Converter tests:

Change test data files extension to "java"

j2k/tests/testData/ast/annotations/jetbrainsNotNull.java

@@ -0,0 +1,30 @@
+//file
+package test;
+
+import org.jetbrains.annotations.NotNull;
+
+public class Test {
+  @NotNull String myStr = "String2";
+
+  public Test(@NotNull String str) {
+    myStr = str;
+  }
+
+  public void sout(@NotNull String str) {
+    System.out.println(str);
+  }
+
+  @NotNull
+  public String dummy(@NotNull String str) {
+    return str;
+  }
+
+  public void test() {
+    sout("String");
+    @NotNull String test = "String2";
+    sout(test);
+    sout(dummy(test));
+
+    new Test(test);
+  }
+}