1 /*
2 * Copyright (C) 2011 The Guava Authors
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
5 * in compliance with the License. You may obtain a copy of the License at
6 *
7 * http://www.apache.org/licenses/LICENSE-2.0
8 *
9 * Unless required by applicable law or agreed to in writing, software distributed under the License
10 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
11 * or implied. See the License for the specific language governing permissions and limitations under
12 * the License.
13 */
14
15 package com.google.common.reflect;
16
17 import static com.google.common.base.Preconditions.checkArgument;
18
19 import com.google.common.annotations.Beta;
20 import java.lang.reflect.Type;
21 import java.lang.reflect.TypeVariable;
22 import javax.annotation.CheckForNull;
23
24 /**
25 * Captures a free type variable that can be used in {@link TypeToken#where}. For example:
26 *
27 * <pre>{@code
28 * static <T> TypeToken<List<T>> listOf(Class<T> elementType) {
29 * return new TypeToken<List<T>>() {}
30 * .where(new TypeParameter<T>() {}, elementType);
31 * }
32 * }</pre>
33 *
34 * @author Ben Yu
35 * @since 12.0
36 */
37 @Beta
38 @ElementTypesAreNonnullByDefault
39 /*
40 * A nullable bound would let users create a TypeParameter instance for a parameter with a nullable
41 * bound. However, it would also let them create `new TypeParameter<@Nullable T>() {}`, which
42 * wouldn't behave as users might expect. Additionally, it's not clear how the TypeToken API could
43 * support even a "normal" `TypeParameter<T>` when `<T>` has a nullable bound. (See the discussion
44 * on TypeToken.where.) So, in the interest of failing fast and encouraging the user to switch to a
45 * non-null bound if possible, let's require a non-null bound here.
46 *
47 * TODO(cpovirk): Elaborate on "wouldn't behave as users might expect."
48 */
49 public abstract class TypeParameter<T> extends TypeCapture<T> {
50
51 final TypeVariable<?> typeVariable;
52
53 protected TypeParameter() {
54 Type type = capture();
55 checkArgument(type instanceof TypeVariable, "%s should be a type variable.", type);
56 this.typeVariable = (TypeVariable<?>) type;
57 }
58
59 @Override
60 public final int hashCode() {
61 return typeVariable.hashCode();
62 }
63
64 @Override
65 public final boolean equals(@CheckForNull Object o) {
66 if (o instanceof TypeParameter) {
67 TypeParameter<?> that = (TypeParameter<?>) o;
68 return typeVariable.equals(that.typeVariable);
69 }
70 return false;
71 }
72
73 @Override
74 public String toString() {
75 return typeVariable.toString();
76 }
77 }