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Guava is a suite of core and expanded libraries that include utility classes, google's collections, io classes, and much much more. Guava has only one code dependency - javax.annotation, per the JSR-305 spec.

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/*
 * Copyright (C) 2011 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.base;

import static com.google.common.base.Preconditions.checkNotNull;

import com.google.common.annotations.GwtCompatible;

import java.util.Collections;
import java.util.Set;

import javax.annotation.Nullable;

/**
 * Implementation of an {@link Optional} containing a reference.
 */
@GwtCompatible
final class Present extends Optional {
  private final T reference;

  Present(T reference) {
    this.reference = reference;
  }

  @Override public boolean isPresent() {
    return true;
  }

  @Override public T get() {
    return reference;
  }

  @Override public T or(T defaultValue) {
    checkNotNull(defaultValue, "use Optional.orNull() instead of Optional.or(null)");
    return reference;
  }

  @Override public Optional or(Optional secondChoice) {
    checkNotNull(secondChoice);
    return this;
  }

  @Override public T or(Supplier supplier) {
    checkNotNull(supplier);
    return reference;
  }

  @Override public T orNull() {
    return reference;
  }

  @Override public Set asSet() {
    return Collections.singleton(reference);
  }
  
  @Override public  Optional transform(Function function) {
    return new Present(checkNotNull(function.apply(reference),
        "the Function passed to Optional.transform() must not return null."));
  }

  @Override public boolean equals(@Nullable Object object) {
    if (object instanceof Present) {
      Present other = (Present) object;
      return reference.equals(other.reference);
    }
    return false;
  }

  @Override public int hashCode() {
    return 0x598df91c + reference.hashCode();
  }

  @Override public String toString() {
    return "Optional.of(" + reference + ")";
  }

  private static final long serialVersionUID = 0;
}