Package com.google.common.reflect

Class TypeToken<T>

  • All Implemented Interfaces:
    Serializable
    @Beta
public abstract class TypeToken<T>
extends Object
implements Serializable
    A Type with generics.

    Operations that are otherwise only available in Class are implemented to support Type, for example isSubtypeOf(com.google.common.reflect.TypeToken<?>), isArray() and getComponentType(). It also provides additional utilities such as getTypes(), resolveType(java.lang.reflect.Type), etc.

    There are three ways to get a TypeToken instance:

    • Wrap a Type obtained via reflection. For example: TypeToken.of(method.getGenericReturnType()).
    • Capture a generic type with a (usually anonymous) subclass. For example: 
      
 new TypeToken<List<String>>() {}

      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 <T> type variable of the listType() method signature; while <String> is lost in erasure: 

      
 class Util {
   static <T> TypeToken<List<T>> listType() {
     return new TypeToken<List<T>>() {};
   }
 }

 TypeToken<List<String>> stringListType = Util.<String>listType();
    • 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: 
      
 abstract class IKnowMyType<T> {
   TypeToken<T> type = new TypeToken<T>(getClass()) {};
 }
 new IKnowMyType<String>() {}.type => String

    TypeToken is serializable when no type variable is contained in the type.

    Note to Guice users: TypeToken is similar to Guice's TypeLiteral class except that it is serializable and offers numerous additional utility methods.

    Since:
    12.0
    Author:
    Bob Lee, Sven Mawson, Ben Yu
    See Also:
    Serialized Form

    • Nested Class Summary

      Nested Classes 
      Modifier and Type Class Description
      class  TypeToken.TypeSet
      The set of interfaces and classes that T is or is a subtype of.

    • Constructor Summary

      Constructors 
      Modifier Constructor Description
      protected TypeToken()
      Constructs a new type token of T.
      protected TypeToken​(Class<?> declaringClass)
      Constructs a new type token of T while resolving free type variables in the context of declaringClass.

    • Constructor Detail

      • TypeToken

        protected TypeToken()
        Constructs a new type token of T.

        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.

        For example: 

        
 TypeToken<List<String>> t = new TypeToken<List<String>>() {};

      • TypeToken

        protected TypeToken​(Class<?> declaringClass)
        Constructs a new type token of T while resolving free type variables in the context of declaringClass.

        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.

        For example: 

        
 abstract class IKnowMyType<T> {
   TypeToken<T> getMyType() {
     return new TypeToken<T>(getClass()) {};
   }
 }

 new IKnowMyType<String>() {}.getMyType() => String

    • Method Detail

      • of

        public static <T> TypeToken<T> of​(Class<T> type)
        Returns an instance of type token that wraps type.

      • of

        public static TypeToken<?> of​(Type type)
        Returns an instance of type token that wraps type.

      • getRawType

        public final Class<? super T> getRawType()
        Returns the raw type of T. Formally speaking, if T is returned by Method.getGenericReturnType(), the raw type is what's returned by Method.getReturnType() of the same method object. Specifically:
        • If T is a Class itself, T itself is returned.
        • If T is a ParameterizedType, the raw type of the parameterized type is returned.
        • If T is a GenericArrayType, the returned type is the corresponding array class. For example: List<Integer>[] => List[].
        • If T is a type variable or a wildcard type, the raw type of the first upper bound is returned. For example: <X extends Foo> => Foo.

      • getType

        public final Type getType()
        Returns the represented type.

      • where

        public final <X> TypeToken<T> where​(TypeParameter<X> typeParam,
                                    TypeToken<X> typeArg)
        Returns a new TypeToken where type variables represented by typeParam are substituted by typeArg. For example, it can be used to construct Map<K, V> for any K and V type: 
        
 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);
 }
        Type Parameters:
        X - The parameter type
        Parameters:
        typeParam - the parameter type variable
        typeArg - the actual type to substitute

      • where

        public final <X> TypeToken<T> where​(TypeParameter<X> typeParam,
                                    Class<X> typeArg)
        Returns a new TypeToken where type variables represented by typeParam are substituted by typeArg. For example, it can be used to construct Map<K, V> for any K and V type: 
        
 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);
 }
        Type Parameters:
        X - The parameter type
        Parameters:
        typeParam - the parameter type variable
        typeArg - the actual type to substitute

      • resolveType

        public final TypeToken<?> resolveType​(Type type)
        Resolves the given type against the type context represented by this type. For example: 
        
 new TypeToken<List<String>>() {}.resolveType(
     List.class.getMethod("get", int.class).getGenericReturnType())
 => String.class

      • getTypes

        public final TypeToken.TypeSet getTypes()
        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.

        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.

        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).

      • getSupertype

        public final TypeToken<? super T> getSupertype​(Class<? super T> superclass)
        Returns the generic form of superclass. For example, if this is ArrayList<String>, Iterable<String> is returned given the input Iterable.class.

      • getSubtype

        public final TypeToken<? extends T> getSubtype​(Class<?> subclass)
        Returns subtype of this with subclass as the raw class. For example, if this is Iterable<String> and subclass is List, List<String> is returned.

      • isSupertypeOf

        public final boolean isSupertypeOf​(Type type)
        Returns true if this type is a supertype of the given type. "Supertype" is defined according to the rules for type arguments introduced with Java generics.
        Since:
        19.0

      • isSubtypeOf

        public final boolean isSubtypeOf​(Type supertype)
        Returns true if this type is a subtype of the given type. "Subtype" is defined according to the rules for type arguments introduced with Java generics.
        Since:
        19.0

      • isArray

        public final boolean isArray()
        Returns true if this type is known to be an array type, such as int[], T[], <? extends Map<String, Integer>[]> etc.

      • isPrimitive

        public final boolean isPrimitive()
        Returns true if this type is one of the nine primitive types (including void).
        Since:
        15.0

      • wrap

        public final TypeToken<T> wrap()
        Returns the corresponding wrapper type if this is a primitive type; otherwise returns this itself. Idempotent.
        Since:
        15.0

      • unwrap

        public final TypeToken<T> unwrap()
        Returns the corresponding primitive type if this is a wrapper type; otherwise returns this itself. Idempotent.
        Since:
        15.0

      • getComponentType

        public final @Nullable TypeToken<?> getComponentType()
        Returns the array component type if this type represents an array (int[], T[], <? extends Map<String, Integer>[]> etc.), or else null is returned.

      • equals

        public boolean equals​(@Nullable Object o)
        Returns true if o is another TypeToken that represents the same Type.
        Overrides:
        equals in class Object
        Parameters:
        o - the reference object with which to compare.
        Returns:
        true if this object is the same as the obj argument; false otherwise.
        See Also:
        Object.hashCode(), HashMap

      • hashCode

        public int hashCode()
        Description copied from class: java.lang.Object
        Returns a hash code value for the object. This method is supported for the benefit of hash tables such as those provided by HashMap.

        The general contract of hashCode is:

        • Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.
        • If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.
        • It is not required that if two objects are unequal according to the Object.equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

        As much as is reasonably practical, the hashCode method defined by class Object does return distinct integers for distinct objects. (The hashCode may or may not be implemented as some function of an object's memory address at some point in time.)

        Overrides:
        hashCode in class Object
        Returns:
        a hash code value for this object.
        See Also:
        Object.equals(java.lang.Object), System.identityHashCode(java.lang.Object)

      • toString

        public String toString()
        Description copied from class: java.lang.Object
        Returns a string representation of the object. In general, the toString method returns a string that "textually represents" this object. The result should be a concise but informative representation that is easy for a person to read. It is recommended that all subclasses override this method.

        The toString method for class Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of: 

         getClass().getName() + '@' + Integer.toHexString(hashCode())
        Overrides:
        toString in class Object
        Returns:
        a string representation of the object.

      • writeReplace

        protected Object writeReplace()
        Implemented to support serialization of subclasses.