java - 无法对非静态现场学生进行静态引用

Question

我正在开发一个项目，该项目创建一个部分填充的 Student 类型数组，该数组永远不会超过 100,000 个 Student 对象。我正在尝试对“列表”部分进行编码，该部分应该以数组的顺序打印出所有学生。

由于静态引用问题，我无法进行编译。我正在尝试使用 print() 方法，该方法已在第二个文件中创建，但无法执行此操作。

这是我到目前为止的代码:

import java.util.Scanner; // for keyboard access


public class Lab6 {
final int LENGTH = 100001;
private Student[] student = new Student[LENGTH];
private int currSize;

/**
 * promptMenu - dumps the menu to the screen and gets the user's selection 
 *       from the Scanner (usually keyboard), returning it
 * @param usingInputStream - where to place the menu text (usually
 * @return caharacter representing the user's (lowercased) selection
 */
public static char promptMenu(Scanner usingInputStream)
{
    // dump the menu ...
    System.out.println("Choose one:");
    System.out.println("  (L)ist all students.");
    System.out.println("  (A)dd a student.");
    System.out.println("  (R)ead many students from file.");
    System.out.println("  Sort Students by (F)inal exam score.");
    System.out.println("  Sort Students by final a(V)erage.");
    System.out.println("  Sort Students by (N)ame.");
    System.out.println("  (Q)uit.");

    // get the user's request ...
    System.out.print("===>");
    return Character.toLowerCase(usingInputStream.next().charAt(0));        
}
public Lab6()
{
student = new Student[LENGTH];
}

public static void main(String[] args) {

    // allow for keyboard input
    Scanner kbd = new Scanner(System.in);

    char choice; // holds the user's chosen option

    // repeat until the user quits. 
    do
    {
        // print menu and get user's selection form menu 
        choice = promptMenu(kbd);

        // based on user's choice, do something ....
        switch(choice)
        {

        /***************************************************************************
         * YOU WILL WANT TO ADD CODE FOR EACH OF CASES l, a, r, f, v, and n  !!!!! *
         ***************************************************************************/

        case 'l':  // list array contents in current ordering (if any)
            if(student != null)
            {
                for(int i=0; i<currSize; i++)
                student.toString();
                System.out.print(student);
            }
            else
                System.out.println("The array is empty.");
            break;

        case 'a':  // prompt user to add a single student and data from the keyboard
            System.out.print("Please add a single student and data from the keyboard: ");

            student = new Student[kbd.nextInt()];

            for(int i=0; i< student.length; i++)
                student[i] = kbd.nextInt();
            break;
        case 'r':  // read data from file
            break;
        case 'f':  // sort data by final exam score (numerically high to low)
            break;
        case 'v':  // sort data by final average (numerically high to low)
            break;
        case 'n':  // sort data by student name (alphabetically low to high)
            break;

        case 'q':  // do nothing ...
            break;  
        default:   // uh, oh ... user gave a bad input. 
            System.out.println("Bad choice ("+choice+") try again!");
        }
    }
    while (choice!='q'); // if user selected quit, then get out of loop.

    // print terminating message (so user knows program has not "hung". 
    System.out.println("Thank you for using student record keeper!");
}

}

//以下是我在单独文件中给出的其他方法:

import java.util.Scanner;  //code uses Scanner class

public class Student {

private String name;  // student name (no spaces)
private int exam1;    // student exam #1 score
private int exam2;    // student exam #2 score
private int finalExam;// student final exam score

/**
 * Constructor for Student class, given a student's pertinent data 
 * @param studentName    - first name of student, as a String.
 * @param exam1Score     - student's score on exam #1
 * @param exam2Score     - student's score on exam #2
 * @param finalExamScore - student's score on final exam
 */
public Student(String studentName, 
               int exam1Score, int exam2Score, int finalExamScore)
{
    name = new String(studentName); // don't want reference copy
    exam1 = exam1Score;
    exam2 = exam2Score;
    finalExam = finalExamScore;
}   

/**
 * Constructor for Student class, to be read from a Scanner. The Scanner
 *   could be the keybard or an open file, etc...
 *   
 * @param s  - the Scanner from which to read a student's data. 
 * 
 * NOTE: very important that data is provided in the order:
 *    name   exam1    exam2    final
 */
public Student(Scanner s)
{
    // for each instance varaiable, just read the associated data from the Scanner
    name = s.next();
    exam1 = s.nextInt();
    exam2 = s.nextInt();
    finalExam = s.nextInt();
}

// Accessors

/**
 * getName 
 * @return the Student's name
 */
public String getName() {return name;}
/**
 * getExam1
 * @return the Student's score on exam #1
 */
public int getExam1() {return exam1;}
/**
 * getExam2
 * @return the Student's score on exam #2
 */
public int getExam2() {return exam2;}
/**
 * getFinal
 * @return the Student's score on the final exam
 */
public int getFinal() {return finalExam;}
/**
 * getAvergae
 * @return the Student's overall average
 */
public double getAverage()
{
    double avg = (2.0/7)*exam1 + (2.0/7)*exam2 + (3.0/7)*finalExam;
    return avg;
}

// Mutators

/**
 * setName
 * @param toName the new name for the student
 */
public void setName(String toName) {name=new String(toName);/*again, don't want reference copy*/}
/**
 * setExam1
 * @param toScore the new exam1 score
 */
public void setExam1(int toScore) {exam1=toScore;}
/**
 * setExam2
 * @param toScore the new exam2 score
 */
public void setExam2(int toScore) {exam2=toScore;}
/**
 * setFinal
 * @param toScore the new final exam score
 */
public void setFinal(int toScore) {finalExam=toScore;}

//utility methods

/**
 * toString
 * @return String representing the Student data. Suitable for use when listing students.
 * 
 * note that adding a toString() method to any class allows you to use an object of that
 *   anywhere a String could go. The most common place for such is when printing, and
 *   the result would be that what is printed is what is returned by the toString method.
 *   So, the following code:
 *       Student almostPerfect = new Student("Perfection", 99, 98, 100);
 *       System.out.println(almostPerfect);
 *   would print something like:
 *                              Perfection:   99   98  100   99.14
 */
public String toString()
{
    String result = String.format("%40s:%5d%5d%5d%8.2f", 
                                  name, exam1, exam2, finalExam,
                                  getAverage());
    return result;
}

/**
 * print - simply dumps the STudent to the screen, using format found above in toString()
 */
public void print()
{
    System.out.print(this);
}

}

Answer 1

哦，亲爱的，我很抱歉你的代码真的很乱而且不合适。也许开始使用 IDE(Eclipse？)。

无论哪种方式，这一行都是问题

private Student[] student = new Student[LENGTH];
// it needs to be changed to static since its in the main method and that is a static
private static Student[] student = new Student[LENGTH]; 

尽管我真的建议您研究一下 static 修饰符的作用......以及如何正确初始化静态对象。

我将尝试快速概述静态修饰符，

1) 可以从任何其他类调用静态方法，具体取决于可见性限制(私有(private)、公共(public)、 protected 、默认)。要从具有正确访问权限的另一个类调用静态方法，您可以执行 NAMEOFCLASSWHEREMETHODIS.METHODNAME();

2) 静态方法/对象可以被调用，因为它不需要类的实例。因此不需要执行 CLASSNAME classn = new CLASSNAME()；缺点是静态对象在启动时初始化。

我会继续，但那里有比我更好的指南。请查看它并继续努力编写您的代码!记得拿个ide，咳https://www.eclipse.org/downloads/

编辑: 在Student.java中

 @Override
public String toString() {
   return this.name + " " + this.score (and so on) ...; 
}

然后你可以进去做类似的事情

System.out.println(student);

或者如果你想打印数组

for(Student s: student) 
   System.out.println(s)