[英]Java Why am I getting a NullPointerException while instantiating my array
我是编程新手,当我尝试初始化 n、numInt 和 arrayMenu 时,我不明白为什么程序会给我一个 NullPointerException 的运行时错误。 没有一个似乎工作。 该程序的工作是收集一组随机整数以存储在一个数组中,并允许用户选择要选择的排序。 谢谢阅读。
import java.util.Scanner;
import java.util.Random;
public class VariousSortsHS
{
private static int[] arrayMenu;
private static Random generator;
/**
* Constructor for objects of class VariousSortsHS.
*/
public VariousSortsHS(int n) //The error starts here
{
arrayMenu = new int[n]; //I don't get why it says null in the array when
//i am initializing the length of the array to n
/*Assigns a random number between 0 too 100.*/
for(int i = 0; i < n; i++)
{
int temp = generator.nextInt(100);
arrayMenu[n] = temp;
}
}
/**
* Selection Sort method.
*/
public static void selection(int n)
{
for(int i = 0; i < arrayMenu.length - 1; i++)
{
int minPos = i;
for(int j = i + 1; j < arrayMenu.length; j++)
{
if(arrayMenu[j] < arrayMenu[minPos]) minPos = j;
}
int temp = arrayMenu[i];
arrayMenu[i] = arrayMenu[minPos];
arrayMenu[minPos] = temp;
System.out.print(temp + " ");
}
}
/**
* Insertion Sort method.
*/
public static void insertion(int n)
{
for(int i = 1; i < arrayMenu.length; i++)
{
int next = arrayMenu[i];
int j = i;
while(j > 0 && arrayMenu[j - 1] > next)
{
arrayMenu[j] = arrayMenu[j - 1];
j--;
}
arrayMenu[j] = next;
System.out.print(next + " ");
}
}
/**
* Quick Sort method.
*/
public static void quick(int n)
{
int pivot = arrayMenu[0];
int i = 0 - 1;
int j = n + 1;
while(i < j)
{
i++; while(arrayMenu[i] < pivot) i++;
j++; while(arrayMenu[j] > pivot) j++;
if(i < j)
{
int temp = arrayMenu[i];
arrayMenu[i] = arrayMenu[j];
arrayMenu[j] = temp;
System.out.print(temp + " ");
}
}
}
/**
* Main method that allows user to input data.
*/
public static void main(String[] args)
{
Scanner in = new Scanner(System.in);
System.out.println("Do you wish to sort random integers? (Yes or No) ");
String answer = in.next();
String answer2 = answer.toLowerCase();
do
{
/*Prompts for array length.*/
System.out.println("How many random integers do you wish to sort?");
int numInt = in.nextInt();
/*Promps for sort selection choice.*/
System.out.println("Select a sort to use: \n\t1)Selection\n\t2)Insertion\n\t3)Quick");
String sort = in.next();
String sort2 = sort.toLowerCase();
if(sort2.equals("selection"))
{
selection(numInt);
}
else if(sort2.equals("insertion"))
{
insertion(numInt);
}
else if(sort2.equals("quick"))
{
quick(numInt);
}
else
{
System.out.println("You have entered the wrong input.");
}
} while(!answer2.equals("no"));
}
}
null
。 考虑将您的构造函数代码改为静态初始化块。generator
永远不会被设置为任何东西,所以它也是 null 并且你不能在它上面调用nextInt
arrayMenu[n]
而不是arrayMenu[i]
当你调用insertion(numInt);
, 方法public static void insertion(int n)
被调用,然后你试图做这样的 for 循环for(int i = 1; i < arrayMenu.length; i++)
但是, arrayMenu
没有初始化,它是空的。 当您尝试在 null 上调用长度时,您会收到 NullPointerException。
您需要添加一个静态构造函数并使用静态 int 初始化大小
//set parameter = n
public static int parameter;
static
{
arrayMenu = new int[parameter];
}
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