001/*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.reflect;
016
017import static com.google.common.base.Preconditions.checkArgument;
018
019import java.lang.reflect.Type;
020import java.lang.reflect.TypeVariable;
021import javax.annotation.CheckForNull;
022
023/**
024 * Captures a free type variable that can be used in {@link TypeToken#where}. For example:
025 *
026 * <pre>{@code
027 * static <T> TypeToken<List<T>> listOf(Class<T> elementType) {
028 *   return new TypeToken<List<T>>() {}
029 *       .where(new TypeParameter<T>() {}, elementType);
030 * }
031 * }</pre>
032 *
033 * @author Ben Yu
034 * @since 12.0
035 */
036@ElementTypesAreNonnullByDefault
037/*
038 * A nullable bound would let users create a TypeParameter instance for a parameter with a nullable
039 * bound. However, it would also let them create `new TypeParameter<@Nullable T>() {}`, which
040 * wouldn't behave as users might expect. Additionally, it's not clear how the TypeToken API could
041 * support even a "normal" `TypeParameter<T>` when `<T>` has a nullable bound. (See the discussion
042 * on TypeToken.where.) So, in the interest of failing fast and encouraging the user to switch to a
043 * non-null bound if possible, let's require a non-null bound here.
044 *
045 * TODO(cpovirk): Elaborate on "wouldn't behave as users might expect."
046 */
047public abstract class TypeParameter<T> extends TypeCapture<T> {
048
049  final TypeVariable<?> typeVariable;
050
051  protected TypeParameter() {
052    Type type = capture();
053    checkArgument(type instanceof TypeVariable, "%s should be a type variable.", type);
054    this.typeVariable = (TypeVariable<?>) type;
055  }
056
057  @Override
058  public final int hashCode() {
059    return typeVariable.hashCode();
060  }
061
062  @Override
063  public final boolean equals(@CheckForNull Object o) {
064    if (o instanceof TypeParameter) {
065      TypeParameter<?> that = (TypeParameter<?>) o;
066      return typeVariable.equals(that.typeVariable);
067    }
068    return false;
069  }
070
071  @Override
072  public String toString() {
073    return typeVariable.toString();
074  }
075}