为变量赋值

Question

import java.util.Scanner;
public abstract class Person {  
public String firstName ;
public String lastName;
public String emailID;
public long phoneNumber;
Person(){}
public void readInfo() {    
    Scanner sc;
}
public void printInfo() {   
    System.out.println(firstName + lastName + emailID + phoneNumber);
}       
}




public interface Policies { 
double MAX_MARKS = 100;
double MAX_GPA = 4.0;   
public void calculateGPA(double[] marks) ;
}




import java.util.Scanner;
public class Student extends Person  implements Policies {
public int studentNumber;
public String programName;
public double gpa;
Student(){}
@Override
public void readInfo() {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter program name: ");
    programName = sc.next();
    System.out.print("Enter student number: ");
    studentNumber = sc.nextInt();
    System.out.print("Enter first name: ");
    firstName = sc.next();
    System.out.print("Enter last name: ");
    lastName = sc.next();
    System.out.print("Enter email ID: ");
    emailID = sc.next();
    System.out.print("Enter phone number: ");
    phoneNumber = sc.nextLong();
    readMarks();
   }
    private void readMarks() {
    Scanner sc = new Scanner(System.in);
    System.out.print("Enter number of courses: ");
    int numCourses = sc.nextInt();
    double marks[] = new double[numCourses];
    for(int i = 0; i < numCourses; i++) {
        System.out.print("Enter mark " + (i+1) + ":");
        marks[i] = sc.nextDouble();
    }

    calculateGPA(marks);        
   }
   @Override
    public void calculateGPA(double marks[]) {
    double totalMarks =0;
    for(int i=0; i < marks.length; i++)
        totalMarks = totalMarks +marks[i];
    gpa = (totalMarks/marks.length *MAX_MARKS)*MAX_GPA;
  }
  @Override
   public void printInfo() {
    super.printInfo();
    System.out.printf("%25d  | %25s  | %25s  | %25s  | %25s  | %25.2f  |\n", studentNumber 
,programName , firstName, lastName, emailID,  gpa);
}
 }




import java.util.Scanner;
public class FulltimeStudent extends Student {  
double tuitionFees; 
FulltimeStudent(){} 
@Override
public void readInfo() {    
    Scanner sc = new Scanner (System.in);   
    super.readInfo();   
    System.out.println("Enter tuition fees");
    tuitionFees = sc.nextDouble();  
}
@Override
public void printInfo() {   
    super.printInfo();  
    System.out.printf("%25d |\n",tuitionFees);  
}
}




import java.util.Scanner;
public class ParttimeStudent extends Student {  
double courseFeesTotal;
double credits; 
ParttimeStudent(){}
@Override
public void readInfo() {
    Scanner sc = new Scanner (System.in);
    super.readInfo();    
    System.out.println("Enter total course fees:");
    courseFeesTotal = sc.nextDouble(); 
    System.out.println("Enter credit hours:");
    credits = sc.nextDouble();
}
@Override
public void printInfo() {   
    super.printInfo();  
    System.out.printf("%25d | %25d\n",courseFeesTotal, credits);    
}   
}






import java.util.Scanner;
public class College {
String name;
Student [] students;
int numStudents;
College(){}
College(String name, int num){
    this.name = name;
    students = new Student[num];
}
public void printTitle() {
    System.out.println(name + " - List of Students");
    for(int i = 0; i< 40; i++ )
        System.out.print("*");
    System.out.printf("%n%15s  | %15s  | %15s  | %15s  | %15s  | %15s  | %15s  | %15s  |\n", 
 "Program" , "Student#" , "Name" , "Email" , "Phone" , "GPA", "Fees", "Credits");
}
public void ReadStudentDetails() {
    Scanner sc = new Scanner (System.in);
    System.out.println("1 - Fulltime Student \n2 - Parttime Student");
    System.out.print("Enter student type:");
    int type = sc.nextInt();
    if (type == 1) {    
        FulltimeStudent fts = new FulltimeStudent();
        fts.readInfo();
    }
    else if (type == 2) {
        ParttimeStudent pts = new ParttimeStudent();
        pts.readInfo();
    }
    else if (type <1 || type >2) { 
        System.out.println("*** Wrong student type... PLease re-enter ***");
    }
    while (type <1 || type >2) {
        System.out.println("1 - Fulltime Student \n2 - Parttime Student");
        System.out.print("Enter student type:");
        type = sc.nextInt();
        if (type == 1) {    
            FulltimeStudent fts = new FulltimeStudent();
            fts.readInfo();
        }
        else if (type == 2) {
            ParttimeStudent pts = new ParttimeStudent();
            pts.readInfo();
        }
        else if (type <1 || type >2) { 
            System.out.println("*** Wrong student type... PLease re-enter ***");
        }
    }
    numStudents++;
}
public void printStudentDetails() {
    if (numStudents == 0) {
        System.out.println("*** No students to print ***");
    }
    else {
        printTitle();
        for (int i = 0; i < numStudents; i++) {
            if (students[i] != null) 
                students[i].printInfo();
        }
    }
}
}




import java.util.Scanner;
 public class CollegeSystemTest {
public static void main(String[] args) {    
    Scanner sc = new Scanner (System.in);
    System.out.println("Enter name of college:");
    String name = sc.next();
    System.out.println("Enter number of students:");
    int numStudents = sc.nextInt(); 
    College c = new College(name, numStudents);     
    int option;     
    do {
        System.out.println("1. Read student \n2. Print details of all students \n3. Exit 
  \nEnter your option:");
        option = sc.nextInt();
        if (option == 1) {
            c.ReadStudentDetails();     
        }
        else if (option == 2) { 
            //c.printTitle();
            c.printStudentDetails() ;
        }
    }while(option != 3); //a do-while loop is used to display menu items and will keep running 
  until option 3 is chosen
    sc.close(); 
}
 }

Answer 1

让我们从printstudentdetails()开始解决这个问题

你说什么都没有输出（大概是...除了标题）。

    for (int i = 0; i < numStudents; i++) {
        if (students[i] != null) 
            students[i].printInfo();
    }

问：为什么那不会打印什么？

null

Student [] students;
int numStudents;
College(){}

College(String name, int num) {
    this.name = name;
    students = new Student[num];
}

int type = sc.nextInt();
if (type == 1) {    
    FulltimeStudent fts = new FulltimeStudent();
    fts.readInfo();
}
...

答：你把信息读进去，然后....没什么。

问：它如何进入student数组？

A：没有！！

null