Optional类（FROM JDK8）的使用

梦里梦外; 2023-06-08 03:20 7阅读 0赞

Optional类自从jdk1.8才诞生，主要用于对可能是null值的处理。案例如下：

public static void main(String[] args) {
            // 其value值为null
            Optional optcar = Optional.empty();
    
            // 取出括号里的值，null值抛出NPE异常
            Optional optBike = Optional.of(null);
    
            // 取出括号里的值，null值返回为null，其他返回原值
            Optional optTrain = Optional.ofNullable("hha");
    
            // isPresent()括号中的值为null时，返回false，反之false
            if (optTrain.isPresent()) {
                System.out.println("hh");
            } else {
                System.out.println("ee");
            }
      
        }

Optional类的源码如下：

package java.util;
 
 import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.function.Predicate;
 import java.util.function.Supplier;
 
 /**
 * A container object which may or may not contain a non-null value.
 * If a value is present, {@code isPresent()} will return {@code true} and
 * {@code get()} will return the value.
 *
 * Additional methods that depend on the presence or absence of a contained
 * value are provided, such as {@link #orElse(java.lang.Object) orElse()}
 * (return a default value if value not present) and
 * {@link #ifPresent(java.util.function.Consumer) ifPresent()} (execute a block
 * of code if the value is present).
 *
 * This is a <a href="../lang/doc-files/ValueBased.html">value-based</a>
 * class; use of identity-sensitive operations (including reference equality
 * ({@code ==}), identity hash code, or synchronization) on instances of
 * {@code Optional} may have unpredictable results and should be avoided.
 *
 * @since 1.8
 */
 public final class Optional<T> {
 /**
 * Common instance for {@code empty()}.
 */
 private static final Optional<?> EMPTY = new Optional<>();
 
 /**
 * If non-null, the value; if null, indicates no value is present
 */
 private final T value;
 
 /**
 * Constructs an empty instance.
 *
 * @implNote Generally only one empty instance, {@link Optional#EMPTY},
 * should exist per VM.
 */
 private Optional() {
 this.value = null;
 }
 
 /**
 * Returns an empty {@code Optional} instance. No value is present for this
 * Optional.
 *
 * @apiNote Though it may be tempting to do so, avoid testing if an object
 * is empty by comparing with {@code ==} against instances returned by
 * {@code Option.empty()}. There is no guarantee that it is a singleton.
 * Instead, use {@link #isPresent()}.
 *
 * @param <T> Type of the non-existent value
 * @return an empty {@code Optional}
 */
 public static<T> Optional<T> empty() {
 @SuppressWarnings("unchecked")
 Optional<T> t = (Optional<T>) EMPTY;
 return t;
 }
 
 /**
 * Constructs an instance with the value present.
 *
 * @param value the non-null value to be present
 * @throws NullPointerException if value is null
 */
 private Optional(T value) {
 this.value = Objects.requireNonNull(value);
 }
 
 /**
 * Returns an {@code Optional} with the specified present non-null value.
 *
 * @param <T> the class of the value
 * @param value the value to be present, which must be non-null
 * @return an {@code Optional} with the value present
 * @throws NullPointerException if value is null
 */
 public static <T> Optional<T> of(T value) {
 return new Optional<>(value);
 }
 
 /**
 * Returns an {@code Optional} describing the specified value, if non-null,
 * otherwise returns an empty {@code Optional}.
 *
 * @param <T> the class of the value
 * @param value the possibly-null value to describe
 * @return an {@code Optional} with a present value if the specified value
 * is non-null, otherwise an empty {@code Optional}
 */
 public static <T> Optional<T> ofNullable(T value) {
 return value == null ? empty() : of(value);
 }
 
 /**
 * If a value is present in this {@code Optional}, returns the value,
 * otherwise throws {@code NoSuchElementException}.
 *
 * @return the non-null value held by this {@code Optional}
 * @throws NoSuchElementException if there is no value present
 *
 * @see Optional#isPresent()
 */
 public T get() {
 if (value == null) {
 throw new NoSuchElementException("No value present");
 }
 return value;
 }
 
 /**
 * Return {@code true} if there is a value present, otherwise {@code false}.
 *
 * @return {@code true} if there is a value present, otherwise {@code false}
 */
 public boolean isPresent() {
 return value != null;
 }
 
 /**
 * If a value is present, invoke the specified consumer with the value,
 * otherwise do nothing.
 *
 * @param consumer block to be executed if a value is present
 * @throws NullPointerException if value is present and {@code consumer} is
 * null
 */
 public void ifPresent(Consumer<? super T> consumer) {
 if (value != null)
 consumer.accept(value);
 }
 
 /**
 * If a value is present, and the value matches the given predicate,
 * return an {@code Optional} describing the value, otherwise return an
 * empty {@code Optional}.
 *
 * @param predicate a predicate to apply to the value, if present
 * @return an {@code Optional} describing the value of this {@code Optional}
 * if a value is present and the value matches the given predicate,
 * otherwise an empty {@code Optional}
 * @throws NullPointerException if the predicate is null
 */
 public Optional<T> filter(Predicate<? super T> predicate) {
 Objects.requireNonNull(predicate);
 if (!isPresent())
 return this;
 else
 return predicate.test(value) ? this : empty();
 }
 
 /**
 * If a value is present, apply the provided mapping function to it,
 * and if the result is non-null, return an {@code Optional} describing the
 * result. Otherwise return an empty {@code Optional}.
 *
 * @apiNote This method supports post-processing on optional values, without
 * the need to explicitly check for a return status. For example, the
 * following code traverses a stream of file names, selects one that has
 * not yet been processed, and then opens that file, returning an
 * {@code Optional<FileInputStream>}:
 *
 * <pre>{@code
 * Optional<FileInputStream> fis =
 * names.stream().filter(name -> !isProcessedYet(name))
 * .findFirst()
 * .map(name -> new FileInputStream(name));
 * }</pre>
 *
 * Here, {@code findFirst} returns an {@code Optional<String>}, and then
 * {@code map} returns an {@code Optional<FileInputStream>} for the desired
 * file if one exists.
 *
 * @param The type of the result of the mapping function
 * @param mapper a mapping function to apply to the value, if present
 * @return an {@code Optional} describing the result of applying a mapping
 * function to the value of this {@code Optional}, if a value is present,
 * otherwise an empty {@code Optional}
 * @throws NullPointerException if the mapping function is null
 */
 public Optional map(Function<? super T, ? extends U> mapper) {
 Objects.requireNonNull(mapper);
 if (!isPresent())
 return empty();
 else {
 return Optional.ofNullable(mapper.apply(value));
 }
 }
 
 /**
 * If a value is present, apply the provided {@code Optional}-bearing
 * mapping function to it, return that result, otherwise return an empty
 * {@code Optional}. This method is similar to {@link #map(Function)},
 * but the provided mapper is one whose result is already an {@code Optional},
 * and if invoked, {@code flatMap} does not wrap it with an additional
 * {@code Optional}.
 *
 * @param The type parameter to the {@code Optional} returned by
 * @param mapper a mapping function to apply to the value, if present
 * the mapping function
 * @return the result of applying an {@code Optional}-bearing mapping
 * function to the value of this {@code Optional}, if a value is present,
 * otherwise an empty {@code Optional}
 * @throws NullPointerException if the mapping function is null or returns
 * a null result
 */
 public Optional flatMap(Function<? super T, Optional> mapper) {
 Objects.requireNonNull(mapper);
 if (!isPresent())
 return empty();
 else {
 return Objects.requireNonNull(mapper.apply(value));
 }
 }
 
 /**
 * Return the value if present, otherwise return {@code other}.
 *
 * @param other the value to be returned if there is no value present, may
 * be null
 * @return the value, if present, otherwise {@code other}
 */
 public T orElse(T other) {
 return value != null ? value : other;
 }
 
 /**
 * Return the value if present, otherwise invoke {@code other} and return
 * the result of that invocation.
 *
 * @param other a {@code Supplier} whose result is returned if no value
 * is present
 * @return the value if present otherwise the result of {@code other.get()}
 * @throws NullPointerException if value is not present and {@code other} is
 * null
 */
 public T orElseGet(Supplier<? extends T> other) {
 return value != null ? value : other.get();
 }
 
 /**
 * Return the contained value, if present, otherwise throw an exception
 * to be created by the provided supplier.
 *
 * @apiNote A method reference to the exception constructor with an empty
 * argument list can be used as the supplier. For example,
 * {@code IllegalStateException::new}
 *
 * @param <X> Type of the exception to be thrown
 * @param exceptionSupplier The supplier which will return the exception to
 * be thrown
 * @return the present value
 * @throws X if there is no value present
 * @throws NullPointerException if no value is present and
 * {@code exceptionSupplier} is null
 */
 public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
 if (value != null) {
 return value;
 } else {
 throw exceptionSupplier.get();
 }
 }
 
 /**
 * Indicates whether some other object is "equal to" this Optional. The
 * other object is considered equal if:
 * <ul>
 * <li>it is also an {@code Optional} and;
 * <li>both instances have no value present or;
 * <li>the present values are "equal to" each other via {@code equals()}.
 * </ul>
 *
 * @param obj an object to be tested for equality
 * @return {code true} if the other object is "equal to" this object
 * otherwise {@code false}
 */
 @Override
 public boolean equals(Object obj) {
 if (this == obj) {
 return true;
 }
 
 if (!(obj instanceof Optional)) {
 return false;
 }
 
 Optional<?> other = (Optional<?>) obj;
 return Objects.equals(value, other.value);
 }
 
 /**
 * Returns the hash code value of the present value, if any, or 0 (zero) if
 * no value is present.
 *
 * @return hash code value of the present value or 0 if no value is present
 */
 @Override
 public int hashCode() {
 return Objects.hashCode(value);
 }
 
 /**
 * Returns a non-empty string representation of this Optional suitable for
 * debugging. The exact presentation format is unspecified and may vary
 * between implementations and versions.
 *
 * @implSpec If a value is present the result must include its string
 * representation in the result. Empty and present Optionals must be
 * unambiguously differentiable.
 *
 * @return the string representation of this instance
 */
 @Override
 public String toString() {
 return value != null
 ? String.format("Optional[%s]", value)
 : "Optional.empty";
 }
 }