fix #394: ensures that defaults are copied

annotations/builder/src/main/java/io/sundr/builder/internal/functions/ClazzAs.java

@@ -619,6 +619,10 @@ private static List<Statement> toInstanceConstructorBody(TypeDef clazz, TypeDef
       statements.add(new StringStatement("this.fluent = " + fluent + "; "));
     }
 
+    if (hasDefaultConstructor(clazz)) {
+      statements.add(new StringStatement("instance = (instance != null ? instance : new " + clazz.getName() + "());\n"));
+    }
+
     StringBuilder builder = new StringBuilder("if (instance != null) {\n");
 
     for (Property property : constructor.getArguments()) {

tests/buildable-fields/src/main/java/io/sundr/tests/WithDefaults.java

@@ -0,0 +1,38 @@
+/**
+ * Copyright 2015 The original authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+**/
+
+package io.sundr.tests;
+
+import io.sundr.builder.annotations.Buildable;
+
+/**
+ * A simple Pojo that demonstrates default fields are preserved
+ */
+@Buildable
+public class WithDefaults {
+
+  private String field = "Hello";
+
+  public String getField() {
+    return field;
+  }
+
+  public void setField(String field) {
+    this.field = field;
+  }
+
+}

tests/buildable-fields/src/test/java/io/sundr/tests/WithDefaultsTest.java

@@ -0,0 +1,32 @@
+/**
+ * Copyright 2015 The original authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+**/
+
+package io.sundr.tests;
+
+import static org.junit.Assert.assertEquals;
+
+import org.junit.Test;
+
+public class WithDefaultsTest {
+
+  @Test
+  public void testGeneratedBuilder() {
+    WithDefaults withDefaults = new WithDefaultsBuilder((WithDefaults) null).build();
+    assertEquals("Hello", withDefaults.getField());
+  }
+
+}