坚持从另一个类调用ArrayList

Question

I have looked here and there are answers to similar questions, but I cannot conform them to my code. My code:

import java.util.*;

class Student {

    private String StuID;
    private String FName;
    private String LName;
    private String Email;
    private int Age;
    private double Grade1;
    private double Grade2;
    private double Grade3;

    public Student (String stuid, String fname, String lname, String email,
        int age, double grade1, double grade2, double grade3)
    {
        this.StuID = stuid;
        this.FName = fname;
        this.LName = lname;
        this.Email = email;
        this.Age = age;
        this.Grade1 = grade1;
        this.Grade2 = grade2;
        this.Grade3 = grade3;
    }

    public String getStuID(){ return this.StuID;}

    public String getFName(){return this.FName;}

    public String getLName(){return this.LName;}

    public String getEmail(){return this.Email;}

    public int getAge(){return this.Age;}

    public double getGrade1(){return this.Grade1;}

    public double getGrade2(){return this.Grade2;}

    public double getGrade3(){return this.Grade3;}

    public String setStuID(String newStuID){return (this.StuID= newStuID);}

    public String setFName(String newFName){return (this.FName= newFName);}

    public String setLName(String newLName){return (this.LName= newLName);}

    public String setEmail(String newEmail){return (this.Email= newEmail);}

    public int setAge(int newAge){return (this.Age= newAge);}

    public double setGrade1(double newGrade1){return (this.Grade1= newGrade1);}

    public double setGrade2(double newGrade2){return (this.Grade2= newGrade2);}

    public double setGrade3(double newGrade3){return (this.Grade1= newGrade3);}

    public String toString() {
        return String.format("StuID: %s\t First Name: %s\t Last Name: %s\t EMail: %s\t Age: %s\t Grade1: %s\t Grade2: %s\t Grade3: %s\t", 
            this.StuID, this.FName, this.LName, this.Email,
            this.Age, this.Grade1, this.Grade2, this.Grade3);

    }
}

public class StudentArrayList {

    public static void main(String[]args){

        ArrayList<Student> StudentArray = new ArrayList<Student>();

        StudentArray.add(new Student("1","John","Smith","John1989@gmail.com", 20, 88, 79, 59));
        StudentArray.add(new Student("2","Susan","Erickson","Erickson_1990@gmail.com", 19, 91, 72, 85));
        StudentArray.add(new Student("3","Jack","Napoli","The_lawyer99yahoo.com", 19, 85, 84, 87));
        StudentArray.add(new Student("4","Erin","Black","Erin.black@comcast.net", 22, 91, 98, 82));
        StudentArray.add(new Student("5","Tom","Gaye","tgay@att.net", 65, 99, 98, 97));

        //Example of printing specific student data using getters.
        System.out.println("");
        for (Student a: StudentArray) {
            System.out.println(a.getStuID());
            System.out.println(a.getFName());
            System.out.println(a.getLName());

        }
    }
}

public class Roster {
    public static void main(String[]args){
        System.out.println("");
        for (Student s: StudentArray) { //THIS IS WHERE IT ERRORS
            System.out.printf("%s\n",s);
        }
    }
}

The problem I am having is I am trying to perform a System.out.print in the Roster class. The error I am getting is "StudentArray cannot be resolved to a variable" in the Roster class. If I do this under the main method in the StudentArrayList , the print method works just fine. I know I have to have a way to call the array, just unsure what to do. Would that be a getter?

Answer 1

"StudentArray cannot be resolved to a variable"

That means that you haven't declared the variable StudentArray .

public class Roster {

    public static void main(String[]args){
        ArrayList<Student> StudentArray = new ArrayList<Student>();

        for (Student s: StudentArray) { // loop through the array
            System.out.printf("%s\n",s);
        }
    }
}

Answer 2

You need to first declare the studentArray

ArrayList<Student> studentArray = new ArrayList<Student>();

populate it adding some students, for example:

Student stud1 = new Student("stud params");
//other students declaration
//you could do this with a loop if data are asked to user
studentArray.add(stud1);

and then print the whole list:

for(Student s: studentArray){
     System.out.println(%s\n",s);
}

Answer 3

I don't think you need the Roster class at all, coz what you want the program to do is being done in the Student class itself. You cannot have two classes with main , both the Student Class and the Roster Class has Main.

The Main in student class is doing the right thing , it is initializing the students and printing the roster.

So you could do two things delete the Roster class or take out this code

public class StudentArrayList {

public static void main(String[]args){

    ArrayList<Student> StudentArray = new ArrayList<Student>();

    StudentArray.add(new Student("1","John","Smith","John1989@gmail.com", 20, 88, 79, 59));
    StudentArray.add(new Student("2","Susan","Erickson","Erickson_1990@gmail.com", 19, 91, 72, 85));
    StudentArray.add(new Student("3","Jack","Napoli","The_lawyer99yahoo.com", 19, 85, 84, 87));
    StudentArray.add(new Student("4","Erin","Black","Erin.black@comcast.net", 22, 91, 98, 82));
    StudentArray.add(new Student("5","Tom","Gaye","tgay@att.net", 65, 99, 98, 97));

    //Example of printing specific student data using getters.
    System.out.println("");
        for (Student a: StudentArray) {
            System.out.println(a.getStuID());
            System.out.println(a.getFName());
            System.out.println(a.getLName());

        }   

}  

to Roster, that will do the job.