--- /dev/null
+package org.simantics.db.function;\r
+\r
+import java.util.NoSuchElementException;\r
+import java.util.Objects;\r
+\r
+import org.simantics.db.exception.DatabaseException;\r
+\r
+/**\r
+ * A container object which may or may not contain a non-null value. If a value\r
+ * is present, {@code isPresent()} will return {@code true} and {@code get()}\r
+ * will return the value.\r
+ *\r
+ * <p>\r
+ * Additional methods that depend on the presence or absence of a contained\r
+ * value are provided, such as {@link #orElse(java.lang.Object) orElse()}\r
+ * (return a default value if value not present) and\r
+ * {@link #ifPresent(org.simantics.db.function.DbConsumer) ifPresent()} (execute\r
+ * a block of code if the value is present).\r
+ *\r
+ * <p>\r
+ * This is a value-based class; use of identity-sensitive operations (including\r
+ * reference equality ({@code ==}), identity hash code, or synchronization) on\r
+ * instances of {@code DbOptional} may have unpredictable results and should be\r
+ * avoided.\r
+ *\r
+ * @since 1.25\r
+ */\r
+public final class DbOptional<T> {\r
+ /**\r
+ * Common instance for {@code empty()}.\r
+ */\r
+ private static final DbOptional<?> EMPTY = new DbOptional<>();\r
+\r
+ /**\r
+ * If non-null, the value; if null, indicates no value is present\r
+ */\r
+ private final T value;\r
+\r
+ /**\r
+ * Constructs an empty instance.\r
+ *\r
+ * @implNote Generally only one empty instance, {@link Optional#EMPTY},\r
+ * should exist per VM.\r
+ */\r
+ private DbOptional() {\r
+ this.value = null;\r
+ }\r
+\r
+ /**\r
+ * Returns an empty {@code Optional} instance. No value is present for this\r
+ * Optional.\r
+ *\r
+ * @apiNote Though it may be tempting to do so, avoid testing if an object\r
+ * is empty by comparing with {@code ==} against instances returned by\r
+ * {@code Option.empty()}. There is no guarantee that it is a singleton.\r
+ * Instead, use {@link #isPresent()}.\r
+ *\r
+ * @param <T> Type of the non-existent value\r
+ * @return an empty {@code Optional}\r
+ */\r
+ public static<T> DbOptional<T> empty() {\r
+ @SuppressWarnings("unchecked")\r
+ DbOptional<T> t = (DbOptional<T>) EMPTY;\r
+ return t;\r
+ }\r
+\r
+ /**\r
+ * Constructs an instance with the value present.\r
+ *\r
+ * @param value the non-null value to be present\r
+ * @throws NullPointerException if value is null\r
+ */\r
+ private DbOptional(T value) {\r
+ this.value = Objects.requireNonNull(value);\r
+ }\r
+\r
+ /**\r
+ * Returns an {@code Optional} with the specified present non-null value.\r
+ *\r
+ * @param <T> the class of the value\r
+ * @param value the value to be present, which must be non-null\r
+ * @return an {@code Optional} with the value present\r
+ * @throws NullPointerException if value is null\r
+ */\r
+ public static <T> DbOptional<T> of(T value) {\r
+ return new DbOptional<>(value);\r
+ }\r
+\r
+ /**\r
+ * Returns an {@code Optional} describing the specified value, if non-null,\r
+ * otherwise returns an empty {@code Optional}.\r
+ *\r
+ * @param <T> the class of the value\r
+ * @param value the possibly-null value to describe\r
+ * @return an {@code Optional} with a present value if the specified value\r
+ * is non-null, otherwise an empty {@code Optional}\r
+ */\r
+ public static <T> DbOptional<T> ofNullable(T value) {\r
+ return value == null ? empty() : of(value);\r
+ }\r
+\r
+ /**\r
+ * If a value is present in this {@code Optional}, returns the value,\r
+ * otherwise throws {@code NoSuchElementException}.\r
+ *\r
+ * @return the non-null value held by this {@code Optional}\r
+ * @throws NoSuchElementException if there is no value present\r
+ *\r
+ * @see Optional#isPresent()\r
+ */\r
+ public T get() {\r
+ if (value == null) {\r
+ throw new NoSuchElementException("No value present");\r
+ }\r
+ return value;\r
+ }\r
+\r
+ /**\r
+ * Return {@code true} if there is a value present, otherwise {@code false}.\r
+ *\r
+ * @return {@code true} if there is a value present, otherwise {@code false}\r
+ */\r
+ public boolean isPresent() {\r
+ return value != null;\r
+ }\r
+\r
+ /**\r
+ * If a value is present, invoke the specified consumer with the value,\r
+ * otherwise do nothing.\r
+ *\r
+ * @param consumer block to be executed if a value is present\r
+ * @throws DatabaseException \r
+ * @throws NullPointerException if value is present and {@code consumer} is\r
+ * null\r
+ */\r
+ public void ifPresent(DbConsumer<? super T> consumer) throws DatabaseException {\r
+ if (value != null)\r
+ consumer.accept(value);\r
+ }\r
+\r
+ /**\r
+ * If a value is present, and the value matches the given predicate,\r
+ * return an {@code Optional} describing the value, otherwise return an\r
+ * empty {@code Optional}.\r
+ *\r
+ * @param predicate a predicate to apply to the value, if present\r
+ * @return an {@code Optional} describing the value of this {@code Optional}\r
+ * if a value is present and the value matches the given predicate,\r
+ * otherwise an empty {@code Optional}\r
+ * @throws DatabaseException \r
+ * @throws NullPointerException if the predicate is null\r
+ */\r
+ public DbOptional<T> filter(DbPredicate<? super T> predicate) throws DatabaseException {\r
+ Objects.requireNonNull(predicate);\r
+ if (!isPresent())\r
+ return this;\r
+ else\r
+ return predicate.test(value) ? this : empty();\r
+ }\r
+\r
+ /**\r
+ * If a value is present, apply the provided mapping function to it,\r
+ * and if the result is non-null, return an {@code Optional} describing the\r
+ * result. Otherwise return an empty {@code Optional}.\r
+ *\r
+ * @apiNote This method supports post-processing on optional values, without\r
+ * the need to explicitly check for a return status. For example, the\r
+ * following code traverses a stream of file names, selects one that has\r
+ * not yet been processed, and then opens that file, returning an\r
+ * {@code Optional<FileInputStream>}:\r
+ *\r
+ * <pre>{@code\r
+ * Optional<FileInputStream> fis =\r
+ * names.stream().filter(name -> !isProcessedYet(name))\r
+ * .findFirst()\r
+ * .map(name -> new FileInputStream(name));\r
+ * }</pre>\r
+ *\r
+ * Here, {@code findFirst} returns an {@code Optional<String>}, and then\r
+ * {@code map} returns an {@code Optional<FileInputStream>} for the desired\r
+ * file if one exists.\r
+ *\r
+ * @param <U> The type of the result of the mapping function\r
+ * @param mapper a mapping function to apply to the value, if present\r
+ * @return an {@code Optional} describing the result of applying a mapping\r
+ * function to the value of this {@code Optional}, if a value is present,\r
+ * otherwise an empty {@code Optional}\r
+ * @throws DatabaseException \r
+ * @throws NullPointerException if the mapping function is null\r
+ */\r
+ public<U> DbOptional<U> map(DbFunction<? super T, ? extends U> mapper) throws DatabaseException {\r
+ Objects.requireNonNull(mapper);\r
+ if (!isPresent())\r
+ return empty();\r
+ else {\r
+ return DbOptional.ofNullable(mapper.apply(value));\r
+ }\r
+ }\r
+\r
+ /**\r
+ * If a value is present, apply the provided {@code Optional}-bearing\r
+ * mapping function to it, return that result, otherwise return an empty\r
+ * {@code Optional}. This method is similar to {@link #map(Function)},\r
+ * but the provided mapper is one whose result is already an {@code Optional},\r
+ * and if invoked, {@code flatMap} does not wrap it with an additional\r
+ * {@code Optional}.\r
+ *\r
+ * @param <U> The type parameter to the {@code Optional} returned by\r
+ * @param mapper a mapping function to apply to the value, if present\r
+ * the mapping function\r
+ * @return the result of applying an {@code Optional}-bearing mapping\r
+ * function to the value of this {@code Optional}, if a value is present,\r
+ * otherwise an empty {@code Optional}\r
+ * @throws DatabaseException \r
+ * @throws NullPointerException if the mapping function is null or returns\r
+ * a null result\r
+ */\r
+ public<U> DbOptional<U> flatMap(DbFunction<? super T, DbOptional<U>> mapper) throws DatabaseException {\r
+ Objects.requireNonNull(mapper);\r
+ if (!isPresent())\r
+ return empty();\r
+ else {\r
+ return Objects.requireNonNull(mapper.apply(value));\r
+ }\r
+ }\r
+\r
+ /**\r
+ * Return the value if present, otherwise return {@code other}.\r
+ *\r
+ * @param other the value to be returned if there is no value present, may\r
+ * be null\r
+ * @return the value, if present, otherwise {@code other}\r
+ */\r
+ public T orElse(T other) {\r
+ return value != null ? value : other;\r
+ }\r
+\r
+ /**\r
+ * Return the value if present, otherwise invoke {@code other} and return\r
+ * the result of that invocation.\r
+ *\r
+ * @param other a {@code Supplier} whose result is returned if no value\r
+ * is present\r
+ * @return the value if present otherwise the result of {@code other.get()}\r
+ * @throws DatabaseException \r
+ * @throws NullPointerException if value is not present and {@code other} is\r
+ * null\r
+ */\r
+ public T orElseGet(DbSupplier<? extends T> other) throws DatabaseException {\r
+ return value != null ? value : other.get();\r
+ }\r
+\r
+ /**\r
+ * Return the contained value, if present, otherwise throw an exception\r
+ * to be created by the provided supplier.\r
+ *\r
+ * @apiNote A method reference to the exception constructor with an empty\r
+ * argument list can be used as the supplier. For example,\r
+ * {@code IllegalStateException::new}\r
+ *\r
+ * @param <X> Type of the exception to be thrown\r
+ * @param exceptionSupplier The supplier which will return the exception to\r
+ * be thrown\r
+ * @return the present value\r
+ * @throws X if there is no value present\r
+ * @throws DatabaseException \r
+ * @throws NullPointerException if no value is present and\r
+ * {@code exceptionSupplier} is null\r
+ */\r
+ public <X extends Throwable> T orElseThrow(DbSupplier<? extends X> exceptionSupplier) throws X, DatabaseException {\r
+ if (value != null) {\r
+ return value;\r
+ } else {\r
+ throw exceptionSupplier.get();\r
+ }\r
+ }\r
+\r
+ /**\r
+ * Indicates whether some other object is "equal to" this Optional. The\r
+ * other object is considered equal if:\r
+ * <ul>\r
+ * <li>it is also an {@code Optional} and;\r
+ * <li>both instances have no value present or;\r
+ * <li>the present values are "equal to" each other via {@code equals()}.\r
+ * </ul>\r
+ *\r
+ * @param obj an object to be tested for equality\r
+ * @return {code true} if the other object is "equal to" this object\r
+ * otherwise {@code false}\r
+ */\r
+ @Override\r
+ public boolean equals(Object obj) {\r
+ if (this == obj) {\r
+ return true;\r
+ }\r
+\r
+ if (!(obj instanceof DbOptional)) {\r
+ return false;\r
+ }\r
+\r
+ DbOptional<?> other = (DbOptional<?>) obj;\r
+ return Objects.equals(value, other.value);\r
+ }\r
+\r
+ /**\r
+ * Returns the hash code value of the present value, if any, or 0 (zero) if\r
+ * no value is present.\r
+ *\r
+ * @return hash code value of the present value or 0 if no value is present\r
+ */\r
+ @Override\r
+ public int hashCode() {\r
+ return Objects.hashCode(value);\r
+ }\r
+\r
+ /**\r
+ * Returns a non-empty string representation of this Optional suitable for\r
+ * debugging. The exact presentation format is unspecified and may vary\r
+ * between implementations and versions.\r
+ *\r
+ * @implSpec If a value is present the result must include its string\r
+ * representation in the result. Empty and present DbOptionals must be\r
+ * unambiguously differentiable.\r
+ *\r
+ * @return the string representation of this instance\r
+ */\r
+ @Override\r
+ public String toString() {\r
+ return value != null\r
+ ? String.format("DbOptional[%s]", value)\r
+ : "DbOptional.empty";\r
+ }\r
+}\r
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