找不到java.lang.NullPointerException [英] Can't find java.lang.NullPointerException

问题描述

我已经对该程序进行了编码

这是稀疏矩阵类

I have coded this program

This is sparse matrix class

public class MatrixTerm {
 public int row,col,value;   
}

class SparsMatrix {
  private int Row,Col,Term;
 public  MatrixTerm SmArray[];
  int count=0;
  
  
  public SparsMatrix(int r,int c,int t){
      Row=r;
      Col=c;
      Term=t;
       SmArray =new MatrixTerm [20];
        }
  
  
  
  public void ReadSparse(){
        Scanner input= new Scanner(System.in);
      System.out.println("lotfan satr va  soton matrix vorodi ra vared konid ");
      Row=input.nextInt();
      Col=input.nextInt();
        int[][] matrix = new int [Row][Col];    
               for (int i=0;i<row;++i){>
                   for(int j=0;j<col;++j){>
                  System.out.println("lotfan "+"("+(i+1)+","+(j+1)+")"+" ra vared konid");     
                   matrix[i][j]=input.nextInt();
                   }}
               
              
                for (int i=0;i<Row;++i){>
                   for(int j=0;j<Col;++j){>
                       if(matrix[i][j]!=0){
                 SmArray[i].row=i;/// <big>HERE SAY java.lang.NullPointerException</big>
                 SmArray[i].col=j;
                 SmArray[i].value=matrix[i][j];
                           ++count;
                       }
                       
                   }
               }
  }
               
      public void show(){
          for(int i=0;i<count;++i){>
              System.out.println(SmArray[i]);
          }
    
}
               
               
  }

当我在第36行测试程序时，它说java.lang.NullPointerException
请帮助我

when i test the program in line 36 it say java.lang.NullPointerException
plz help me

推荐答案

用调试器检查i的值. Row值是动态的，但SmArray的声明是硬编码的20.SmArray =new MatrixTerm [20];

首先，从不调用SparsMatrix，因此不会初始化数组.其次，您可能会考虑使用列表而不是数组，因为列表会根据需要扩展，因此效率更高.

添加以下导入:

Check the value of i with the debugger. The Row value is dynamic but the declaration of the SmArray is hardcoded 20. SmArray =new MatrixTerm [20];

First of all, SparsMatrix is never called so the array isn''t initialized. Second, you might consider using a list instead of an array because a list will expand as needed and therefor more efficient.

Add these imports:

import java.util.Arrays;
import java.util.List;
import java.util.Iterator;

将数组声明更改为通用列表:

Change the array declaration to a generic list:

List smArray<matrixterm> = new ArrayList<matrixterm>();</matrixterm></matrixterm>

更改MatrixTerm以添加构造函数:

Change the MatrixTerm to add a constructor:

public class MatrixTerm {
  public int row,col,value;
 
  public MatrixTerm(int row, int col, int value) {
    this.row = row;
    this.col = col;
    this.value = value;
  }
}

像这样向smArray添加值:

Add values to smArray like this:

smArray.add(new MatrixTerm(row, col, value));

祝你好运！

Good luck!

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