跟踪实例化对象数量的Java类变量

将studentID设为静态成员

静态成员将在整个类的每个实例中保留，无论有多少个clas实例。

   public class Student extends Person{      static int studentID = 0;      int level;      public Student(){      }      public Student(String fName, String lName, int gLevel){        super(fName, lName);        if(gLevel >= 0 && gLevel <= 12){          level = gLevel;        }        studentID++;      }      public int getLevel(){        return level;      }      public String toString(){        String toReturn;        toReturn = super.toString() + "n   Grade Level: " + level + "n   ID #: " + studentID;        return toReturn;      }    }