访问单独类中的私有变量？

Question

This is homework, so I don't expect you to do it for me.

I have a program that sorts students by their student ID numbers. The problem is that the student class that creates the students (provided by my teacher, can't change) has the ID's set as private, so I can't access them in the calling function.

This is the student class:

public class StudentQ {
  private String name;
  private int id;
  private double avg;

  StudentQ(String sname, int sid, double average){
    name=sname;
    id=sid;
    avg=average;
  }

  // Note this uses the String static method format to produce a string
  // in the same way that System.out.printf does
  public String toString(){ 
    return String.format("%15s [%6d]:%7.2f", name,id,avg);
  }

  public String getName(){
    return name;
  }

  public int getID(){
    return id;
  }

  public double getAverage(){
    return avg;
  }

  public void setAverage(double newavg){
    avg=newavg;
  }
}

and this is my sorting class:

static void sortByID(StudentQ[] students) {


  for (int lastPlace = students.length-1; lastPlace > 0; lastPlace--) {   
    int maxLoc = 0;
    for (int j = 1; j <= lastPlace; j++) {
      if (students[j].getID() > students[maxLoc].getID()) {
        maxLoc = j;  
      }
    }
    int temp = students[maxLoc].getID();
    *students[maxLoc].id = students[lastPlace].id;
    students[lastPlace].id= temp;*

  }

}

The way it is now, it gives me and error saying that the field StudentQ.id is not visable, and I can't use .getID() because that is trying to assign a value to a method.

Thank you.

Answer 1

Use the supplied getID method of the Student class, that's what it's there for (AKA a getter)

students[lastPlace].getID();

The next problem you will have is the fact that there is no setter for it...so you can't (and shouldn't) assign the ID's

Instead, you should swap the actual object references...

StudentQ temp = students[maxLoc];
students[maxLoc] = students[lastPlace];
students[lastPlace] = temp;

Answer 2

The Student class has a "getter" method for the ID variable. This is a common Java workaround for private variables that need to be accessed by other classes. Instead of students[j].id , call students[j].getID()

Answer 3

You shouldn't have to change the student IDs at all, in order to sort the objects. If you truly do, you must define a "setter" for the variable (like the getter that you already have), but my guess is that really you should be rethinking your strategy: sorting objects generally should not involve altering the objects themselves! Instead, move the objects themselves , eg students[maxLoc] = students[lastPlace] .