

	@Override
	public Integer getValue() {
		return value;
	}

	public int getImplementationValue() {
		return implementationValue;
	}
}

FixtureMonkey fixture = FixtureMonkey.builder()
	.objectIntrospector(
		ConstructorPropertiesArbitraryIntrospector.INSTANCE) // used for instantiate implementations of ObjectValueSupplier
	.plugin(
		new InterfacePlugin()
			.interfaceImplements(
				ObjectValueSupplier.class,
				List.of(StringValueSupplier.class, IntegerValueSupplier.class)
            )
	)
	.build();

IntegerValueSupplier integerValueSupplier = (IntegerValueSupplier)fixture.giveMeBuilder(ObjectValueSupplier.class)
	.set("$", new IntegerValueSupplier(-1203))
	.sample();

You can also change the properties of the implementation.

IntegerValueSupplier integerValueSupplier = (IntegerValueSupplier)fixture.giveMeBuilder(ObjectValueSupplier.class)
	.set("$", new IntegerValueSupplier(-1203, 1203))
	.set("implementationValue", 1) // it works.
	.sample();

Without the option #

You have to use the set API in a specific way with Values.just.

public interface ObjectValueSupplier {
	Object getValue();
}

public class StringValueSupplier implements ObjectValueSupplier {
	private final String value;

	@ConstructorProperties("value") // It is not needed if you are using Lombok.
	public StringValueSupplier(String value) {
		this.value = value;
	}

	@Override
	public String getValue() {
		return value;
	}
}

public class IntegerValueSupplier implements ObjectValueSupplier {
	private final int value;

	@ConstructorProperties("value") // It is not needed if you are using Lombok.
	public IntegerValueSupplier(int value) {
		this.value = value;
	}

	@Override
	public Integer getValue() {
		return value;
	}
}

FixtureMonkey fixture = FixtureMonkey.builder()
	.objectIntrospector(ConstructorPropertiesArbitraryIntrospector.INSTANCE) // used for instantiate implementations of ObjectValueSupplier
	.build();

IntegerValueSupplier integerValueSupplier = (IntegerValueSupplier)fixture.giveMeBuilder(ObjectValueSupplier.class)
	.set("$", Values.just(new IntegerValueSupplier(-1203)))
	.sample();

The implementation is not used by interfaceImplements, Fixture Monkey does not know the properties of the implementation. You cannot set the properties of the implementation.

IntegerValueSupplier integerValueSupplier = (IntegerValueSupplier)fixture.giveMeBuilder(ObjectValueSupplier.class)
    .set("$", Values.just(new IntegerValueSupplier(-1203)))
    .set("value", 1) // not works
    .sample();