Add ResolveHandler filter (#120)

src/main/java/com/github/joselion/maybe/ResolveHandler.java

@@ -3,6 +3,7 @@
 import java.util.Optional;
 import java.util.function.Consumer;
 import java.util.function.Function;
+import java.util.function.Predicate;
 
 import org.eclipse.jdt.annotation.Nullable;
 
@@ -123,8 +124,9 @@ public <X extends E> ResolveHandler<T, E> catchError(final Class<X> errorType, f
    * If neither the value nor the error is present, it returns an empty handler.
    * 
    * @param <U> the type the value will be mapped to
-   * @param mapper a function to map the current value to another (if present)
-   * @return a new handler with the mapped value, the previous error, or nothing
+   * @param mapper a function to map the value to another (if present)
+   * @return a new handler with either the mapped value, the previous error, or
+   *         nothing
    */
   public <U> ResolveHandler<U, E> map(final Function<T, U> mapper) {
     if (success.isPresent()) {
@@ -138,6 +140,34 @@ public <U> ResolveHandler<U, E> map(final Function<T, U> mapper) {
     return withNothing();
   }
 
+  /**
+   * If a value is present, and the value matches the given {@code predicate},
+   * returns a new handler with the value, otherwise returns an empty handler.
+   * If the error is present, the {@code predicate} is never applied and, the
+   * next handler will still contain the error.
+   * <p>
+   * If neither the value nor the error is present, it returns an empty handler.
+   * 
+   * @param predicate a predicate to apply to the value (if present)
+   * @return a new handler with either the value if it matched the predicate,
+   *         the previous error, or nothing
+   */
+  public ResolveHandler<T, E> filter(final Predicate<T> predicate) {
+    if (success.isPresent()) {
+      final T value = success.get();
+
+      return predicate.test(value)
+        ? withSuccess(value)
+        : withNothing();
+    }
+
+    if (error.isPresent()) {
+      return withError(error.get());
+    }
+
+    return withNothing();
+  }
+
   /**
    * Returns the value resolved/handled if present. A default value otherwise.
    * 

src/test/java/com/github/joselion/maybe/ResolveHandlerTest.java

@@ -11,6 +11,7 @@
 import java.io.IOException;
 import java.util.ArrayList;
 import java.util.List;
+import java.util.Objects;
 import java.util.function.Function;
 
 import com.github.joselion.maybe.helpers.UnitTest;
@@ -240,7 +241,7 @@
   @Nested class map {
     @Nested class when_the_value_is_present {
       @Test void returns_a_new_handler_applying_the_mapper_function() {
-        final ResolveHandler<Integer, Exception> handler = ResolveHandler.withSuccess("Hello world!")
+        final ResolveHandler<Integer, ?> handler = ResolveHandler.withSuccess("Hello world!")
           .map(String::length);
 
         assertThat(handler)
@@ -284,6 +285,70 @@
     }
   }
 
+  @Nested class filter {
+    @Nested class when_the_value_is_present {
+      @Nested class and_the_predicate_matches {
+        @Test void returns_a_new_handler_with_the_value() {
+          final ResolveHandler<String, ?> handler = ResolveHandler.withSuccess("Hello world!")
+            .filter(it -> it.contains("world"));
+
+          assertThat(handler)
+            .extracting(SUCCESS, optional(String.class))
+            .contains("Hello world!");
+
+          assertThat(handler)
+            .extracting(ERROR, optional(Exception.class))
+            .isEmpty();
+        }
+      }
+
+      @Nested class and_the_predicate_does_NOT_match {
+        @Test void returns_an_empty_handler() {
+          final ResolveHandler<String, ?> handler = ResolveHandler.withSuccess("Hello world!")
+            .filter(it -> it.contains("planet"));
+
+          assertThat(handler)
+            .extracting(SUCCESS, optional(String.class))
+            .isEmpty();
+
+          assertThat(handler)
+            .extracting(ERROR, optional(Exception.class))
+            .isEmpty();
+        }
+      }
+    }
+
+    @Nested class when_the_error_is_present {
+      @Test void returns_a_new_handler_with_the_previous_error() {
+        final ResolveHandler<?, IOException> handler = ResolveHandler.withError(FAIL_EXCEPTION)
+          .filter(Objects::isNull);
+
+        assertThat(handler)
+          .extracting(SUCCESS, optional(String.class))
+          .isEmpty();
+
+        assertThat(handler)
+          .extracting(ERROR, optional(IOException.class))
+          .contains(FAIL_EXCEPTION);
+      }
+    }
+
+    @Nested class when_neither_the_value_nor_the_error_is_present {
+      @Test void returns_an_empty_handler() {
+        final ResolveHandler<?, ?> handler = ResolveHandler.withNothing()
+          .filter(Objects::isNull);
+
+        assertThat(handler)
+          .extracting(SUCCESS, optional(String.class))
+          .isEmpty();
+
+        assertThat(handler)
+          .extracting(ERROR, optional(Exception.class))
+          .isEmpty();
+      }
+    }
+  }
+
   @Nested class orDefault {
     @Nested class when_the_value_is_present {
       @Test void returns_the_value() {