Merge remote-tracking branch 'origin/57-null-to-empty-string-converter' into dev

Author: greenrobot

objectbox-java-api/src/main/java/io/objectbox/annotation/DefaultValue.java

@@ -0,0 +1,18 @@
+package io.objectbox.annotation;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * Defines the Java code of the default value to use for a property, when getting an existing entity and the database
+ * value for the property is null.
+ * <p>
+ * Currently only {@code @DefaultValue("")} is supported.
+ */
+@Retention(RetentionPolicy.CLASS)
+@Target({ElementType.FIELD})
+public @interface DefaultValue {
+    String value();
+}

objectbox-java/src/main/java/io/objectbox/converter/NullToEmptyStringConverter.java

@@ -0,0 +1,22 @@
+package io.objectbox.converter;
+
+import javax.annotation.Nullable;
+
+/**
+ * Used as a converter if a property is annotated with {@link io.objectbox.annotation.DefaultValue @DefaultValue("")}.
+ */
+public class NullToEmptyStringConverter implements PropertyConverter<String, String> {
+
+    @Override
+    public String convertToDatabaseValue(String entityProperty) {
+        return entityProperty;
+    }
+
+    @Override
+    public String convertToEntityProperty(@Nullable String databaseValue) {
+        if (databaseValue == null) {
+            return "";
+        }
+        return databaseValue;
+    }
+}