此函数中的Nullpointerexception [英] Nullpointerexception in this function

问题描述

Hello CodeProject社区，



我正在尝试实例化一个元素矩阵。但是当我调用矩阵中的某个元素时，它会返回Null（或者在调用方法时返回NullPointerException）。这是我的代码的简化版本，仍然返回Null。我想知道导致这个问题的原因。谢谢！



祝你好运



我的尝试： < br $> b $ b

  public   class  MyProgram  extends  ConsoleProgram 
 {
 
  public   void  run（）
 {
 someClass [] [] sc =  new  someClass [< span class =code-digit> 1 ] [ 2 ]; 
 System.out.println（sc [ 0 ] [ 0 ]。getX（）） ; 
} 
 
} 
 
  public   class  someClass 
 {
 
  private   int  x =  0 ; 
  public  someClass（）
 {
 
} 
 
  public   int  getX（）
 {
  return  x; 
} 
} 

解决方案

你的问题在于功能运行



这就是你所做的事情

  public   void  run（）
 {
 someClass [] [] sc =  new  someClass [ 1 ] [ 2 ];  //  您已为数组本身分配了一些空间;  
 System.out.println（ sc [ 0 ] [ 0 ]。getX（））;  //  访问此处是违规行为 
  / /  因为你没有为每个对象初始化空间;因此，在你开始访问之前，你需要初始化。 
} 
 

换句话说

< pre lang =java> sc [ 0 ] [ 0 ] = new someClass（）;

当声明一个原始变量数组时，java会为该变量设置默认值。对于所有类和String，默认为null。按照示例更好地理解

  public   class  Klass {
  public   int  x = 10; 
  public   static   void  main （ String  [] args）{
  int  [] i =  new   int  [ 2 ]; 
  char  [] c = new  char  [ 2 ]; 
 Klass [] k = new Klass [ 2 ]; 
 字符串 s [] =  new   String  [ 2 ]; 
 System.out.println（i [ 0 ] +   + i [ 1 ]）; 
 System.out.println（c [ 0 ] +   + c [ 1 ]）;  //  在这种情况下，c设置为零 
 System.out.println（s [  0 ] +   + s [  1 ]）; 
 System.out.println（k [ 0 ] +   + k [ 1 ]）; 
 尝试 {
 System.out.println（k [ 0 ]。x ）;  //  因为k为null，它将引发异常 
}  catch （NullPointerException e）{
 e.printStackTrace（）; 
} 
 k [ 0 ] =  new  Klass（）; 
 System.out.println（k [ 0 ]。x）;  //  您会看到一些结果 
} 
} 

Hello CodeProject community,

I'm trying to instantiate a matrix of an element. But when I call a certain element in the matrix, it would return a Null (or a NullPointerException when calling a method). Here's a simplified version of my code which still returns Null. I wonder what causes this problem. Thank you!

Best regards

What I have tried:

public class MyProgram extends ConsoleProgram
{

    public void run()
    {
        someClass[][] sc = new someClass[1][2];
        System.out.println(sc[0][0].getX());
    }
    
}

public class someClass
{
    
    private int x = 0;
    public someClass()
    {
 
    }
    
    public int getX()
    {
        return x;
    }
}

解决方案

your problem is with function run

This is what you have done

public void run()
{
       someClass[][] sc = new someClass[1][2]; // you have allocated some space for the array itself;
       System.out.println(sc[0][0].getX()); // accessing here is a violation
       // since you have not initialize space for each object; hence before you start accessing you need to initialize.
}

in other word

sc[0][0] = new someClass();

When you declare an array of primitive variables, java set default value for that variable. And for all classes and String default is null. Follow the example to understand better

public class Klass {
    public int x=10;
    public static void main(String []args) {
        int []i = new int[2];
        char []c=new char[2];
        Klass []k=new Klass[2];
        String s[] = new String[2];
        System.out.println(i[0] + " " + i[1]);
        System.out.println(c[0] + " " + c[1]); // in this case c is set to zero
        System.out.println(s[0] + " " + s[1]);
        System.out.println(k[0] + " " + k[1]);
        try {
          System.out.println(k[0].x); // since k is null it will thow exception
        } catch(NullPointerException e) {
            e.printStackTrace();
        }
        k[0] = new Klass();
        System.out.println(k[0].x); // You will see some result
    }
}

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