Java this.classProperty versus classProperty incase argument variable name clashes?

Can someone please explain output 0 in case of first constructor without this ?

If the argument variable name is same as class property name and i am using that property inside the method. What does java interpret that "class property" or "the argument variable" ?

Without this:

 public User(int userId){
     userId = userId;
 
  }
 

With this:

 public User(int userId){
     this.userId = userId;
 
  }
 
 
  public void PrintUserId(){
     System.out.println(this.userId);
 }
 
  User firstUser = new User(123);
  firstUser.PrintUserId(); 
 

// 0 without this

//123 with this

---