Question

我已使用输入获取值并将其添加到ArrayList。当我第二次输入数据时，它会在输入第二个数据后覆盖第一个数据。无法修复它。

学生班

public class Student {
static int rollNumber, age;
static String firstName, lastName, gender;
public Student(int rollNumber, String firstName, String lastName, int age, String gender) 
 {
   this.rollNumber = rollNumber;
   this.firstName = firstName;
   this.lastName = lastName;
   this.age = age;
   this.gender = gender;
  }
}

InputParticipant类

import java.util.*;
public class InputParticipant extends Student{
        public static String sportsEvent;
        public InputParticipant(int rollNumb, String firstName, String lastName,
        int age, String gender, String sportsEvent) 
        {
            super(rollNumber, firstName, lastName, age, gender);
            this.sportsEvent=sportsEvent;
        }
        public void elegibility()
        {
            System.out.print("Eligibility:");
            //50 meter race
            if(age>=10 && age<=12 && gender.equals("female") && sportsEvent.equals("50meterrace"))
            {
                System.out.println(" Yes ");
            }
            else if((age>=10 && age<=15) && sportsEvent.equals("100meterrace"))//100 meter race
            {
                System.out.println(" yes ");
            }
            else if(age>=15 && (sportsEvent.equals("400meterrace") || sportsEvent.equals("Juvelin") || sportsEvent.equals("discus")))//400 meter race, Juvelin and discus
            {
                System.out.println(" Yes ");
            }
            else
            {
                System.out.println(" No (age:Min 10-12 for 50m race-female only, Min 10-15 for 100m race and Min 15 for 400m race, Juvelin and discus)");
            }

         }
         public static void main(String[] args) 
         {
             List<InputParticipant> stud=new ArrayList<InputParticipant>();
             InputParticipant i;
             i=new InputParticipant(rollNumber, firstName, lastName, age, gender, sportsEvent);
             Scanner in=new Scanner(System.in);
             while(true)
             {
                System.out.println("Enter the Student details:");
                System.out.print("roll number:");
                rollNumber=in.nextInt();
                System.out.print("First name:");
                firstName=in.next();
                System.out.print("Last name:");
                lastName=in.next();
                System.out.print("age:");
                age=in.nextInt();
                System.out.print("gender:");
                gender=in.next();
                System.out.print("sports event(50meterrace/100meterrace/400meterrace/Juvelin/discus):");
                sportsEvent=in.next();
                stud.add(i);
                List<InputParticipant> l=new ArrayList<InputParticipant>();
                l.addAll(stud);
                Iterator<InputParticipant> disp= l.iterator();
                while(disp.hasNext())
                {
                    InputParticipant st=disp.next();
                    System.out.println("Roll Number:"+st.rollNumber+"\nName: "+st.firstName+" "+st.lastName+"\nGender: "+st.gender+"\nAge: "+st.age+"\nSports Event:"+st.sportsEvent);
                    st.elegibility();
          System.out.println("-----------------------------------------");
                }
            }
        }
   }

Answer 1

因为你在循环中创建了新对象： List l = new ArrayList（）;

尝试这样的事情：

public static void main(String[] args) {
    List<InputParticipant> stud=new ArrayList<InputParticipant>();
    //List<InputParticipant> l=new ArrayList<InputParticipant>();

    Scanner in = new Scanner(System.in);
    while(true){
        System.out.println("Enter the Student details:");
        System.out.print("roll number:");
    rollNumber=in.nextInt();
    System.out.print("First name:");
    firstName=in.next();
    System.out.print("Last name:");
    lastName=in.next();
    System.out.print("age:");
    age=in.nextInt();
    System.out.print("gender:");
    gender=in.next();
    System.out.print("sports event(50meterrace/100meterrace/400meterrace/Juvelin/discus):");
    sportsEvent=in.next();
    stud.add(i);
    InputParticipant i = new InputParticipant(rollNumber, firstName, lastName, age, gender, sportsEvent);

    Iterator<InputParticipant> disp = l.iterator();
    while(disp.hasNext()){
        InputParticipant st=disp.next();
        System.out.println("Roll Number:"+st.rollNumber+"\nName: "+st.firstName+" "+st.lastName+"\nGender: "+st.gender+"\nAge: "+st.age+"\nSports Event:"+st.sportsEvent);
        st.elegibility();
        System.out.println("-----------------------------------------");
    }
    }
    }

Answer 2

可能尝试使用Java POJO。

在您的情况下，从Scanner获取值后，创建一个新Object，然后将该对象添加到ArrayList。

i = new InputParticipant(rollNumber, firstName, lastName, age, gender, sportsEvent);
stud.add(i);

不确定为什么要在while循环中创建另一个ArrayList，因为while应仅用于收集输入。

Answer 3

我无法真正帮助您修复代码，因为很多事情都是很奇怪。

为什么看起来你总是重写可能与创建Student static 的所有类成员有关，这意味着Student类的每个实例都具有相同的名称，firstname，...

- 也可以从创建一个Student类开始，该类只包含带有getter和setter的私有成员以及所有这些属性的构造函数。

- 然后在另一个Main类中创建main方法，这将阻止你做你不应该做的事情。

- 在该方法实例化一个List，此后开始一个循环。

- 在该循环中，您将1名学生的数据收集到局部变量中（String name = in.readNext（）; String firstName = ....）

- 您使用构造函数实例化Student对象，只需将本地变量传递给它。

- 将新创建的Student对象添加到列表中。

- 循环完成后，您应该有一个填充学生的列表。

Answer 4

Student类和InputParticipant类的成员都是静态的。阅读static members。换句话说，ArrayList没有被覆盖，静态成员就像是同一个类的不同对象之间的共享变量。如果将某个内容分配给一个对象中的静态成员，则该成员将在所有对象中具有新值。只需删除static关键字并使其成为实例成员。

public class Student {
public int rollNumber, age;
public String firstName, lastName, gender;
public Student(int rollNumber, String firstName, String lastName, int age, String gender) 
 {
   this.rollNumber = rollNumber;
   this.firstName = firstName;
   this.lastName = lastName;
   this.age = age;
   this.gender = gender;
  }
}

如何停止覆盖arraylist？

4 个答案: