java - Extending or passing as argument -

which 1 better practice, doing this

public class {     private b b;      public a() {          b = new b(this);     } }  public class b {     private a;      public b(a a) {         this.a = a;     } } 

or extending class, im confused difference besides having 'a' field.

it depends on particular situation.

lets assume a class represents car,
if b engine should use composition (car has engine)
if b racecar should use inheritance (racecar better car)

if in doubt, current trend in software development prefer composition on inheritance.