如何从另一个类中定义的类创建数组

问题描述：

我是Java的新手程序员，正在尝试使用在其他文件中定义的类.因此，我编写了以下两个.java文件:

I am a novice Java programmer trying to use classes defined in a different file. So, I've written these two .java files:

首先，有MyLibrary.java:

package mymainprogram;

public class MyLibrary {
    public class MyRecord {
        int number;
        char letter;
    }

    public static int TriplePlusThree(int input_number) {            
        return ((input_number*3) + 3);
    }
}

然后是MyMainProgram.java:

package mymainprogram;

import java.util.Scanner;

public class MyMainProgram {
    public static void main(String[] args) {
        Scanner keyread = new Scanner(System.in);

        System.out.print("Enter Number to Process: ");
        int num = keyread.nextInt();  
        int result = MyLibrary.TriplePlusThree(num);
        System.out.println("3x + 3 = "+result);

        String letters = "ABCDEFGHIJ";
        MyLibrary.MyRecord[] TenRecs = new MyLibrary.MyRecord[10];

        for (int i = 0; i < 10; i++) {
            TenRecs[i].number = i;      //NullPointerException here
            TenRecs[i].letter = letters.charAt(i);           
        }
    }
}

我完全可以使方法正常工作.现在我的目标是创建一个数组，其中数组的每个成员都有一个整数和字符.(注意:我不是在寻找实现此目标的更好方法；我只是在使用这个简单的示例来尝试实现此目的).

I had no problem getting the method to work just fine; now my goal is to create an array where each member of the array has an integer and character. (Note: I'm not looking for better ways to accomplish this objective; I'm merely using this trivial example to try to get this working).

当我尝试运行程序时，我得到了:

When I tried to run my program, I got:

java.lang.NullPointerException

我对此进行了研究，并发现了此页面，其中说:

I researched this, and found this page, which says:

如果即使在创建对象之前尝试访问对象，也会发生运行时错误.例如，以下语句在运行时抛出NullPointerException，它指示[this array]尚未指向[an]对象.必须使用该类的构造函数实例化这些对象，并应通过以下方式将它们的引用分配给数组元素.

If we try to access the objects even before creating them, run time errors would occur. For instance, the following statement throws a NullPointerException during runtime which indicates that [this array] isn't yet pointing to [an] object. The objects have to be instantiated using the constructor of the class and their references should be assigned to the array elements in the following way.

studentArray[0] = new Student();

因此，我尝试在主程序中做到这一点:

So, I tried to do that in my Main Program:

MyRecordArray[0] = new MyLibrary.MyRecord();

但是会出现此错误:

包含MyLibrary.MyRecord的封闭实例是必需的

an enclosing instance that contains MyLibrary.MyRecord is required

该错误消息导致我此Stack Exchange问题，内容为:

That error message led me to this Stack Exchange question, which says:

您必须创建X类(外部类)的对象，然后使用objX.new InnerClass()语法创建Y类的对象.

you have to create an object of X class (outer class) and then use objX.new InnerClass() syntax to create an object of Y class.

X x   = new X();
X.Y y = x.new Y();

因此，根据该答案，我在程序中添加了以下两行:

So, in accordance with that answer, I've added these two lines to my program:

MyLibrary mylibrary         = new MyLibrary();
MyLibrary.MyRecord myrecord = mylibrary.new MyRecord();

这些行不给出任何警告或编译错误，所以我觉得我离它更近了一步，但我仍在尝试弄清楚如何制作一个数组.我知道如果我想创建一个整数数组，我会简单地做到这一点:

Those lines don't give any warnings or compilation errors, so I feel like I'm one step closer, but I'm still trying to figure out how to make an array. I know if I wanted to make an array of integers, I would simply do this:

int[] TenInts = new int[10];

所以，我尝试过类似的事情:

So, I've tried things like:

myrecord[] TenRecs = new myrecord[10];
MyRecord[] TenRecs = new MyRecord[10];

但是没有任何效果，我感觉我现在正在抓稻草.我觉得合适的眼睛可以很快解决这个问题.

But nothing is working, and I feel like I'm grasping at straws now. I get the feeling that the right set of eyes could solve this pretty quickly.

答

您需要将内部类声明为静态.

You need to declare the inner class as static.

您可以按照以下要求修改代码:

You can modify the code as follows to suit your requirements:

这是MyLibrary的代码

This is the code for MyLibrary

public class MyLibrary {

    public static class MyRecord{
        int number;
        char letter;

        public MyRecord(){
            number = 0;
            letter = '\0';
        }

        public MyRecord(int number, char letter){
            this.number = number;
            this.letter = letter;
        }
    }

    public static int TriplePlusThree(int input_number){ 
        return (input_number * 3 + 3);
    }
}

这是MyMainProgram的代码

This is the code for the MyMainProgram

import java.util.Scanner;

public class MyMainProgram {

    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        System.out.println("Enter number to process");
        int num = in.nextInt();
        System.out.println("3x + 3 = " + MyLibrary.TriplePlusThree(num));

        String letters = "ABCDEFGHIJ";
        MyLibrary.MyRecord[] TenRecords = new MyLibrary.MyRecord[2];

        for (int i=0; i<TenRecords.length; i++){
            TenRecords[i] = new MyLibrary.MyRecord();
            TenRecords[i].number = i;
            TenRecords[i].letter = letters.charAt(i); 
        }

        // Printing class records
        for (int i=0; i<TenRecords.length; i++){
            System.out.println("Printing records of record " + i + " : ");
            System.out.println("Number : " + TenRecords[i].number);
            System.out.println("Letter : " + TenRecords[i].letter);
        }
        in.close();
    }
}

您可以如下创建内部类的实例:

You can create the instance of the inner class as follows:

TenRecords[i] = new MyLibrary.MyRecord();

希望这会有所帮助.

如何从另一个类中定义的类创建数组

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