java - 使用数组而不是 Java 中的 ArrayList 从用户输入创建一个新对象

Question

我在使用用户输入动态创建新对象时遇到问题。我知道如何使用 ArrayList 来做到这一点，但我想知道是否可以只使用一个数组？ Object 1 和 Object 2 从 MainObject 扩展而来。

我目前有:

import java.util.Scanner;
public class Main
{
public static void main (String args[]) 
{
MainObject[] Main = new MainObject[99];
//^objects created will be added to this array^
int input;
Scanner scanner = new Scanner(System.in);
do
{
    System.out.println("1. Add a new object 1");
    System.out.println("2. Add a new object 2");
    System.out.println("3. Display all object info");
    System.out.println("4. Quit");

    System.out.print("Please enter either 1 to 4: "); 
    input =(scanner.nextLine());
switch(input) {
    case 1 :
    object1 obj1 = new object1();
    System.out.println("Please enter name of object: ");
    obj1.setName(scanner.nextLine());
    obj1.display();


    case 2 :
    object2 obj2 = new object2();
    System.out.println("Please enter name of object: ");
    obj2.setName(scanner.nextLine());
    obj2.display();

    case 3 :        
    //this is where the for loop should be to display all the info of obj 1 and 2

    case 4 :
    System.out.println("Thank You");
    break;
  }
}
while (input==1 || input==2 || input==3)

所以我像这样将对象添加到数组中

case 1 :
object1 obj1 = new object1();
System.out.println("Please enter name of object: ");
obj1.setName(scanner.nextLine());
obj1.display();
Main[0] = obj1;
break;

case 2 :
object2 obj2 = new object2();
System.out.println("Please enter name of object: ");
obj2.setName(scanner.nextLine());
obj2.display();
Main[1] = obj2;
break;

case 3 :
int x = 0;
for (x=0; x<Main.length; x++)
   {
      Main[x].displayComputer();
   }
break;

编译并运行，它工作正常，但它给了我一个 java.lang.NULLPointerException:null 并且导致问题的突出显示代码是

Main[x].displayComputer();

Answer 1

ArrayList 可以具有可变大小，而数组是静态大小。也就是说，一旦你分配了一个数组，你就不能追加/插入新元素。但是，您可以分配一个大数组，然后逐个填充它。一般来说，这个过程是这样的:

int nextSpot = 0; //next spot to fill in array
while (still_getting_input) {
    if (supposed_to_insert) {
        if (nextSpot_in_valid_range)
            myArray[nextSpot++] = value_to_insert;
        else
            System.out.println("Invalid operation!"); //cannot insert.
    }
}

所以，你的程序应该是这样的:

import java.util.Scanner;
public class Main
{
public static void main (String args[]) 
{
MainObject[] Main = new MainObject[99];
//^objects created will be added to this array^
String input;
Scanner scanner = new Scanner(System.in);
int nextSpot = 0;
do
{
    System.out.println("1. Add a new object 1");
    System.out.println("2. Add a new object 2");
    System.out.println("3. Display all object info");
    System.out.println("4. Quit");

    System.out.print("Please enter either 1 to 4: "); 
    input =(scanner.nextLine());

    switch(input) {
    case 1 :
    if (nextSpot < Main.length) {
        object1 obj1 = new object1();
        System.out.println("Please enter name of object: ");
        obj1.setName(scanner.nextLine());
        obj1.display();
        Main[nextSpot++] = obj1;
    }
    else {
        System.out.println("Error!");
    }
    break;

    // etc.

    }
}
while (input==1 || input==2 || input==3)

您的代码还有一些其他问题(特别是您对 switch 语句的使用；您将遇到失败)，但这回答了您提出的问题。