为什么会有这样的异常？

Question

import java.io.*;
public class KeyInput
{
public static int readInt() throws IOException
{
String s;
InputStreamReader ir;
BufferedReader in;
ir=new InputStreamReader(System.in);
in=new BufferedReader(ir);
System.out.print("： ");
s=in.readLine();
int i=Integer.parseInt(s);

    return i;
}
public static double readDouble() throws IOException
{
   String s;
    InputStreamReader ir;
    BufferedReader in;
    ir=new InputStreamReader(System.in);
    in=new BufferedReader(ir);
    s=in.readLine();
    double d=Double.parseDouble(s);
    return d;
}

}

import java.io.IOException;
public class Matrix{
private int rows;
private int cols;
double[][] data;
public Matrix(){
this(0,0);
}
public Matrix(int rows,int cols){
this.rows = rows;
this.cols = cols;
data = new double[rows][cols];
}
public Matrix(int rows,int cols,double data[][]){
this.rows = rows;
this.cols = cols;
this.data = data;
}
public void setData(int rows,int cols,double value){
this.rows = rows;
this.cols = cols;
data[rows][cols] = Math.random()*value;
}
public double getData(int rows,int cols){
return data[rows][cols];
}
public Matrix multiply(Matrix B){
Matrix A = this;
if(A.cols!=B.rows)
new IllegalArgumentException("矩阵不匹配！");
Matrix C = new Matrix(A.rows,B.cols);

    for(int i=0; i<C.rows; i++)
        for(int j=0; j<C.cols; j++)
            for(int k=0; k<A.cols; k++)
                C.data[i][j] += (A.data[i][k]*B.data[k][j]);
    return C;
}
public String toString(){
    StringBuffer s = new StringBuffer();
    int rows = data.length;
    int cols = data[0].length;
    s.append("Matrix ["+rows+"]["+cols+"] ="+"\n");
        for(int i=0; i<rows; i++){
            for(int j=0; j<cols; j++)
                s.append(" "+data[i][j]+" ");
                s.append("\n");
        }
        s.append("\n");
    return s.toString();
}
public static void main(String args[]) throws IOException{
    int rows,cols;
    rows = KeyInput.readInt();
    cols = KeyInput.readInt();

    Matrix A = new Matrix(rows,cols);
    for(int i=0;i<A.rows;i++){
        for(int j=0;j<A.cols;j++)
    A.setData(i, j, 10.0);
    }

    rows = KeyInput.readInt();
    cols = KeyInput.readInt();
    Matrix B = new Matrix(rows,cols);
    for(int i=0;i<rows;i++){
        for(int j=0;j<cols;j++)
    B.setData(i, j, 10.0);
    }
    System.out.println(A.multiply(B).toString());

}
}

Exception in thread "main" [color=red]java.lang.ArrayIndexOutOfBoundsException[/color]: 0
at test.Matrix.toString(Matrix.java:48)
at test.Matrix.main(Matrix.java:76）

为什么？

Answer 1

问题在toString方法中，设置一个断点单步跟踪debug一下吧