common/reflect/TypeToken.java

/*
 * Copyright (C) 2006 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.reflect;

import static com.google.common.base.Preconditions.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;

import com.google.common.annotations.Beta;
import com.google.common.annotations.VisibleForTesting;
import com.google.common.base.Joiner;
import com.google.common.base.Predicate;
import com.google.common.collect.FluentIterable;
import com.google.common.collect.ForwardingSet;
import com.google.common.collect.ImmutableList;
import com.google.common.collect.ImmutableMap;
import com.google.common.collect.ImmutableSet;
import com.google.common.collect.Maps;
import com.google.common.collect.Ordering;
import com.google.common.primitives.Primitives;

import java.io.Serializable;
import java.lang.reflect.Constructor;
import java.lang.reflect.GenericArrayType;
import java.lang.reflect.Method;
import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;
import java.lang.reflect.WildcardType;
import java.util.Arrays;
import java.util.Comparator;
import java.util.Map;
import java.util.Set;

import javax.annotation.Nullable;

/**
 * A {@link Type} with generics.
 *
 * <p>Operations that are otherwise only available in {@link Class} are implemented to support
 * {@code Type}, for example {@link #isAssignableFrom}, {@link #isArray} and {@link
 * #getComponentType}. It also provides additional utilities such as {@link #getTypes} and {@link
 * #resolveType} etc.
 *
 * <p>There are three ways to get a {@code TypeToken} instance: <ul>
 * <li>Wrap a {@code Type} obtained via reflection. For example: {@code
 * TypeToken.of(method.getGenericReturnType())}.
 * <li>Capture a generic type with a (usually anonymous) subclass. For example: <pre>   {@code
 *   new TypeToken<List<String>>() {}}</pre>
 * <p>Note that it's critical that the actual type argument is carried by a subclass.
 * The following code is wrong because it only captures the {@code <T>} type variable
 * of the {@code listType()} method signature; while {@code <String>} is lost in erasure:
 * <pre>   {@code
 *   class Util {
 *     static <T> TypeToken<List<T>> listType() {
 *       return new TypeToken<List<T>>() {};
 *     }
 *   }
 *
 *   TypeToken<List<String>> stringListType = Util.<String>listType();}</pre>
 * <li>Capture a generic type with a (usually anonymous) subclass and resolve it against
 * a context class that knows what the type parameters are. For example: <pre>   {@code
 *   abstract class IKnowMyType<T> {
 *     TypeToken<T> type = new TypeToken<T>(getClass()) {};
 *   }
 *   new IKnowMyType<String>() {}.type => String}</pre>
 * </ul>
 *
 * <p>{@code TypeToken} is serializable when no type variable is contained in the type.
 *
 * <p>Note to Guice users: {@code} TypeToken is similar to Guice's {@code TypeLiteral} class
 * except that it is serializable and offers numerous additional utility methods.
 *
 * @author Bob Lee
 * @author Sven Mawson
 * @author Ben Yu
 * @since 12.0
 */
@Beta
@SuppressWarnings("serial") // SimpleTypeToken is the serialized form.
public abstract class TypeToken<T> extends TypeCapture<T> implements Serializable {

  private final Type runtimeType;

  /** Resolver for resolving types with {@link #runtimeType} as context. */
  private transient TypeResolver typeResolver;

  /**
   * Constructs a new type token of {@code T}.
   *
   * <p>Clients create an empty anonymous subclass. Doing so embeds the type
   * parameter in the anonymous class's type hierarchy so we can reconstitute
   * it at runtime despite erasure.
   *
   * <p>For example: <pre>   {@code
   *   TypeToken<List<String>> t = new TypeToken<List<String>>() {};}</pre>
   */
  protected TypeToken() {
    this.runtimeType = capture();
    checkState(!(runtimeType instanceof TypeVariable),
        "Cannot construct a TypeToken for a type variable.\n" +
        "You probably meant to call new TypeToken<%s>(getClass()) " +
        "that can resolve the type variable for you.\n" +
        "If you do need to create a TypeToken of a type variable, " +
        "please use TypeToken.of() instead.", runtimeType);
  }

  /**
   * Constructs a new type token of {@code T} while resolving free type variables in the context of
   * {@code declaringClass}.
   *
   * <p>Clients create an empty anonymous subclass. Doing so embeds the type
   * parameter in the anonymous class's type hierarchy so we can reconstitute
   * it at runtime despite erasure.
   *
   * <p>For example: <pre>   {@code
   *   abstract class IKnowMyType<T> {
   *     TypeToken<T> getMyType() {
   *       return new TypeToken<T>(getClass()) {};
   *     }
   *   }
   *
   *   new IKnowMyType<String>() {}.getMyType() => String}</pre>
   */
  protected TypeToken(Class<?> declaringClass) {
    Type captured = super.capture();
    if (captured instanceof Class) {
      this.runtimeType = captured;
    } else {
      this.runtimeType = of(declaringClass).resolveType(captured).runtimeType;
    }
  }

  private TypeToken(Type type) {
    this.runtimeType = checkNotNull(type);
  }

  /** Returns an instance of type token that wraps {@code type}. */
  public static <T> TypeToken<T> of(Class<T> type) {
    return new SimpleTypeToken<T>(type);
  }

  /** Returns an instance of type token that wraps {@code type}. */
  public static TypeToken<?> of(Type type) {
    return new SimpleTypeToken<Object>(type);
  }

  /**
   * Returns the raw type of {@code T}. Formally speaking, if {@code T} is returned by
   * {@link java.lang.reflect.Method#getGenericReturnType}, the raw type is what's returned by
   * {@link java.lang.reflect.Method#getReturnType} of the same method object. Specifically:
   * <ul>
   * <li>If {@code T} is a {@code Class} itself, {@code T} itself is returned.
   * <li>If {@code T} is a {@link ParameterizedType}, the raw type of the parameterized type is
   *     returned.
   * <li>If {@code T} is a {@link GenericArrayType}, the returned type is the corresponding array
   *     class. For example: {@code List<Integer>[] => List[]}.
   * <li>If {@code T} is a type variable or a wildcard type, the raw type of the first upper bound
   *     is returned. For example: {@code <X extends Foo> => Foo}.
   * </ul>
   */
  public final Class<? super T> getRawType() {
    Class<?> rawType = getRawType(runtimeType);
    @SuppressWarnings("unchecked") // raw type is |T|
    Class<? super T> result = (Class<? super T>) rawType;
    return result;
  }

  /**
   * Returns the raw type of the class or parameterized type; if {@code T} is type variable or
   * wildcard type, the raw types of all its upper bounds are returned.
   */
  private ImmutableSet<Class<? super T>> getImmediateRawTypes() {
    // Cast from ImmutableSet<Class<?>> to ImmutableSet<Class<? super T>>
    @SuppressWarnings({"unchecked", "rawtypes"})
    ImmutableSet<Class<? super T>> result = (ImmutableSet) getRawTypes(runtimeType);
    return result;
  }

  /** Returns the represented type. */
  public final Type getType() {
    return runtimeType;
  }

  /**
   * <p>Returns a new {@code TypeToken} where type variables represented by {@code typeParam}
   * are substituted by {@code typeArg}. For example, it can be used to construct
   * {@code Map<K, V>} for any {@code K} and {@code V} type: <pre>   {@code
   *   static <K, V> TypeToken<Map<K, V>> mapOf(
   *       TypeToken<K> keyType, TypeToken<V> valueType) {
   *     return new TypeToken<Map<K, V>>() {}
   *         .where(new TypeParameter<K>() {}, keyType)
   *         .where(new TypeParameter<V>() {}, valueType);
   *   }}</pre>
   *
   * @param <X> The parameter type
   * @param typeParam the parameter type variable
   * @param typeArg the actual type to substitute
   */
  public final <X> TypeToken<T> where(TypeParameter<X> typeParam, TypeToken<X> typeArg) {
    TypeResolver resolver = new TypeResolver()
        .where(ImmutableMap.of(
            new TypeResolver.TypeVariableKey(typeParam.typeVariable),
            typeArg.runtimeType));
    // If there's any type error, we'd report now rather than later.
    return new SimpleTypeToken<T>(resolver.resolveType(runtimeType));
  }

  /**
   * <p>Returns a new {@code TypeToken} where type variables represented by {@code typeParam}
   * are substituted by {@code typeArg}. For example, it can be used to construct
   * {@code Map<K, V>} for any {@code K} and {@code V} type: <pre>   {@code
   *   static <K, V> TypeToken<Map<K, V>> mapOf(
   *       Class<K> keyType, Class<V> valueType) {
   *     return new TypeToken<Map<K, V>>() {}
   *         .where(new TypeParameter<K>() {}, keyType)
   *         .where(new TypeParameter<V>() {}, valueType);
   *   }}</pre>
   *
   * @param <X> The parameter type
   * @param typeParam the parameter type variable
   * @param typeArg the actual type to substitute
   */
  public final <X> TypeToken<T> where(TypeParameter<X> typeParam, Class<X> typeArg) {
    return where(typeParam, of(typeArg));
  }

  /**
   * <p>Resolves the given {@code type} against the type context represented by this type.
   * For example: <pre>   {@code
   *   new TypeToken<List<String>>() {}.resolveType(
   *       List.class.getMethod("get", int.class).getGenericReturnType())
   *   => String.class}</pre>
   */
  public final TypeToken<?> resolveType(Type type) {
    checkNotNull(type);
    TypeResolver resolver = typeResolver;
    if (resolver == null) {
      resolver = (typeResolver = TypeResolver.accordingTo(runtimeType));
    }
    return of(resolver.resolveType(type));
  }

  private Type[] resolveInPlace(Type[] types) {
    for (int i = 0; i < types.length; i++) {
      types[i] = resolveType(types[i]).getType();
    }
    return types;
  }

  private TypeToken<?> resolveSupertype(Type type) {
    TypeToken<?> supertype = resolveType(type);
    // super types' type mapping is a subset of type mapping of this type.
    supertype.typeResolver = typeResolver;
    return supertype;
  }

  /**
   * Returns the generic superclass of this type or {@code null} if the type represents
   * {@link Object} or an interface. This method is similar but different from {@link
   * Class#getGenericSuperclass}. For example, {@code
   * new TypeToken<StringArrayList>() {}.getGenericSuperclass()} will return {@code
   * new TypeToken<ArrayList<String>>() {}}; while {@code
   * StringArrayList.class.getGenericSuperclass()} will return {@code ArrayList<E>}, where {@code E}
   * is the type variable declared by class {@code ArrayList}.
   *
   * <p>If this type is a type variable or wildcard, its first upper bound is examined and returned
   * if the bound is a class or extends from a class. This means that the returned type could be a
   * type variable too.
   */
  @Nullable
  final TypeToken<? super T> getGenericSuperclass() {
    if (runtimeType instanceof TypeVariable) {
      // First bound is always the super class, if one exists.
      return boundAsSuperclass(((TypeVariable<?>) runtimeType).getBounds()[0]);
    }
    if (runtimeType instanceof WildcardType) {
      // wildcard has one and only one upper bound.
      return boundAsSuperclass(((WildcardType) runtimeType).getUpperBounds()[0]);
    }
    Type superclass = getRawType().getGenericSuperclass();
    if (superclass == null) {
      return null;
    }
    @SuppressWarnings("unchecked") // super class of T
    TypeToken<? super T> superToken = (TypeToken<? super T>) resolveSupertype(superclass);
    return superToken;
  }

  @Nullable private TypeToken<? super T> boundAsSuperclass(Type bound) {
    TypeToken<?> token = of(bound);
    if (token.getRawType().isInterface()) {
      return null;
    }
    @SuppressWarnings("unchecked") // only upper bound of T is passed in.
    TypeToken<? super T> superclass = (TypeToken<? super T>) token;
    return superclass;
  }

  /**
   * Returns the generic interfaces that this type directly {@code implements}. This method is
   * similar but different from {@link Class#getGenericInterfaces()}. For example, {@code
   * new TypeToken<List<String>>() {}.getGenericInterfaces()} will return a list that contains
   * {@code new TypeToken<Iterable<String>>() {}}; while {@code List.class.getGenericInterfaces()}
   * will return an array that contains {@code Iterable<T>}, where the {@code T} is the type
   * variable declared by interface {@code Iterable}.
   *
   * <p>If this type is a type variable or wildcard, its upper bounds are examined and those that
   * are either an interface or upper-bounded only by interfaces are returned. This means that the
   * returned types could include type variables too.
   */
  final ImmutableList<TypeToken<? super T>> getGenericInterfaces() {
    if (runtimeType instanceof TypeVariable) {
      return boundsAsInterfaces(((TypeVariable<?>) runtimeType).getBounds());
    }
    if (runtimeType instanceof WildcardType) {
      return boundsAsInterfaces(((WildcardType) runtimeType).getUpperBounds());
    }
    ImmutableList.Builder<TypeToken<? super T>> builder = ImmutableList.builder();
    for (Type interfaceType : getRawType().getGenericInterfaces()) {
      @SuppressWarnings("unchecked") // interface of T
      TypeToken<? super T> resolvedInterface = (TypeToken<? super T>)
          resolveSupertype(interfaceType);
      builder.add(resolvedInterface);
    }
    return builder.build();
  }

  private ImmutableList<TypeToken<? super T>> boundsAsInterfaces(Type[] bounds) {
    ImmutableList.Builder<TypeToken<? super T>> builder = ImmutableList.builder();
    for (Type bound : bounds) {
      @SuppressWarnings("unchecked") // upper bound of T
      TypeToken<? super T> boundType = (TypeToken<? super T>) of(bound);
      if (boundType.getRawType().isInterface()) {
        builder.add(boundType);
      }
    }
    return builder.build();
  }

  /**
   * Returns the set of interfaces and classes that this type is or is a subtype of. The returned
   * types are parameterized with proper type arguments.
   *
   * <p>Subtypes are always listed before supertypes. But the reverse is not true. A type isn't
   * necessarily a subtype of all the types following. Order between types without subtype
   * relationship is arbitrary and not guaranteed.
   *
   * <p>If this type is a type variable or wildcard, upper bounds that are themselves type variables
   * aren't included (their super interfaces and superclasses are).
   */
  public final TypeSet getTypes() {
    return new TypeSet();
  }

  /**
   * Returns the generic form of {@code superclass}. For example, if this is
   * {@code ArrayList<String>}, {@code Iterable<String>} is returned given the
   * input {@code Iterable.class}.
   */
  public final TypeToken<? super T> getSupertype(Class<? super T> superclass) {
    checkArgument(superclass.isAssignableFrom(getRawType()),
        "%s is not a super class of %s", superclass, this);
    if (runtimeType instanceof TypeVariable) {
      return getSupertypeFromUpperBounds(superclass, ((TypeVariable<?>) runtimeType).getBounds());
    }
    if (runtimeType instanceof WildcardType) {
      return getSupertypeFromUpperBounds(superclass, ((WildcardType) runtimeType).getUpperBounds());
    }
    if (superclass.isArray()) {
      return getArraySupertype(superclass);
    }
    @SuppressWarnings("unchecked") // resolved supertype
    TypeToken<? super T> supertype = (TypeToken<? super T>)
        resolveSupertype(toGenericType(superclass).runtimeType);
    return supertype;
  }

  /**
   * Returns subtype of {@code this} with {@code subclass} as the raw class.
   * For example, if this is {@code Iterable<String>} and {@code subclass} is {@code List},
   * {@code List<String>} is returned.
   */
  public final TypeToken<? extends T> getSubtype(Class<?> subclass) {
    checkArgument(!(runtimeType instanceof TypeVariable),
        "Cannot get subtype of type variable <%s>", this);
    if (runtimeType instanceof WildcardType) {
      return getSubtypeFromLowerBounds(subclass, ((WildcardType) runtimeType).getLowerBounds());
    }
    checkArgument(getRawType().isAssignableFrom(subclass),
        "%s isn't a subclass of %s", subclass, this);
    // unwrap array type if necessary
    if (isArray()) {
      return getArraySubtype(subclass);
    }
    @SuppressWarnings("unchecked") // guarded by the isAssignableFrom() statement above
    TypeToken<? extends T> subtype = (TypeToken<? extends T>)
        of(resolveTypeArgsForSubclass(subclass));
    return subtype;
  }

  /** Returns true if this type is assignable from the given {@code type}. */
  public final boolean isAssignableFrom(TypeToken<?> type) {
    return isAssignableFrom(type.runtimeType);
  }

  /** Check if this type is assignable from the given {@code type}. */
  public final boolean isAssignableFrom(Type type) {
    return isAssignable(checkNotNull(type), runtimeType);
  }

  /**
   * Returns true if this type is known to be an array type, such as {@code int[]}, {@code T[]},
   * {@code <? extends Map<String, Integer>[]>} etc.
   */
  public final boolean isArray() {
    return getComponentType() != null;
  }

  /**
   * Returns true if this type is one of the nine primitive types (including {@code void}).
   *
   * @since 15.0
   */
  public final boolean isPrimitive() {
    return (runtimeType instanceof Class) && ((Class<?>) runtimeType).isPrimitive();
  }

  /**
   * Returns the corresponding wrapper type if this is a primitive type; otherwise returns
   * {@code this} itself. Idempotent.
   *
   * @since 15.0
   */
  public final TypeToken<T> wrap() {
    if (isPrimitive()) {
      @SuppressWarnings("unchecked") // this is a primitive class
      Class<T> type = (Class<T>) runtimeType;
      return TypeToken.of(Primitives.wrap(type));
    }
    return this;
  }

  private boolean isWrapper() {
    return Primitives.allWrapperTypes().contains(runtimeType);
  }

  /**
   * Returns the corresponding primitive type if this is a wrapper type; otherwise returns
   * {@code this} itself. Idempotent.
   *
   * @since 15.0
   */
  public final TypeToken<T> unwrap() {
    if (isWrapper()) {
      @SuppressWarnings("unchecked") // this is a wrapper class
      Class<T> type = (Class<T>) runtimeType;
      return TypeToken.of(Primitives.unwrap(type));
    }
    return this;
  }

  /**
   * Returns the array component type if this type represents an array ({@code int[]}, {@code T[]},
   * {@code <? extends Map<String, Integer>[]>} etc.), or else {@code null} is returned.
   */
  @Nullable public final TypeToken<?> getComponentType() {
    Type componentType = Types.getComponentType(runtimeType);
    if (componentType == null) {
      return null;
    }
    return of(componentType);
  }

  /**
   * Returns the {@link Invokable} for {@code method}, which must be a member of {@code T}.
   *
   * @since 14.0
   */
  public final Invokable<T, Object> method(Method method) {
    checkArgument(of(method.getDeclaringClass()).isAssignableFrom(this),
        "%s not declared by %s", method, this);
    return new Invokable.MethodInvokable<T>(method) {
      @Override Type getGenericReturnType() {
        return resolveType(super.getGenericReturnType()).getType();
      }
      @Override Type[] getGenericParameterTypes() {
        return resolveInPlace(super.getGenericParameterTypes());
      }
      @Override Type[] getGenericExceptionTypes() {
        return resolveInPlace(super.getGenericExceptionTypes());
      }
      @Override public TypeToken<T> getOwnerType() {
        return TypeToken.this;
      }
      @Override public String toString() {
        return getOwnerType() + "." + super.toString();
      }
    };
  }

  /**
   * Returns the {@link Invokable} for {@code constructor}, which must be a member of {@code T}.
   *
   * @since 14.0
   */
  public final Invokable<T, T> constructor(Constructor<?> constructor) {
    checkArgument(constructor.getDeclaringClass() == getRawType(),
        "%s not declared by %s", constructor, getRawType());
    return new Invokable.ConstructorInvokable<T>(constructor) {
      @Override Type getGenericReturnType() {
        return resolveType(super.getGenericReturnType()).getType();
      }
      @Override Type[] getGenericParameterTypes() {
        return resolveInPlace(super.getGenericParameterTypes());
      }
      @Override Type[] getGenericExceptionTypes() {
        return resolveInPlace(super.getGenericExceptionTypes());
      }
      @Override public TypeToken<T> getOwnerType() {
        return TypeToken.this;
      }
      @Override public String toString() {
        return getOwnerType() + "(" + Joiner.on(", ").join(getGenericParameterTypes()) + ")";
      }
    };
  }

  /**
   * The set of interfaces and classes that {@code T} is or is a subtype of. {@link Object} is not
   * included in the set if this type is an interface.
   */
  public class TypeSet extends ForwardingSet<TypeToken<? super T>> implements Serializable {

    private transient ImmutableSet<TypeToken<? super T>> types;

    TypeSet() {}

    /** Returns the types that are interfaces implemented by this type. */
    public TypeSet interfaces() {
      return new InterfaceSet(this);
    }

    /** Returns the types that are classes. */
    public TypeSet classes() {
      return new ClassSet();
    }

    @Override protected Set<TypeToken<? super T>> delegate() {
      ImmutableSet<TypeToken<? super T>> filteredTypes = types;
      if (filteredTypes == null) {
        // Java has no way to express ? super T when we parameterize TypeToken vs. Class.
        @SuppressWarnings({"unchecked", "rawtypes"})
        ImmutableList<TypeToken<? super T>> collectedTypes = (ImmutableList)
            TypeCollector.FOR_GENERIC_TYPE.collectTypes(TypeToken.this);
        return (types = FluentIterable.from(collectedTypes)
                .filter(TypeFilter.IGNORE_TYPE_VARIABLE_OR_WILDCARD)
                .toSet());
      } else {
        return filteredTypes;
      }
    }

    /** Returns the raw types of the types in this set, in the same order. */
    public Set<Class<? super T>> rawTypes() {
      // Java has no way to express ? super T when we parameterize TypeToken vs. Class.
      @SuppressWarnings({"unchecked", "rawtypes"})
      ImmutableList<Class<? super T>> collectedTypes = (ImmutableList)
          TypeCollector.FOR_RAW_TYPE.collectTypes(getImmediateRawTypes());
      return ImmutableSet.copyOf(collectedTypes);
    }

    private static final long serialVersionUID = 0;
  }

  private final class InterfaceSet extends TypeSet {

    private transient final TypeSet allTypes;
    private transient ImmutableSet<TypeToken<? super T>> interfaces;

    InterfaceSet(TypeSet allTypes) {
      this.allTypes = allTypes;
    }

    @Override protected Set<TypeToken<? super T>> delegate() {
      ImmutableSet<TypeToken<? super T>> result = interfaces;
      if (result == null) {
        return (interfaces = FluentIterable.from(allTypes)
            .filter(TypeFilter.INTERFACE_ONLY)
            .toSet());
      } else {
        return result;
      }
    }

    @Override public TypeSet interfaces() {
      return this;
    }

    @Override public Set<Class<? super T>> rawTypes() {
      // Java has no way to express ? super T when we parameterize TypeToken vs. Class.
      @SuppressWarnings({"unchecked", "rawtypes"})
      ImmutableList<Class<? super T>> collectedTypes = (ImmutableList)
          TypeCollector.FOR_RAW_TYPE.collectTypes(getImmediateRawTypes());
      return FluentIterable.from(collectedTypes)
          .filter(new Predicate<Class<?>>() {
            @Override public boolean apply(Class<?> type) {
              return type.isInterface();
            }
          })
          .toSet();
    }

    @Override public TypeSet classes() {
      throw new UnsupportedOperationException("interfaces().classes() not supported.");
    }

    private Object readResolve() {
      return getTypes().interfaces();
    }

    private static final long serialVersionUID = 0;
  }

  private final class ClassSet extends TypeSet {

    private transient ImmutableSet<TypeToken<? super T>> classes;

    @Override protected Set<TypeToken<? super T>> delegate() {
      ImmutableSet<TypeToken<? super T>> result = classes;
      if (result == null) {
        @SuppressWarnings({"unchecked", "rawtypes"})
        ImmutableList<TypeToken<? super T>> collectedTypes = (ImmutableList)
            TypeCollector.FOR_GENERIC_TYPE.classesOnly().collectTypes(TypeToken.this);
        return (classes = FluentIterable.from(collectedTypes)
            .filter(TypeFilter.IGNORE_TYPE_VARIABLE_OR_WILDCARD)
            .toSet());
      } else {
        return result;
      }
    }

    @Override public TypeSet classes() {
      return this;
    }

    @Override public Set<Class<? super T>> rawTypes() {
      // Java has no way to express ? super T when we parameterize TypeToken vs. Class.
      @SuppressWarnings({"unchecked", "rawtypes"})
      ImmutableList<Class<? super T>> collectedTypes = (ImmutableList)
          TypeCollector.FOR_RAW_TYPE.classesOnly().collectTypes(getImmediateRawTypes());
      return ImmutableSet.copyOf(collectedTypes);
    }

    @Override public TypeSet interfaces() {
      throw new UnsupportedOperationException("classes().interfaces() not supported.");
    }

    private Object readResolve() {
      return getTypes().classes();
    }

    private static final long serialVersionUID = 0;
  }

  private enum TypeFilter implements Predicate<TypeToken<?>> {

    IGNORE_TYPE_VARIABLE_OR_WILDCARD {
      @Override public boolean apply(TypeToken<?> type) {
        return !(type.runtimeType instanceof TypeVariable
            || type.runtimeType instanceof WildcardType);
      }
    },
    INTERFACE_ONLY {
      @Override public boolean apply(TypeToken<?> type) {
        return type.getRawType().isInterface();
      }
    }
  }

  /**
   * Returns true if {@code o} is another {@code TypeToken} that represents the same {@link Type}.
   */
  @Override public boolean equals(@Nullable Object o) {
    if (o instanceof TypeToken) {
      TypeToken<?> that = (TypeToken<?>) o;
      return runtimeType.equals(that.runtimeType);
    }
    return false;
  }

  @Override public int hashCode() {
    return runtimeType.hashCode();
  }

  @Override public String toString() {
    return Types.toString(runtimeType);
  }

  /** Implemented to support serialization of subclasses. */
  protected Object writeReplace() {
    // TypeResolver just transforms the type to our own impls that are Serializable
    // except TypeVariable.
    return of(new TypeResolver().resolveType(runtimeType));
  }

  /**
   * Ensures that this type token doesn't contain type variables, which can cause unchecked type
   * errors for callers like {@link TypeToInstanceMap}.
   */
  final TypeToken<T> rejectTypeVariables() {
    new TypeVisitor() {
      @Override void visitTypeVariable(TypeVariable<?> type) {
        throw new IllegalArgumentException(
            runtimeType + "contains a type variable and is not safe for the operation");
      }
      @Override void visitWildcardType(WildcardType type) {
        visit(type.getLowerBounds());
        visit(type.getUpperBounds());
      }
      @Override void visitParameterizedType(ParameterizedType type) {
        visit(type.getActualTypeArguments());
        visit(type.getOwnerType());
      }
      @Override void visitGenericArrayType(GenericArrayType type) {
        visit(type.getGenericComponentType());
      }
    }.visit(runtimeType);
    return this;
  }

  private static boolean isAssignable(Type from, Type to) {
    if (to.equals(from)) {
      return true;
    }
    if (to instanceof WildcardType) {
      return isAssignableToWildcardType(from, (WildcardType) to);
    }
    // if "from" is type variable, it's assignable if any of its "extends"
    // bounds is assignable to "to".
    if (from instanceof TypeVariable) {
      return isAssignableFromAny(((TypeVariable<?>) from).getBounds(), to);
    }
    // if "from" is wildcard, it'a assignable to "to" if any of its "extends"
    // bounds is assignable to "to".
    if (from instanceof WildcardType) {
      return isAssignableFromAny(((WildcardType) from).getUpperBounds(), to);
    }
    if (from instanceof GenericArrayType) {
      return isAssignableFromGenericArrayType((GenericArrayType) from, to);
    }
    // Proceed to regular Type assignability check
    if (to instanceof Class) {
      return isAssignableToClass(from, (Class<?>) to);
    } else if (to instanceof ParameterizedType) {
      return isAssignableToParameterizedType(from, (ParameterizedType) to);
    } else if (to instanceof GenericArrayType) {
      return isAssignableToGenericArrayType(from, (GenericArrayType) to);
    } else { // to instanceof TypeVariable
      return false;
    }
  }

  private static boolean isAssignableFromAny(Type[] fromTypes, Type to) {
    for (Type from : fromTypes) {
      if (isAssignable(from, to)) {
        return true;
      }
    }
    return false;
  }

  private static boolean isAssignableToClass(Type from, Class<?> to) {
    return to.isAssignableFrom(getRawType(from));
  }

  private static boolean isAssignableToWildcardType(
      Type from, WildcardType to) {
    // if "to" is <? extends Foo>, "from" can be:
    // Foo, SubFoo, <? extends Foo>, <? extends SubFoo>, <T extends Foo> or
    // <T extends SubFoo>.
    // if "to" is <? super Foo>, "from" can be:
    // Foo, SuperFoo, <? super Foo> or <? super SuperFoo>.
    return isAssignable(from, supertypeBound(to)) && isAssignableBySubtypeBound(from, to);
  }

  private static boolean isAssignableBySubtypeBound(Type from, WildcardType to) {
    Type toSubtypeBound = subtypeBound(to);
    if (toSubtypeBound == null) {
      return true;
    }
    Type fromSubtypeBound = subtypeBound(from);
    if (fromSubtypeBound == null) {
      return false;
    }
    return isAssignable(toSubtypeBound, fromSubtypeBound);
  }

  private static boolean isAssignableToParameterizedType(Type from, ParameterizedType to) {
    Class<?> matchedClass = getRawType(to);
    if (!matchedClass.isAssignableFrom(getRawType(from))) {
      return false;
    }
    Type[] typeParams = matchedClass.getTypeParameters();
    Type[] toTypeArgs = to.getActualTypeArguments();
    TypeToken<?> fromTypeToken = of(from);
    for (int i = 0; i < typeParams.length; i++) {
      // If "to" is "List<? extends CharSequence>"
      // and "from" is StringArrayList,
      // First step is to figure out StringArrayList "is-a" List<E> and <E> is
      // String.
      // typeParams[0] is E and fromTypeToken.get(typeParams[0]) will resolve to
      // String.
      // String is then matched against <? extends CharSequence>.
      Type fromTypeArg = fromTypeToken.resolveType(typeParams[i]).runtimeType;
      if (!matchTypeArgument(fromTypeArg, toTypeArgs[i])) {
        return false;
      }
    }
    return true;
  }

  private static boolean isAssignableToGenericArrayType(Type from, GenericArrayType to) {
    if (from instanceof Class) {
      Class<?> fromClass = (Class<?>) from;
      if (!fromClass.isArray()) {
        return false;
      }
      return isAssignable(fromClass.getComponentType(), to.getGenericComponentType());
    } else if (from instanceof GenericArrayType) {
      GenericArrayType fromArrayType = (GenericArrayType) from;
      return isAssignable(fromArrayType.getGenericComponentType(), to.getGenericComponentType());
    } else {
      return false;
    }
  }

  private static boolean isAssignableFromGenericArrayType(GenericArrayType from, Type to) {
    if (to instanceof Class) {
      Class<?> toClass = (Class<?>) to;
      if (!toClass.isArray()) {
        return toClass == Object.class; // any T[] is assignable to Object
      }
      return isAssignable(from.getGenericComponentType(), toClass.getComponentType());
    } else if (to instanceof GenericArrayType) {
      GenericArrayType toArrayType = (GenericArrayType) to;
      return isAssignable(from.getGenericComponentType(), toArrayType.getGenericComponentType());
    } else {
      return false;
    }
  }

  private static boolean matchTypeArgument(Type from, Type to) {
    if (from.equals(to)) {
      return true;
    }
    if (to instanceof WildcardType) {
      return isAssignableToWildcardType(from, (WildcardType) to);
    }
    return false;
  }

  private static Type supertypeBound(Type type) {
    if (type instanceof WildcardType) {
      return supertypeBound((WildcardType) type);
    }
    return type;
  }

  private static Type supertypeBound(WildcardType type) {
    Type[] upperBounds = type.getUpperBounds();
    if (upperBounds.length == 1) {
      return supertypeBound(upperBounds[0]);
    } else if (upperBounds.length == 0) {
      return Object.class;
    } else {
      throw new AssertionError(
          "There should be at most one upper bound for wildcard type: " + type);
    }
  }

  @Nullable private static Type subtypeBound(Type type) {
    if (type instanceof WildcardType) {
      return subtypeBound((WildcardType) type);
    } else {
      return type;
    }
  }

  @Nullable private static Type subtypeBound(WildcardType type) {
    Type[] lowerBounds = type.getLowerBounds();
    if (lowerBounds.length == 1) {
      return subtypeBound(lowerBounds[0]);
    } else if (lowerBounds.length == 0) {
      return null;
    } else {
      throw new AssertionError(
          "Wildcard should have at most one lower bound: " + type);
    }
  }

  @VisibleForTesting static Class<?> getRawType(Type type) {
    // For wildcard or type variable, the first bound determines the runtime type.
    return getRawTypes(type).iterator().next();
  }

  @VisibleForTesting static ImmutableSet<Class<?>> getRawTypes(Type type) {
    checkNotNull(type);
    final ImmutableSet.Builder<Class<?>> builder = ImmutableSet.builder();
    new TypeVisitor() {
      @Override void visitTypeVariable(TypeVariable<?> t) {
        visit(t.getBounds());
      }
      @Override void visitWildcardType(WildcardType t) {
        visit(t.getUpperBounds());
      }
      @Override void visitParameterizedType(ParameterizedType t) {
        builder.add((Class<?>) t.getRawType());
      }
      @Override void visitClass(Class<?> t) {
        builder.add(t);
      }
      @Override void visitGenericArrayType(GenericArrayType t) {
        builder.add(Types.getArrayClass(getRawType(t.getGenericComponentType())));
      }

    }.visit(type);
    return builder.build();
  }

  /**
   * Returns the type token representing the generic type declaration of {@code cls}. For example:
   * {@code TypeToken.getGenericType(Iterable.class)} returns {@code Iterable<T>}.
   *
   * <p>If {@code cls} isn't parameterized and isn't a generic array, the type token of the class is
   * returned.
   */
  @VisibleForTesting static <T> TypeToken<? extends T> toGenericType(Class<T> cls) {
    if (cls.isArray()) {
      Type arrayOfGenericType = Types.newArrayType(
          // If we are passed with int[].class, don't turn it to GenericArrayType
          toGenericType(cls.getComponentType()).runtimeType);
      @SuppressWarnings("unchecked") // array is covariant
      TypeToken<? extends T> result = (TypeToken<? extends T>) of(arrayOfGenericType);
      return result;
    }
    TypeVariable<Class<T>>[] typeParams = cls.getTypeParameters();
    if (typeParams.length > 0) {
      @SuppressWarnings("unchecked") // Like, it's Iterable<T> for Iterable.class
      TypeToken<? extends T> type = (TypeToken<? extends T>)
          of(Types.newParameterizedType(cls, typeParams));
      return type;
    } else {
      return of(cls);
    }
  }

  private TypeToken<? super T> getSupertypeFromUpperBounds(
      Class<? super T> supertype, Type[] upperBounds) {
    for (Type upperBound : upperBounds) {
      @SuppressWarnings("unchecked") // T's upperbound is <? super T>.
      TypeToken<? super T> bound = (TypeToken<? super T>) of(upperBound);
      if (of(supertype).isAssignableFrom(bound)) {
        @SuppressWarnings({"rawtypes", "unchecked"}) // guarded by the isAssignableFrom check.
        TypeToken<? super T> result = bound.getSupertype((Class) supertype);
        return result;
      }
    }
    throw new IllegalArgumentException(supertype + " isn't a super type of " + this);
  }

  private TypeToken<? extends T> getSubtypeFromLowerBounds(Class<?> subclass, Type[] lowerBounds) {
    for (Type lowerBound : lowerBounds) {
      @SuppressWarnings("unchecked") // T's lower bound is <? extends T>
      TypeToken<? extends T> bound = (TypeToken<? extends T>) of(lowerBound);
      // Java supports only one lowerbound anyway.
      return bound.getSubtype(subclass);
    }
    throw new IllegalArgumentException(subclass + " isn't a subclass of " + this);
  }

  private TypeToken<? super T> getArraySupertype(Class<? super T> supertype) {
    // with component type, we have lost generic type information
    // Use raw type so that compiler allows us to call getSupertype()
    @SuppressWarnings("rawtypes")
    TypeToken componentType = checkNotNull(getComponentType(),
        "%s isn't a super type of %s", supertype, this);
    // array is covariant. component type is super type, so is the array type.
    @SuppressWarnings("unchecked") // going from raw type back to generics
    TypeToken<?> componentSupertype = componentType.getSupertype(supertype.getComponentType());
    @SuppressWarnings("unchecked") // component type is super type, so is array type.
    TypeToken<? super T> result = (TypeToken<? super T>)
        // If we are passed with int[].class, don't turn it to GenericArrayType
        of(newArrayClassOrGenericArrayType(componentSupertype.runtimeType));
    return result;
  }

  private TypeToken<? extends T> getArraySubtype(Class<?> subclass) {
    // array is covariant. component type is subtype, so is the array type.
    TypeToken<?> componentSubtype = getComponentType()
        .getSubtype(subclass.getComponentType());
    @SuppressWarnings("unchecked") // component type is subtype, so is array type.
    TypeToken<? extends T> result = (TypeToken<? extends T>)
        // If we are passed with int[].class, don't turn it to GenericArrayType
        of(newArrayClassOrGenericArrayType(componentSubtype.runtimeType));
    return result;
  }

  private Type resolveTypeArgsForSubclass(Class<?> subclass) {
    if (runtimeType instanceof Class) {
      // no resolution needed
      return subclass;
    }
    // class Base<A, B> {}
    // class Sub<X, Y> extends Base<X, Y> {}
    // Base<String, Integer>.subtype(Sub.class):

    // Sub<X, Y>.getSupertype(Base.class) => Base<X, Y>
    // => X=String, Y=Integer
    // => Sub<X, Y>=Sub<String, Integer>
    TypeToken<?> genericSubtype = toGenericType(subclass);
    @SuppressWarnings({"rawtypes", "unchecked"}) // subclass isn't <? extends T>
    Type supertypeWithArgsFromSubtype = genericSubtype
        .getSupertype((Class) getRawType())
        .runtimeType;
    return new TypeResolver().where(supertypeWithArgsFromSubtype, runtimeType)
        .resolveType(genericSubtype.runtimeType);
  }

  /**
   * Creates an array class if {@code componentType} is a class, or else, a
   * {@link GenericArrayType}. This is what Java7 does for generic array type
   * parameters.
   */
  private static Type newArrayClassOrGenericArrayType(Type componentType) {
    return Types.JavaVersion.JAVA7.newArrayType(componentType);
  }

  private static final class SimpleTypeToken<T> extends TypeToken<T> {

    SimpleTypeToken(Type type) {
      super(type);
    }

    private static final long serialVersionUID = 0;
  }

  /**
   * Collects parent types from a sub type.
   *
   * @param <K> The type "kind". Either a TypeToken, or Class.
   */
  private abstract static class TypeCollector<K> {

    static final TypeCollector<TypeToken<?>> FOR_GENERIC_TYPE =
        new TypeCollector<TypeToken<?>>() {
          @Override Class<?> getRawType(TypeToken<?> type) {
            return type.getRawType();
          }

          @Override Iterable<? extends TypeToken<?>> getInterfaces(TypeToken<?> type) {
            return type.getGenericInterfaces();
          }

          @Nullable
          @Override TypeToken<?> getSuperclass(TypeToken<?> type) {
            return type.getGenericSuperclass();
          }
        };

    static final TypeCollector<Class<?>> FOR_RAW_TYPE =
        new TypeCollector<Class<?>>() {
          @Override Class<?> getRawType(Class<?> type) {
            return type;
          }

          @Override Iterable<? extends Class<?>> getInterfaces(Class<?> type) {
            return Arrays.asList(type.getInterfaces());
          }

          @Nullable
          @Override Class<?> getSuperclass(Class<?> type) {
            return type.getSuperclass();
          }
        };

    /** For just classes, we don't have to traverse interfaces. */
    final TypeCollector<K> classesOnly() {
      return new ForwardingTypeCollector<K>(this) {
        @Override Iterable<? extends K> getInterfaces(K type) {
          return ImmutableSet.of();
        }
        @Override ImmutableList<K> collectTypes(Iterable<? extends K> types) {
          ImmutableList.Builder<K> builder = ImmutableList.builder();
          for (K type : types) {
            if (!getRawType(type).isInterface()) {
              builder.add(type);
            }
          }
          return super.collectTypes(builder.build());
        }
      };
    }

    final ImmutableList<K> collectTypes(K type) {
      return collectTypes(ImmutableList.of(type));
    }

    ImmutableList<K> collectTypes(Iterable<? extends K> types) {
      // type -> order number. 1 for Object, 2 for anything directly below, so on so forth.
      Map<K, Integer> map = Maps.newHashMap();
      for (K type : types) {
        collectTypes(type, map);
      }
      return sortKeysByValue(map, Ordering.natural().reverse());
    }

    /** Collects all types to map, and returns the total depth from T up to Object. */
    private int collectTypes(K type, Map<? super K, Integer> map) {
      Integer existing = map.get(this);
      if (existing != null) {
        // short circuit: if set contains type it already contains its supertypes
        return existing;
      }
      int aboveMe = getRawType(type).isInterface()
          ? 1 // interfaces should be listed before Object
          : 0;
      for (K interfaceType : getInterfaces(type)) {
        aboveMe = Math.max(aboveMe, collectTypes(interfaceType, map));
      }
      K superclass = getSuperclass(type);
      if (superclass != null) {
        aboveMe = Math.max(aboveMe, collectTypes(superclass, map));
      }
      /*
       * TODO(benyu): should we include Object for interface?
       * Also, CharSequence[] and Object[] for String[]?
       *
       */
      map.put(type, aboveMe + 1);
      return aboveMe + 1;
    }

    private static <K, V> ImmutableList<K> sortKeysByValue(
        final Map<K, V> map, final Comparator<? super V> valueComparator) {
      Ordering<K> keyOrdering = new Ordering<K>() {
        @Override public int compare(K left, K right) {
          return valueComparator.compare(map.get(left), map.get(right));
        }
      };
      return keyOrdering.immutableSortedCopy(map.keySet());
    }

    abstract Class<?> getRawType(K type);
    abstract Iterable<? extends K> getInterfaces(K type);
    @Nullable abstract K getSuperclass(K type);

    private static class ForwardingTypeCollector<K> extends TypeCollector<K> {

      private final TypeCollector<K> delegate;

      ForwardingTypeCollector(TypeCollector<K> delegate) {
        this.delegate = delegate;
      }

      @Override Class<?> getRawType(K type) {
        return delegate.getRawType(type);
      }

      @Override Iterable<? extends K> getInterfaces(K type) {
        return delegate.getInterfaces(type);
      }

      @Override K getSuperclass(K type) {
        return delegate.getSuperclass(type);
      }
    }
  }
}