001/*
002 * Copyright (C) 2007 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.base;
016
017import com.google.common.annotations.GwtCompatible;
018import org.checkerframework.checker.nullness.qual.Nullable;
019
020/**
021 * A class that can supply objects of a single type; a pre-Java-8 version of {@link
022 * java.util.function.Supplier java.util.function.Supplier}. Semantically, this could be a factory,
023 * generator, builder, closure, or something else entirely. No guarantees are implied by this
024 * interface.
025 *
026 * <p>The {@link Suppliers} class provides common suppliers and related utilities.
027 *
028 * <p>See the Guava User Guide article on <a href=
029 * "https://github.com/google/guava/wiki/FunctionalExplained">the use of functional types</a>.
030 *
031 * <h3>For Java 8+ users</h3>
032 *
033 * <p>This interface is now a legacy type. Use {@code java.util.function.Supplier} (or the
034 * appropriate primitive specialization such as {@code IntSupplier}) instead whenever possible.
035 * Otherwise, at least reduce <i>explicit</i> dependencies on this type by using lambda expressions
036 * or method references instead of classes, leaving your code easier to migrate in the future.
037 *
038 * <p>To use an existing supplier instance (say, named {@code supplier}) in a context where the
039 * <i>other type</i> of supplier is expected, use the method reference {@code supplier::get}. A
040 * future version of {@code com.google.common.base.Supplier} will be made to <i>extend</i> {@code
041 * java.util.function.Supplier}, making conversion code necessary only in one direction. At that
042 * time, this interface will be officially discouraged.
043 *
044 * @author Harry Heymann
045 * @since 2.0
046 */
047@GwtCompatible
048@ElementTypesAreNonnullByDefault
049public interface Supplier<T extends @Nullable Object> {
050  /**
051   * Retrieves an instance of the appropriate type. The returned object may or may not be a new
052   * instance, depending on the implementation.
053   *
054   * @return an instance of the appropriate type
055   */
056  @ParametricNullness
057  T get();
058}