    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 = Validate.notNull(TypeUtils.getTypeArguments(getClass(), TypeLiteral.class).get(T),
    093                "%s does not assign type parameter %s", getClass(), TypeUtils.toLongString(T));
    094        this.toString = String.format("%s<%s>", TypeLiteral.class.getSimpleName(), TypeUtils.toString(value));
    095    }
    096
    097    @Override
    098    public final boolean equals(final Object obj) {
    099        if (obj == this) {
    100            return true;
    101        }
    102        if (!(obj instanceof TypeLiteral)) {
    103            return false;
    104        }
    105        final TypeLiteral<?> other = (TypeLiteral<?>) obj;
    106        return TypeUtils.equals(value, other.value);
    107    }
    108
    109    @Override
    110    public Type getType() {
    111        return value;
    112    }
    113
    114    @Override
    115    public int hashCode() {
    116        return 37 << 4 | value.hashCode();
    117    }
    118
    119    @Override
    120    public String toString() {
    121        return toString;
    122    }
    123}