2D数组空指针异常错误 [英] 2D array Null Pointer Exception error
问题描述
public class DoubleMatrix
{
private double[][] doubMatrix;
public DoubleMatrix(int row, int col)
{
if(row > 0 && col > 0)
{
makeDoubMatrix(row,col);
}
else
{
row = 1;
col = 1;
}
}
public DoubleMatrix(double[][] tempArray)
{
if(tempArray != null)
{
for(int i = 0; i < tempArray.length-1;i++)
{
if(tempArray[i].length == tempArray[i+1].length)
{
tempArray = doubMatrix;
}
}
}
else
{
makeDoubMatrix(1,1);
}
}
public int getDim1()
{
return doubMatrix.length;
}
public int getDim2()
{
return doubMatrix[0].length;
}
private void makeDoubMatrix(int row, int col)
{
double[][] tempArray = new double[row][col];
for (int i = 0;i < row;i++)
{
for(int j = 0;j < col;j++)
{
tempArray[i][j] = Math.random() * (100);
doubMatrix[i][j] = tempArray[i][j];
}
}
}
public DoubleMatrix addMatrix(DoubleMatrix secondMatrix)
{
//this. doubMatrix = doubMatrix;
double[][] tempArray;
if(secondMatrix.doubMatrix.length == doubMatrix.length)
if(secondMatrix.doubMatrix[0].length == doubMatrix[0].length)
{
tempArray = new double[doubMatrix.length][doubMatrix[0].length];
for(int i = 0; i< secondMatrix.doubMatrix.length;i++)
for(int j = 0; j< secondMatrix.doubMatrix[i].length;j++ )
{
tempArray[i][j] = secondMatrix.doubMatrix[i][j] + doubMatrix[i][j];// add two matrices
}//end for
return new DoubleMatrix (tempArray);
}
return new DoubleMatrix(1,1);
}
public DoubleMatrix getTransposedMatrix()
{
double[][] tempArray = new double[doubMatrix.length][doubMatrix[0].length];
for(int i = 0;i < doubMatrix.length;i++)
for(int j = 0;j < doubMatrix[i].length;j++)
{
tempArray[j][i] = doubMatrix[i][j];// transposed matrix2
}//end for
return new DoubleMatrix(tempArray);
}
public DoubleMatrix multiplyingMatrix(DoubleMatrix secondMatrix)
{
double[][] tempArray = new double[secondMatrix.doubMatrix.length][doubMatrix[0].length];
//check if dimension of matrix1 equal to dimension of matrix2
if(secondMatrix.doubMatrix[0].length == doubMatrix.length)
if(doubMatrix.length == secondMatrix.doubMatrix[0].length)
{
for (int i = 0; i <secondMatrix.doubMatrix.length; i++)
{
for(int j = 0; j < doubMatrix[0].length; j++)
{
for (int k = 0; k < doubMatrix.length; k++)
{
tempArray[i][j] = tempArray[i][j] + secondMatrix.doubMatrix[i][k]*doubMatrix[k][j]; // multiply 2 matrices
}
}
}//end for
}// end if
return new DoubleMatrix(1,1);
}
public void printMatrix(String text)
{
System.out.println(text);// output string
for(int i = 0; i< doubMatrix.length;i++)
{
for(int j = 0; j< doubMatrix[i].length;j++ ) {
System.out.printf("%9.1f", doubMatrix[i][j]);// out put value for matrices
}
System.out.println();
}
}
}
public class Program3
{
public static void main(String[] args)
{
int num1 = (int) (Math.random()*(10-3+1)+3);
int num2 = (int) (Math.random()*(10-3+1)+3);
DoubleMatrix doubMatObj1 = new DoubleMatrix(num1,num2);
DoubleMatrix doubMatObj2 = new DoubleMatrix(doubMatObj1.getDim1(),doubMatObj1.getDim2());
DoubleMatrix doubMatObj3;
doubMatObj2.getDim1();
doubMatObj3 = doubMatObj1.addMatrix(doubMatObj2);
doubMatObj1.printMatrix("First Matrix Object");
doubMatObj2.printMatrix("Second Matrix Object");
doubMatObj3.printMatrix("Result of Adding Matrix Objects");
doubMatObj2 = doubMatObj2.getTransposedMatrix();
doubMatObj2.printMatrix("Result of inverting Matrix Object");
doubMatObj3 = doubMatObj1.multiplyingMatrix(doubMatObj2);
doubMatObj3.printMatrix("Result of Multiplying Matrix Objects");
}
}
我有一个 makeDoubMatrix方法的最后一行语句中出现NullPointerException 错误，以及第一个构造函数的if语句中调用makedoubMatrix。
doubMatrix在我已经初始化时似乎为空。我将如何解决此问题？
Hi, I have a NullPointerException error in the last line statement of the makeDoubMatrix method as well the call makedoubMatrix in side if statement of the first constructor.
doubMatrix seems to be null when I already initialize it. How will I be able to fix this problem ?
推荐答案
您要初始化数组，即：
private double[][] doubMatrix = new double[size1][size2];
其中size1和size2是任意大小。您可能想要的是：
where size1 and size2 are arbitrary sizes. What you probably want is:
if(row > 0 && col > 0)
{
doubMatrix = new double[row][col];
makeDoubMatrix(row,col);
}
else
{
doubMatrix = new double[1][1];
makeDoubMatrix(1,1);
}
如果行和行都将数组doubMatrix初始化为row * col的大小col大于0，否则为1 * 1，然后使用其初始化大小调用makeDoubMatrix（您可以在if-else之后使用 doubMatrix.size 调用此方法和 doubMatrix [0] .size ，但我认为它现在更具可读性。
which initializes the array doubMatrix to a size of row*col if both row and col are greather than 0, and to 1*1 otherwise, then calls makeDoubMatrix with its initialized size (you could have this method call after the if-else, using doubMatrix.size and doubMatrix[0].size, but I think it's more readable now).
更改第二个构造函数（使用相同的推理，它需要一个二维数组。
Change the second constructor (which takes a 2D array) using the same reasoning.
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