Each method parameter in the java_new_object() / java_call_method() can be either the value itself or a two-element vector. When it is a two-element vector it's first element is the JNI type signature string, e.g.: for integer - I, for array of integers - [I], for java.lang.String - Ljava/lang/String. The signature is important because the Java VM JNI API needs it in order to find the appropriate constructor/method/property.