Add an ObjectGenerator<>, and related machinery (also a mapOf generator)

18 files changed, 567 insertions, 100 deletions

diff --git a/src/main/java/au/id/zancanaro/javacheck/DataSourceHelper.java b/src/main/java/au/id/zancanaro/javacheck/DataSourceHelper.java
new file mode 100644
index 0000000..522fd26
--- /dev/null
+++ b/src/main/java/au/id/zancanaro/javacheck/DataSourceHelper.java
@@ -0,0 +1,109 @@
+package au.id.zancanaro.javacheck;
+
+import au.id.zancanaro.javacheck.annotations.DataSource;
+
+import java.lang.reflect.Field;
+import java.lang.reflect.Modifier;
+import java.lang.reflect.ParameterizedType;
+import java.lang.reflect.Type;
+import java.util.*;
+
+public class DataSourceHelper {
+    private final Class<?> classObject;
+    private final Map<Type, Generator<?>> generators;
+
+    public DataSourceHelper(Class<?> classObject) {
+        this.classObject = classObject;
+        this.generators = new HashMap<>();
+    }
+
+    public static Set<Type> validateGeneratorFields(Class<?> classObject, List<Throwable> errors) {
+        Set<Type> result = new HashSet<>();
+
+        for (Field field : classObject.getDeclaredFields()) {
+            if (field.isAnnotationPresent(DataSource.class)) {
+                boolean error = false;
+                if (!Modifier.isStatic(field.getModifiers())) {
+                    errors.add(new Error("@DataSource field " + field.getName() + " must be static"));
+                    error = true;
+                }
+                if (!Modifier.isPublic(field.getModifiers())) {
+                    errors.add(new Error("@DataSource field " + field.getName() + " must be public"));
+                    error = true;
+                }
+
+                Type type = field.getGenericType();
+                ParameterizedType parameterizedType;
+                if (type instanceof ParameterizedType) {
+                    parameterizedType = (ParameterizedType) type;
+                    if (parameterizedType.getRawType() instanceof Class) {
+                        Class<?> c = (Class) parameterizedType.getRawType();
+                        if (c == Generator.class) {
+                            if (!error) {
+                                result.add(parameterizedType.getActualTypeArguments()[0]);
+                            }
+                        } else {
+                            errors.add(new Error("@DataSource fields must be of type Generator<T>"));
+                        }
+                    } else {
+                        errors.add(new Error("@DataSource fields must be of type Generator<T>"));
+                    }
+                } else {
+                    errors.add(new Error("@DataSource fields must be of type Generator<T>"));
+                }
+            }
+        }
+
+        return result;
+    }
+
+
+    private static final Map<Type, Type> rawTypes;
+
+    static {
+        Map<Type, Type> types = new HashMap<>();
+        types.put(Double.class, Double.TYPE);
+        types.put(Float.class, Float.TYPE);
+        types.put(Long.class, Long.TYPE);
+        types.put(Integer.class, Integer.TYPE);
+        types.put(Short.class, Short.TYPE);
+        types.put(Byte.class, Byte.TYPE);
+        types.put(Character.class, Character.TYPE);
+        types.put(Boolean.class, Boolean.TYPE);
+        rawTypes = Collections.unmodifiableMap(types);
+    }
+
+    public Map<Type, Generator<?>> computeGenerators() {
+        if (generators.isEmpty()) {
+            for (Field field : classObject.getDeclaredFields()) {
+                if (!field.isAnnotationPresent(DataSource.class)) {
+                    continue;
+                }
+                Type type = field.getGenericType();
+                if (!(type instanceof ParameterizedType)) {
+                    continue;
+                }
+                ParameterizedType parameterizedType = (ParameterizedType) type;
+                if (!(parameterizedType.getRawType() instanceof Class)) {
+                    continue;
+                }
+                Class<?> c = (Class) parameterizedType.getRawType();
+                if (c != Generator.class) {
+                    continue;
+                }
+                try {
+                    Type target = parameterizedType.getActualTypeArguments()[0];
+                    @SuppressWarnings("unchecked")
+                    Generator<Object> generator = (Generator<Object>) field.get(null);
+                    generators.put(target, generator);
+                    if (rawTypes.containsKey(target)) {
+                        generators.put(rawTypes.get(target), generator);
+                    }
+                } catch (IllegalAccessException ex) {
+                    throw new RuntimeException(ex);
+                }
+            }
+        }
+        return generators;
+    }
+}
diff --git a/src/main/java/au/id/zancanaro/javacheck/Generator.java b/src/main/java/au/id/zancanaro/javacheck/Generator.java
index 24c23ca..ec7a16e 100644
--- a/src/main/java/au/id/zancanaro/javacheck/Generator.java
+++ b/src/main/java/au/id/zancanaro/javacheck/Generator.java
@@ -1,6 +1,5 @@
 package au.id.zancanaro.javacheck;
 
-import java.util.Iterator;
 import java.util.List;
 import java.util.Random;
 import java.util.function.Function;
diff --git a/src/main/java/au/id/zancanaro/javacheck/Generators.java b/src/main/java/au/id/zancanaro/javacheck/Generators.java
index f4ac025..63d4ece 100644
--- a/src/main/java/au/id/zancanaro/javacheck/Generators.java
+++ b/src/main/java/au/id/zancanaro/javacheck/Generators.java
@@ -1,8 +1,12 @@
 package au.id.zancanaro.javacheck;
 
+import au.id.zancanaro.javacheck.object.GeneratorProvider;
+import au.id.zancanaro.javacheck.object.ObjectGenerator;
+
 import java.util.*;
 import java.util.function.Consumer;
 import java.util.function.Function;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 import java.util.stream.StreamSupport;
 
@@ -75,6 +79,7 @@ public final class Generators {
     private static ShrinkStrategy<Long> longShrinkStrategy(final long bound) {
         return value -> StreamSupport.stream(new Spliterators.AbstractSpliterator<Long>(Long.MAX_VALUE, Spliterator.ORDERED) {
             long curr = value - bound;
+
             @Override
             public boolean tryAdvance(Consumer<? super Long> action) {
                 if (curr == 0) {
@@ -173,6 +178,22 @@ public final class Generators {
         };
     }
 
+    @SuppressWarnings("unchecked")
+    public static <K, V> Generator<Map<K, V>> mapOf(Generator<K> keyGen, Generator<V> valueGen) {
+        return (random, size) -> {
+            Generator<Integer> countGen = sized(s -> integer(0, s));
+            int count = countGen.generate(random, size).getValue();
+            return Generator.list(count, Generator.tuple(keyGen, valueGen))
+                    .generate(random, size)
+                    .map(pairs -> pairs.stream()
+                            .collect(Collectors.toMap(
+                                    pair -> (K) pair.get(0),
+                                    pair -> (V) pair.get(1),
+                                    (first, second) -> second)))
+                    .map(Collections::unmodifiableMap);
+        };
+    }
+
     public static Generator<Character> character() {
         return integer(0, 256).map(i -> (char) i.intValue());
     }
@@ -216,4 +237,12 @@ public final class Generators {
             return String.valueOf(chars);
         });
     }
+
+    public static <T> Generator<T> ofType(Class<T> type) {
+        return new ObjectGenerator<>(type);
+    }
+
+    public static <T> Generator<T> ofType(Class<T> type, GeneratorProvider provider) {
+        return new ObjectGenerator<>(type, provider);
+    }
 }
diff --git a/src/main/java/au/id/zancanaro/javacheck/junit/Properties.java b/src/main/java/au/id/zancanaro/javacheck/junit/Properties.java
index 9d50af0..94032bc 100644
--- a/src/main/java/au/id/zancanaro/javacheck/junit/Properties.java
+++ b/src/main/java/au/id/zancanaro/javacheck/junit/Properties.java
@@ -1,9 +1,9 @@
 package au.id.zancanaro.javacheck.junit;
 
+import au.id.zancanaro.javacheck.DataSourceHelper;
 import au.id.zancanaro.javacheck.Generator;
 import au.id.zancanaro.javacheck.ShrinkResult;
 import au.id.zancanaro.javacheck.ShrinkTree;
-import au.id.zancanaro.javacheck.annotations.DataSource;
 import au.id.zancanaro.javacheck.annotations.Property;
 import au.id.zancanaro.javacheck.annotations.Seed;
 import org.junit.AssumptionViolatedException;
@@ -13,64 +13,26 @@ import org.junit.runners.model.InitializationError;
 import org.junit.runners.model.Statement;
 import org.junit.runners.model.TestClass;
 
-import java.lang.reflect.*;
+import java.lang.reflect.Method;
+import java.lang.reflect.Type;
 import java.util.*;
 
 @SuppressWarnings("WeakerAccess")
 public class Properties extends BlockJUnit4ClassRunner {
-    private final Map<Type, Generator<?>> generators = new HashMap<>();
+    private final DataSourceHelper helper;
 
     public Properties(Class<?> classObject) throws InitializationError {
         super(classObject);
+        helper = new DataSourceHelper(classObject);
     }
 
     @Override
     protected void collectInitializationErrors(List<Throwable> errors) {
         super.collectInitializationErrors(errors);
-        Set<Type> generated = validateGeneratorFields(errors);
+        Set<Type> generated = DataSourceHelper.validateGeneratorFields(getTestClass().getJavaClass(), errors);
         validateTestMethodParameters(errors, generated);
     }
 
-    private Set<Type> validateGeneratorFields(List<Throwable> errors) {
-        Set<Type> result = new HashSet<>();
-        Field[] fields = getTestClass().getJavaClass().getDeclaredFields();
-
-        for (Field field : fields) {
-            if (field.isAnnotationPresent(DataSource.class)) {
-                boolean error = false;
-                if (!Modifier.isStatic(field.getModifiers())) {
-                    errors.add(new Error("@DataSource field " + field.getName() + " must be static"));
-                    error = true;
-                }
-                if (!Modifier.isPublic(field.getModifiers())) {
-                    errors.add(new Error("@DataSource field " + field.getName() + " must be public"));
-                    error = true;
-                }
-
-                Type type = field.getGenericType();
-                ParameterizedType parameterizedType;
-                if (type instanceof ParameterizedType) {
-                    parameterizedType = (ParameterizedType) type;
-                    if (parameterizedType.getRawType() instanceof Class) {
-                        Class<?> c = (Class) parameterizedType.getRawType();
-                        if (c == Generator.class) {
-                            if (!error) {
-                                result.add(parameterizedType.getActualTypeArguments()[0]);
-                            }
-                        } else {
-                            errors.add(new Error("@DataSource fields must be of type Generator<T>"));
-                        }
-                    } else {
-                        errors.add(new Error("@DataSource fields must be of type Generator<T>"));
-                    }
-                } else {
-                    errors.add(new Error("@DataSource fields must be of type Generator<T>"));
-                }
-            }
-        }
-        return result;
-    }
-
     private void validateTestMethodParameters(List<Throwable> errors, Set<Type> generated) {
         for (FrameworkMethod each : computeTestMethods()) {
             for (Type type : each.getMethod().getGenericParameterTypes()) {
@@ -82,56 +44,6 @@ public class Properties extends BlockJUnit4ClassRunner {
         }
     }
 
-    private static final Map<Type, Type> rawTypes;
-
-    static {
-        Map<Type, Type> types = new HashMap<>();
-        types.put(Double.class, Double.TYPE);
-        types.put(Float.class, Float.TYPE);
-        types.put(Long.class, Long.TYPE);
-        types.put(Integer.class, Integer.TYPE);
-        types.put(Short.class, Short.TYPE);
-        types.put(Byte.class, Byte.TYPE);
-        types.put(Character.class, Character.TYPE);
-        types.put(Boolean.class, Boolean.TYPE);
-        rawTypes = Collections.unmodifiableMap(types);
-    }
-
-    private Map<Type, Generator<?>> computeGenerators() {
-        if (generators.isEmpty()) {
-            Field[] fields = getTestClass().getJavaClass().getDeclaredFields();
-
-            for (Field field : fields) {
-                if (!field.isAnnotationPresent(DataSource.class)) {
-                    continue;
-                }
-                Type type = field.getGenericType();
-                if (!(type instanceof ParameterizedType)) {
-                    continue;
-                }
-                ParameterizedType parameterizedType = (ParameterizedType) type;
-                if (!(parameterizedType.getRawType() instanceof Class)) {
-                    continue;
-                }
-                Class<?> c = (Class) parameterizedType.getRawType();
-                if (c != Generator.class) {
-                    continue;
-                }
-                try {
-                    Type target = parameterizedType.getActualTypeArguments()[0];
-                    @SuppressWarnings("unchecked")
-                    Generator<Object> generator = (Generator<Object>) field.get(null);
-                    generators.put(target, generator);
-                    if (rawTypes.containsKey(target)) {
-                        generators.put(rawTypes.get(target), generator);
-                    }
-                } catch (IllegalAccessException ex) {
-                    throw new RuntimeException(ex);
-                }
-            }
-        }
-        return generators;
-    }
 
     @Override
     protected void validateConstructor(List<Throwable> errors) {
@@ -161,7 +73,7 @@ public class Properties extends BlockJUnit4ClassRunner {
 
     @Override
     public Statement methodBlock(final FrameworkMethod method) {
-        return new GenerativeTester(method, getTestClass(), computeGenerators());
+        return new GenerativeTester(method, getTestClass(), helper.computeGenerators());
     }
 
     public static class GenerativeTester extends Statement {
diff --git a/src/main/java/au/id/zancanaro/javacheck/junit/PropertyError.java b/src/main/java/au/id/zancanaro/javacheck/junit/PropertyError.java
index 44c071b..d3747b9 100644
--- a/src/main/java/au/id/zancanaro/javacheck/junit/PropertyError.java
+++ b/src/main/java/au/id/zancanaro/javacheck/junit/PropertyError.java
@@ -4,6 +4,7 @@ import au.id.zancanaro.javacheck.ShrinkResult;
 
 import java.util.Arrays;
 import java.util.Iterator;
+import java.util.List;
 
 @SuppressWarnings("WeakerAccess")
 public class PropertyError extends AssertionError {
@@ -19,9 +20,9 @@ public class PropertyError extends AssertionError {
         initCause(shrunk.thrown);
     }
 
-    private static String joinArgs(Object... params) {
+    private static String joinArgs(List<Object> params) {
         StringBuilder sb = new StringBuilder();
-        Iterator<Object> iterator = Arrays.asList(params).iterator();
+        Iterator<Object> iterator = params.iterator();
         while (iterator.hasNext()) {
             Object next = iterator.next();
             sb.append(stringValueOf(next));
diff --git a/src/main/java/au/id/zancanaro/javacheck/object/CharType.java b/src/main/java/au/id/zancanaro/javacheck/object/CharType.java
new file mode 100644
index 0000000..befbd04
--- /dev/null
+++ b/src/main/java/au/id/zancanaro/javacheck/object/CharType.java
@@ -0,0 +1,15 @@
+package au.id.zancanaro.javacheck.object;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+@Target(ElementType.FIELD)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface CharType {
+    public static enum TYPE {
+        ALL, ASCII, ALPHA, ALPHA_NUMERIC
+    };
+    TYPE value();
+}
diff --git a/src/main/java/au/id/zancanaro/javacheck/object/DoubleRange.java b/src/main/java/au/id/zancanaro/javacheck/object/DoubleRange.java
new file mode 100644
index 0000000..ca7c9a9
--- /dev/null
+++ b/src/main/java/au/id/zancanaro/javacheck/object/DoubleRange.java
@@ -0,0 +1,13 @@
+package au.id.zancanaro.javacheck.object;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+@Target(ElementType.FIELD)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface DoubleRange {
+    double min();
+    double max();
+}
diff --git a/src/main/java/au/id/zancanaro/javacheck/object/GeneratorProvider.java b/src/main/java/au/id/zancanaro/javacheck/object/GeneratorProvider.java
new file mode 100644
index 0000000..7e5964f
--- /dev/null
+++ b/src/main/java/au/id/zancanaro/javacheck/object/GeneratorProvider.java
@@ -0,0 +1,117 @@
+package au.id.zancanaro.javacheck.object;
+
+import au.id.zancanaro.javacheck.Generator;
+
+import java.lang.annotation.Annotation;
+import java.lang.reflect.ParameterizedType;
+import java.lang.reflect.Type;
+import java.lang.reflect.TypeVariable;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+
+import static au.id.zancanaro.javacheck.Generators.*;
+
+public interface GeneratorProvider {
+    Generator<?> getGenerator(Type type, Annotation[] annotations, GeneratorProvider topLevel);
+
+    final static GeneratorProvider DEFAULT_PROVIDER = new GeneratorProvider() {
+        @SuppressWarnings("unchecked")
+        private <T> T getAnnotation(Annotation[] annotations, Class<T> type) {
+            for (Annotation ann : annotations) {
+                if (type.isAssignableFrom(ann.getClass())) {
+                    return (T) ann;
+                }
+            }
+            return null;
+        }
+
+        @Override
+        public Generator<?> getGenerator(Type type, Annotation[] annotations, GeneratorProvider provider) {
+            if (type == Integer.TYPE || type == Integer.class) {
+                IntRange range = getAnnotation(annotations, IntRange.class);
+                if (range == null) {
+                    return integer();
+                } else {
+                    return integer(range.min(), range.max());
+                }
+            } else if (type == Long.TYPE || type == Long.class) {
+                LongRange range = getAnnotation(annotations, LongRange.class);
+                if (range == null) {
+                    return longInteger();
+                } else {
+                    return longInteger(range.min(), range.max());
+                }
+            } else if (type == Double.TYPE || type == Double.class) {
+                DoubleRange range = getAnnotation(annotations, DoubleRange.class);
+                if (range == null) {
+                    return doublePrecision();
+                } else {
+                    return doublePrecision(range.min(), range.max());
+                }
+            } else if (type == String.class) {
+                CharType range = getAnnotation(annotations, CharType.class);
+                if (range == null) {
+                    return string();
+                } else {
+                    switch (range.value()) {
+                        case ALPHA:
+                            return stringOf(alphaCharacter());
+                        case ALPHA_NUMERIC:
+                            return stringOf(alphaNumericCharacter());
+                        case ASCII:
+                            return stringOf(asciiCharacter());
+                        case ALL:
+                            return stringOf(character());
+                        default:
+                            return string();
+                    }
+                }
+            } else if (type == List.class) {
+                return listOf(provider.getGenerator(
+                        List.class.getTypeParameters()[0],
+                        new Annotation[0],
+                        provider));
+            } else if (type == Map.class) {
+                TypeVariable<?>[] params = Map.class.getTypeParameters();
+                return mapOf(
+                        provider.getGenerator(params[0], new Annotation[0], provider),
+                        provider.getGenerator(params[1], new Annotation[0], provider));
+            } else if (type instanceof Class) {
+                return ofType((Class<?>) type, provider);
+            } else if (type instanceof ParameterizedType) {
+                ParameterizedType param = (ParameterizedType) type;
+                if (param.getRawType() instanceof Class) {
+                    Class<?> container = (Class) param.getRawType();
+                    TypeVariable<?>[] variables = container.getTypeParameters();
+                    Type[] types = param.getActualTypeArguments();
+                    return provider.getGenerator(
+                            container,
+                            new Annotation[0],
+                            provider.withSubstitutedTypeVariables(variables, types));
+                }
+                throw new ObjectGenerationException("Cannot generate object of type " + type);
+            } else {
+                throw new ObjectGenerationException("Cannot generate object of type " + type);
+            }
+        }
+    };
+
+    default GeneratorProvider withSubstitutedTypeVariables(TypeVariable<?>[] variables, Type[] types) {
+        GeneratorProvider fallback = this;
+        return new GeneratorProvider() {
+            @Override
+            public Generator<?> getGenerator(Type type, Annotation[] annotations, GeneratorProvider provider) {
+                for (int i = 0; i < variables.length; ++i) {
+                    if (Objects.equals(variables[i], type)) {
+                        return provider.getGenerator(
+                                types[i],
+                                new Annotation[0],
+                                this);
+                    }
+                }
+                return fallback.getGenerator(type, annotations, provider);
+            }
+        };
+    }
+}
diff --git a/src/main/java/au/id/zancanaro/javacheck/object/IntRange.java b/src/main/java/au/id/zancanaro/javacheck/object/IntRange.java
new file mode 100644
index 0000000..8dc3f28
--- /dev/null
+++ b/src/main/java/au/id/zancanaro/javacheck/object/IntRange.java
@@ -0,0 +1,13 @@
+package au.id.zancanaro.javacheck.object;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+@Target(ElementType.FIELD)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface IntRange {
+    int max();
+    int min();
+}
diff --git a/src/main/java/au/id/zancanaro/javacheck/object/LongRange.java b/src/main/java/au/id/zancanaro/javacheck/object/LongRange.java
new file mode 100644
index 0000000..dd5a61f
--- /dev/null
+++ b/src/main/java/au/id/zancanaro/javacheck/object/LongRange.java
@@ -0,0 +1,13 @@
+package au.id.zancanaro.javacheck.object;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+@Target(ElementType.FIELD)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface LongRange {
+    long max();
+    long min();
+}
diff --git a/src/main/java/au/id/zancanaro/javacheck/object/ObjectGenerationException.java b/src/main/java/au/id/zancanaro/javacheck/object/ObjectGenerationException.java
new file mode 100644
index 0000000..d2147eb
--- /dev/null
+++ b/src/main/java/au/id/zancanaro/javacheck/object/ObjectGenerationException.java
@@ -0,0 +1,11 @@
+package au.id.zancanaro.javacheck.object;
+
+public class ObjectGenerationException extends RuntimeException {
+    public ObjectGenerationException(String message) {
+        super(message);
+    }
+
+    public ObjectGenerationException(Throwable cause) {
+        super(cause);
+    }
+}
diff --git a/src/main/java/au/id/zancanaro/javacheck/object/ObjectGenerator.java b/src/main/java/au/id/zancanaro/javacheck/object/ObjectGenerator.java
new file mode 100644
index 0000000..a31d990
--- /dev/null
+++ b/src/main/java/au/id/zancanaro/javacheck/object/ObjectGenerator.java
@@ -0,0 +1,103 @@
+package au.id.zancanaro.javacheck.object;
+
+import au.id.zancanaro.javacheck.Generator;
+import au.id.zancanaro.javacheck.ShrinkTree;
+
+import java.lang.annotation.Annotation;
+import java.lang.reflect.*;
+import java.util.*;
+
+public class ObjectGenerator<T> implements Generator<T> {
+    private final Class<T> objectClass;
+    private final GeneratorProvider provider;
+    private final List<Generator<?>> constructorGenerators;
+    private final SortedMap<Field, Generator<?>> fieldGenerators;
+    private transient Constructor<T> constructor = null;
+
+    public ObjectGenerator(Class<T> objectClass) {
+        this(objectClass, GeneratorProvider.DEFAULT_PROVIDER);
+    }
+
+    public ObjectGenerator(Class<T> objectClass, GeneratorProvider provider) {
+        this.objectClass = objectClass;
+        this.provider = provider;
+        this.constructorGenerators = calculateConstructorGenerators();
+        this.fieldGenerators = calculateFieldGenerators();
+    }
+
+    @SuppressWarnings("unchecked")
+    private Constructor<T> getConstructor() {
+        if (this.constructor == null) {
+            Constructor<?>[] constructors = objectClass.getConstructors();
+            if (constructors.length == 1) {
+                return (Constructor<T>) constructors[0];
+            } else {
+                for (Constructor<?> constructor : constructors) {
+                    if (constructor.isAnnotationPresent(UseForGeneration.class)) {
+                        if (this.constructor == null) {
+                            this.constructor = (Constructor<T>) constructor;
+                        } else {
+                            throw new ObjectGenerationException("Multiple constructors are annotated with @UseForGeneration in class" + objectClass);
+                        }
+                    }
+                }
+                if (this.constructor == null) {
+                    throw new ObjectGenerationException("Multiple constructors, but none are annotated with @UseForGeneration, in class" + objectClass);
+                }
+            }
+        }
+        return this.constructor;
+    }
+
+    private List<Generator<?>> calculateConstructorGenerators() {
+        List<Generator<?>> result = new ArrayList<>();
+        Constructor<T> constructor = getConstructor();
+        Type[] parameterTypes = constructor.getGenericParameterTypes();
+        Annotation[][] annotations = constructor.getParameterAnnotations();
+        for (int i = 0; i < parameterTypes.length; ++i) {
+            result.add(provider.getGenerator(parameterTypes[i], annotations[i], provider));
+        }
+        return result;
+    }
+
+    private SortedMap<Field, Generator<?>> calculateFieldGenerators() {
+        Comparator<Field> fieldComparator = Comparator.comparing(Field::getName);
+        SortedMap<Field, Generator<?>> result = new TreeMap<>(fieldComparator);
+        for (Field field : objectClass.getFields()) {
+            if (!Modifier.isFinal(field.getModifiers())
+                    && !Modifier.isStatic(field.getModifiers())) {
+                result.put(field, provider.getGenerator(
+                        field.getGenericType(),
+                        field.getAnnotations(),
+                        provider));
+            }
+        }
+        return result;
+    }
+
+    @Override
+    public ShrinkTree<T> generate(Random random, int size) {
+        Generator<?>[] parameters = new Generator[constructorGenerators.size()];
+        parameters = constructorGenerators.toArray(parameters);
+        Generator<?>[] fields = new Generator[fieldGenerators.size()];
+        fields = fieldGenerators.values().toArray(fields);
+        return Generator.tuple(Generator.tuple(parameters), Generator.tuple(fields))
+                .generate(random, size)
+                .map(objs -> makeObject(
+                        (List<?>) objs.get(0),
+                        (List<?>) objs.get(1)));
+    }
+
+    private T makeObject(List<?> parameters, List<?> fields) {
+        try {
+            T obj = getConstructor().newInstance(parameters.toArray());
+            int i = 0;
+            for (Field key : fieldGenerators.keySet()) {
+                key.set(obj, fields.get(i++));
+            }
+            return obj;
+        } catch (IllegalAccessException | InstantiationException | InvocationTargetException ex) {
+            throw new ObjectGenerationException(ex);
+        }
+    }
+}
diff --git a/src/main/java/au/id/zancanaro/javacheck/object/UseForGeneration.java b/src/main/java/au/id/zancanaro/javacheck/object/UseForGeneration.java
new file mode 100644
index 0000000..68990bf
--- /dev/null
+++ b/src/main/java/au/id/zancanaro/javacheck/object/UseForGeneration.java
@@ -0,0 +1,11 @@
+package au.id.zancanaro.javacheck.object;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+@Target(ElementType.CONSTRUCTOR)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface UseForGeneration {
+}
diff --git a/src/main/java/au/id/zancanaro/javacheck/state/CommandValue.java b/src/main/java/au/id/zancanaro/javacheck/state/CommandValue.java
index 8584fc7..1ca51a9 100644
--- a/src/main/java/au/id/zancanaro/javacheck/state/CommandValue.java
+++ b/src/main/java/au/id/zancanaro/javacheck/state/CommandValue.java
@@ -2,7 +2,6 @@ package au.id.zancanaro.javacheck.state;
 
 import java.util.Map;
 import java.util.NoSuchElementException;
-import java.util.function.Supplier;
 
 public class CommandValue<T> {
     public static interface Action<T> {
diff --git a/src/test/java/au/id/zancanaro/javacheck/SimpleListOperationsTest.java b/src/test/java/au/id/zancanaro/javacheck/SimpleListOperationsTest.java
index 70b2425..fcc8baf 100644
--- a/src/test/java/au/id/zancanaro/javacheck/SimpleListOperationsTest.java
+++ b/src/test/java/au/id/zancanaro/javacheck/SimpleListOperationsTest.java
@@ -6,7 +6,6 @@ import au.id.zancanaro.javacheck.junit.Properties;
 import org.junit.runner.RunWith;
 
 import java.util.ArrayList;
-import java.util.Collections;
 import java.util.List;
 
 import static au.id.zancanaro.javacheck.Generators.*;
diff --git a/src/test/java/au/id/zancanaro/javacheck/object/MyObject.java b/src/test/java/au/id/zancanaro/javacheck/object/MyObject.java
new file mode 100644
index 0000000..ac0c370
--- /dev/null
+++ b/src/test/java/au/id/zancanaro/javacheck/object/MyObject.java
@@ -0,0 +1,50 @@
+package au.id.zancanaro.javacheck.object;
+
+public class MyObject {
+    public final String string;
+    public final int value;
+    public final SubObject<Integer> subObject;
+
+    public MyObject(String string, int value, SubObject<Integer> subObject) {
+        this.string = string;
+        this.value = value;
+        this.subObject = subObject;
+    }
+
+    @UseForGeneration
+    public MyObject(String string, SubObject<Integer> subObject) {
+        this(string, string.length(), subObject);
+    }
+
+    public MyObject add(MyObject other) {
+        return new MyObject(
+                this.string + other.string,
+                this.value + other.value,
+                this.subObject.add(other.subObject));
+    }
+
+    @Override
+    public String toString() {
+        return "{" + string + ", " + value + ", " + subObject + "}";
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+
+        MyObject myObject = (MyObject) o;
+
+        return value == myObject.value
+                && string.equals(myObject.string)
+                && subObject.equals(myObject.subObject);
+    }
+
+    @Override
+    public int hashCode() {
+        int result = string.hashCode();
+        result = 31 * result + value;
+        result = 31 * result + subObject.hashCode();
+        return result;
+    }
+}
diff --git a/src/test/java/au/id/zancanaro/javacheck/object/MyObjectAddTest.java b/src/test/java/au/id/zancanaro/javacheck/object/MyObjectAddTest.java
new file mode 100644
index 0000000..7965449
--- /dev/null
+++ b/src/test/java/au/id/zancanaro/javacheck/object/MyObjectAddTest.java
@@ -0,0 +1,29 @@
+package au.id.zancanaro.javacheck.object;
+
+import au.id.zancanaro.javacheck.Generator;
+import au.id.zancanaro.javacheck.annotations.DataSource;
+import au.id.zancanaro.javacheck.annotations.Property;
+import au.id.zancanaro.javacheck.junit.Properties;
+import org.junit.runner.RunWith;
+
+import java.util.Collections;
+
+import static au.id.zancanaro.javacheck.Generators.ofType;
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+@RunWith(Properties.class)
+public class MyObjectAddTest {
+
+    @DataSource
+    public static Generator<MyObject> source = ofType(MyObject.class);
+
+    @Property
+    public void testAdd(MyObject a, MyObject b) {
+        MyObject added = a.add(b);
+        assertTrue(added.string.length() == a.string.length() + b.string.length());
+        assertTrue(added.value == a.value + b.value);
+        assertEquals(added.subObject, a.subObject.add(b.subObject));
+    }
+
+}
diff --git a/src/test/java/au/id/zancanaro/javacheck/object/SubObject.java b/src/test/java/au/id/zancanaro/javacheck/object/SubObject.java
new file mode 100644
index 0000000..bdf2ce8
--- /dev/null
+++ b/src/test/java/au/id/zancanaro/javacheck/object/SubObject.java
@@ -0,0 +1,44 @@
+package au.id.zancanaro.javacheck.object;
+
+import java.util.*;
+
+public class SubObject<T> {
+    public final Map<String, List<T>> obj;
+
+    public SubObject(Map<String, List<T>> obj) {
+        this.obj = obj;
+    }
+
+    public SubObject<T> add(SubObject<T> other) {
+        Map<String, List<T>> values = new HashMap<>(obj);
+        for (Map.Entry<String, List<T>> entry : other.obj.entrySet()) {
+            String key = entry.getKey();
+            if (values.containsKey(key)) {
+                List<T> result = new ArrayList<>(values.get(key));
+                result.addAll(entry.getValue());
+                values.put(key, Collections.unmodifiableList(result));
+            } else {
+                values.put(key, entry.getValue());
+            }
+        }
+        return new SubObject<>(Collections.unmodifiableMap(values));
+    }
+
+    @Override
+    public String toString() {
+        return obj.toString();
+    }
+
+    @Override
+    public boolean equals(Object o) {
+        if (this == o) return true;
+        if (o == null || getClass() != o.getClass()) return false;
+        SubObject subObject = (SubObject) o;
+        return obj.equals(subObject.obj);
+    }
+
+    @Override
+    public int hashCode() {
+        return obj.hashCode();
+    }
+}