001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *      https://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.lang3.reflect;
018
019import java.lang.reflect.Type;
020import java.lang.reflect.TypeVariable;
021
022import org.apache.commons.lang3.Validate;
023
024/**
025 * Type literal comparable to {@code javax.enterprise.util.TypeLiteral},
026 * made generally available outside the JEE context. Allows the passing around of
027 * a "token" that represents a type in a typesafe manner, as opposed to
028 * passing the (non-parameterized) {@link Type} object itself. Consider:
029 * <p>
030 * You might see such a typesafe API as:
031 * <pre>{@code
032 * class Typesafe {
033 *   <T> T obtain(Class<T> type, ...);
034 * }
035 * }
036 * </pre>
037 * Consumed in the manner of:
038 * <pre>
039 * Foo foo = typesafe.obtain(Foo.class, ...);
040 * </pre>
041 * Yet, you run into problems when you want to do this with a parameterized type:
042 * <pre>{@code
043 * List<String> listOfString = typesafe.obtain(List.class, ...); // could only give us a raw List
044 * }</pre>
045 * {@link java.lang.reflect.Type} might provide some value:
046 * <pre>{@code
047 * Type listOfStringType = ...; // firstly, how to obtain this? Doable, but not straightforward.
048 * List<String> listOfString = (List<String>) typesafe.obtain(listOfStringType, ...); // nongeneric Type would necessitate a cast
049 * }</pre>
050 * The "type literal" concept was introduced to provide an alternative, i.e.:
051 * <pre>{@code
052 * class Typesafe {
053 *   <T> T obtain(TypeLiteral<T> type, ...);
054 * }
055 * }</pre>
056 * Consuming code looks like:
057 * <pre>{@code
058 * List<String> listOfString = typesafe.obtain(new TypeLiteral<List<String>>() {}, ...);
059 * }</pre>
060 * <p>
061 * This has the effect of "jumping up" a level to tie a {@link java.lang.reflect.Type}
062 * to a type variable while simultaneously making it short work to obtain a
063 * {@link Type} instance for any given type, inline.
064 * </p>
065 * <p>Additionally {@link TypeLiteral} implements the {@link Typed} interface which
066 * is a generalization of this concept, and which may be implemented in custom classes.
067 * It is suggested that APIs be defined in terms of the interface, in the following manner:
068 * </p>
069 * <pre>{@code
070 *   <T> T obtain(Typed<T> typed, ...);
071 * }</pre>
072 *
073 * @param <T> the type
074 * @since 3.2
075 */
076public abstract class TypeLiteral<T> implements Typed<T> {
077
078    @SuppressWarnings("rawtypes")
079    private static final TypeVariable<Class<TypeLiteral>> T = TypeLiteral.class.getTypeParameters()[0];
080
081    /**
082     * Represented type.
083     */
084    public final Type value;
085
086    private final String toString;
087
088    /**
089     * Constructs a new instance.
090     */
091    protected TypeLiteral() {
092        this.value =
093            Validate.notNull(TypeUtils.getTypeArguments(getClass(), TypeLiteral.class).get(T),
094                "%s does not assign type parameter %s", getClass(), TypeUtils.toLongString(T));
095
096        this.toString = String.format("%s<%s>", TypeLiteral.class.getSimpleName(), TypeUtils.toString(value));
097    }
098
099    @Override
100    public final boolean equals(final Object obj) {
101        if (obj == this) {
102            return true;
103        }
104        if (!(obj instanceof TypeLiteral)) {
105            return false;
106        }
107        final TypeLiteral<?> other = (TypeLiteral<?>) obj;
108        return TypeUtils.equals(value, other.value);
109    }
110
111    @Override
112    public Type getType() {
113        return value;
114    }
115
116    @Override
117    public int hashCode() {
118        return 37 << 4 | value.hashCode();
119    }
120
121    @Override
122    public String toString() {
123        return toString;
124    }
125}