java - 如何从另一个类中定义的类创建数组

Question

我是一名新手 Java 程序员，试图使用在不同文件中定义的类。所以，我编写了这两个 .java 文件:

首先是 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，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);           
        }
    }
}

我没有遇到任何问题，让该方法正常工作；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:我并不是在寻找更好的方法来实现这一目标；我只是使用这个简单的例子来尝试实现这一目标)。

当我尝试运行我的程序时，我得到:

java.lang.NullPointerException

我对此进行了研究，发现this page ，其中表示:

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，我尝试在主程序中执行此操作:

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

但这给出了这个错误:

an enclosing instance that contains MyLibrary.MyRecord is required

该错误消息将我引导至 this Stack Exchange question ，其中表示:

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();

<小时/>

因此，根据该答案，我将这两行添加到我的程序中:

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

这些行不会给出任何警告或编译错误，所以我觉得我又近了一步，但我仍在试图弄清楚如何创建一个数组。我知道如果我想创建一个整数数组，我只需这样做:

int[] TenInts = new int[10];

所以，我尝试过以下方法:

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

但是没有任何效果，我感觉自己现在就像是捕获了救命稻草。我感觉只要有一双正确的眼睛就能很快解决这个问题。

Answer 1

您需要将内部类声明为静态。

您可以按如下方式修改代码以满足您的要求:

这是 MyLibrary 的代码

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

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();
    }
}

您可以按如下方式创建内部类的实例:

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

希望这有帮助。