Difference between public void, private void and public string

Question

Hi everyone,
My 1st question is how do you know when to use public void, private void and public string? I am mightily cofuse with this.
2ndly, Can anybody explain to me on following code snippet:
Traceback B0;//the starting point  of Traceback
// Traceback objects
abstract class Traceback {
  int i, j;                     // absolute coordinates
// Traceback2 objects for simple gap costs
class Traceback2 extends Traceback {
  public Traceback2(int i, int j)
  { this.i = i; this.j = j; }
}And using the code above is the following allowed:
B[0] = new Traceback2(i-1, 0);
Any replies much appreciated. Thank you.

Answer 1

1)
public and private are access modifiers,
void and String return type declarations.
2)
It's called "inheritance", and "bad design" as well.
You should read the tutorials, you know?