src/share/classes/java/util/Optional.java
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rev 7596 : 8015317: Optional.filter, map, and flatMap
Reviewed-by:
Contributed-by: brian.goetz@oracle.com, henry.jen@oracle.com
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* questions.
*/
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
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* If non-null, the value; if null, indicates no value is present
*/
private final T value;
/**
! * Construct an empty instance.
*
* @implNote Generally only one empty instance, {@link Optional#EMPTY},
* should exist per VM.
*/
private Optional() {
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* 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() {
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Optional<T> t = (Optional<T>) EMPTY;
return t;
}
/**
! * Construct an instance with the value present.
*
* @param value the non-null value to be present
*/
private Optional(T value) {
this.value = Objects.requireNonNull(value);
}
/**
! * Return an {@code Optional} with the specified present value.
*
* @param value the value to be present, which must be non-null
* @return an {@code Optional} with the value present
*/
public static <T> Optional<T> of(T value) {
return new Optional<>(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
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Optional<T> t = (Optional<T>) EMPTY;
return t;
}
/**
! * Constructs an instance with the value present.
*
* @param value the non-null value to be present
*/
private Optional(T value) {
this.value = Objects.requireNonNull(value);
}
/**
! * Returns an {@code Optional} with the specified present non-null value.
*
* @param value the value to be present, which must be non-null
* @return an {@code Optional} with the value present
*/
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 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
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public boolean isPresent() {
return value != null;
}
/**
! * Have the specified consumer accept the value if a value is present,
* 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
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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
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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 <U> 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<U> Optional<U> 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 <U> 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<U> Optional<U> flatMap(Function<? super T, ? extends Optional<U>> 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}