Add ifPresent action on JsonNullable (#16)

Author: plum117

src/main/java/org/openapitools/jackson/nullable/JsonNullable.java

@@ -2,6 +2,7 @@
 
 import java.io.Serializable;
 import java.util.NoSuchElementException;
+import java.util.function.Consumer;
 
 public class JsonNullable<T> implements Serializable {
 
@@ -68,6 +69,20 @@ public boolean isPresent() {
         return isPresent;
     }
 
+    /**
+     * If a value is present, performs the given action with the value,
+     * otherwise does nothing.
+     *
+     * @param action the action to be performed, if a value is present
+     */
+    public void ifPresent(
+            Consumer<? super T> action) {
+
+        if (this.isPresent) {
+            action.accept(value);
+        }
+    }
+
     @Override
     public boolean equals(Object obj) {
         if (this == obj) {

src/main/java/org/openapitools/jackson/nullable/JsonNullableModule.java

@@ -1,6 +1,7 @@
 package org.openapitools.jackson.nullable;
 
 import com.fasterxml.jackson.core.Version;
+import com.fasterxml.jackson.core.json.PackageVersion;
 import com.fasterxml.jackson.databind.Module;
 
 public class JsonNullableModule extends Module {

src/test/java/org/openapitools/jackson/nullable/JsonNullableSimpleTest.java

@@ -57,6 +57,30 @@ public void orElseMissing() {
         assertEquals("world", test.orElse("world"));
     }
 
+    @Test
+    public void ifPresentWithValueNotPresent() {
+        JsonNullable<String> test = JsonNullable.undefined();
+        test.ifPresent(string -> {
+            throw new RuntimeException();
+        });
+    }
+
+    @Test(expected = RuntimeException.class)
+    public void ifPresentWithNullValuePresent() {
+        JsonNullable<String> test = JsonNullable.of(null);
+        test.ifPresent(string -> {
+            throw new RuntimeException();
+        });
+    }
+
+    @Test(expected = RuntimeException.class)
+    public void ifPresentWithNonNullValuePresent() {
+        JsonNullable<String> test = JsonNullable.of("test");
+        test.ifPresent(string -> {
+            throw new RuntimeException();
+        });
+    }
+
     @Test
     public void serializeNonBeanProperty() throws JsonProcessingException {
         assertEquals("null", mapper.writeValueAsString(JsonNullable.of(null)));