001/*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.reflect;
018
019import static com.google.common.base.Preconditions.checkArgument;
020import static com.google.common.base.Preconditions.checkNotNull;
021
022import com.google.common.annotations.Beta;
023
024import java.lang.reflect.Type;
025import java.lang.reflect.TypeVariable;
026
027/**
028 * Captures a free type variable that can be used in {@link TypeToken#where}.
029 * For example: <pre>   {@code
030 *
031 *   static <T> TypeToken<List<T>> listOf(Class<T> elementType) {
032 *     return new TypeToken<List<T>>() {}
033 *         .where(new TypeParameter<T>() {}, elementType);
034 *   }
035 * }</pre>
036 *
037 * @author Ben Yu
038 * @since 12.0
039 */
040@Beta
041public abstract class TypeParameter<T> extends TypeCapture<T> {
042
043  final TypeVariable<?> typeVariable;
044
045  private TypeParameter(TypeVariable<?> typeVariable) {
046    this.typeVariable = checkNotNull(typeVariable);
047  }
048
049  protected TypeParameter() {
050    Type type = capture();
051    checkArgument(type instanceof TypeVariable, "%s should be a type variable.", type);
052    this.typeVariable = (TypeVariable<?>) type;
053  }
054
055  @Override public final int hashCode() {
056    return typeVariable.hashCode();
057  }
058
059  @Override public final boolean equals(Object o) {
060    if (o instanceof TypeParameter) {
061      TypeParameter<?> that = (TypeParameter<?>) o;
062      return typeVariable.equals(that.typeVariable);
063    }
064    return false;
065  }
066
067  @Override public String toString() {
068    return typeVariable.toString();
069  }
070}