How does class inheritance work in java?

Question

Java

Help me understand why the code returns 0 instead of 12:

class App {
  public static void main(String[] args) throws Exception {
    Params params = new Params();
    params.MACDFast = 12;
    params.MACDSlow = 16;
    params.MACDMA = 9;

    DSO dso = new DSO(params);
  }
}

class TSParams {
  public int MACDFast;
}

class Params extends TSParams {
  public int MACDFast, MACDSlow, MACDMA;
}

class TS {
  public TS(TSParams params) {
    System.out.println(params.MACDFast);
  }
}

class DSO extends TS {
  public DSO(Params params) {
    super(params);
  }
}

It seems that the TS class constructor works not with an object, but with a static variable of the TSParams class. Actually, how to derive the true value of MACDFast from the TS class?

Answer 1

In your case, the Params.MACDFast field hid (hided) the TSParams.MACDFast field (there were two of them).
Moreover, the fields are resolved using the compile-time type.
Therefore, in main, you set the Params.MACDFast field, and the TSParams.MACDFast field remained with the default value - 0.
But in the TS constructor, you have the compile-time type - TSParams, so you read the TSParams.MACDFast (0) field, not Params. MACDFast(12).
To fix this in your example, it is enough to remove MACDFast from the Params class.