Question

I’m a C# programmer trying to hack at a Java project. Here’s an anonymized extract from our production code. It works (I think). Note that this is the whole class.

public class Y extends X
{
  public Z m_Z;

  protected void readCustomData (CustomStream reader, boolean forUpdate)
    throws IOException, FTGException
  {
    super.readCustomData (reader, forUpdate) ;
    m_Z.readBinaryData (reader, forUpdate) ;
  }

  protected void writeCustomData (CustomStream writer, int original)
    throws IOException, FTGException
  {
    super.writeCustomData (writer, original) ;
    m_Z.writeBinaryData (writer, original) ;
  }
}

What puzzles me is – where is m_Z initialized? I cannot find it in the entire codebase. So why don’t the readCustomData and writeCustomData methods fail with NullReferenceException – or whatever the equivalent is in Java? Is m_Z somehow automagically constructed along with Y? Or have I missed something after all and there is some deeper magic in the codebase which initializes it?

Answer 1

When a Java class does not declare a constructor, the compiler implicitly adds a no-argument constructor that does nothing but call the superclass no-argument constructor (if there is none such, there will be a compiler error).

However, in your example the field m_Z would be null. If calls to those method succeed, then the field must be set elsewhere. It is public, after all (very bad practice).