【以前】作业思索：内部类以及double相加值

import java.math.BigDecimal;


public class MatrixCal {
    //内部类
	static class Matrix{                                                                                           //第一处
		private double[][] mat;
		private int nr,nc;
		public Matrix(int r,int c){
			if(r > 0 && c > 0)
				mat = new double[nr=r][nc=c];
			else{
				mat = null;nr = nc = 0;
			}
		}
		public Matrix(double[][] m){
			mat = m;
			nr = m.length;
			nc = m[0].length;
		}
		public double getElement(int i, int j){
			return mat[i][j];
		}
		public void setElement(int i,int j,double e){mat[i][j] = e;}
		public int rows(){return nr;}
		public int cols(){return nc;}
		public Matrix times(Matrix b){
			if(nc != b.nr)return null;
			Matrix p = new Matrix(nr,b.nc);
			for(int i = 0; i < nr; i ++)
				for(int j = 0; j < b.nc; j++)
					p.setElement(i, j, rowTimesCol(i,b.mat,j));
			return p;
		}
		private double rowTimesCol(int ri,double[][] b,int ci){
			double sum = 0.0d;
			double[] row = mat[ri];
			for(int k = 0; k < b.length; k++)
				sum += row[k]*b[k][ci];
			return sum;
		}
		public String toString(){
			StringBuffer buf = new StringBuffer();
			for(int i = 0; i < nr; i++){
				buf.append("(");
				for(int j = 0; j < nc - 1; j++)
					buf.append(mat[i][j]+" ");
				buf.append(mat[i][nc-1]+")\n");
			}
			buf.setLength(buf.length()-1);
			return new String(buf);
		}
		/*
		 * Lab要求实现的三个函数
		 */
		public Matrix plus(Matrix m,int mr,int mc){
			double[][] tmp = new double[mr][mc];
			if(nr != mr && nc != mc)return null;
			for(int i = 0; i < mr; i++)
				/**
				 *                                       第二处 采用BigDecimal格式化double值，避免出现错误的结果（比如3.2222222222222229）
				 */
				for(int j = 0; j < mc; j++){
					tmp[i][j] = mat[i][j] + m.mat[i][j];
					BigDecimal bigDecimal = new BigDecimal(tmp[i][j]);
					bigDecimal = bigDecimal.setScale(2,BigDecimal.ROUND_HALF_UP);
					tmp[i][j] = bigDecimal.doubleValue();
				}
			Matrix tmp_m = new Matrix(tmp);
			return tmp_m;
		}
		public Matrix minus(Matrix m,int mr,int mc){
			double[][] tmp = new double[mr][mc];
			if(nr != mr && nc != mc)return null;
			for(int i = 0; i < mr; i++)
				for(int j = 0; j < mc; j++){
					tmp[i][j] = mat[i][j] - m.mat[i][j];
					BigDecimal bigDecimal = new BigDecimal(tmp[i][j]);
					bigDecimal = bigDecimal.setScale(2,BigDecimal.ROUND_HALF_UP);
					tmp[i][j] = bigDecimal.doubleValue();
				}
			Matrix tmp_m = new Matrix(tmp);
			return tmp_m;
		}
		public Matrix minus(){
			for(int i = 0; i < nr; i++)
				for(int j = 0; j < nc; j++)
					this.mat[i][j] *= -1;
			return this;
		}
		public Matrix transpose(){
			Matrix m = new Matrix(nc,nr);
			for(int i = 0; i < nc; i++)
				for(int j = 0; j < nr; j++)
					m.mat[i][j] = this.mat[j][i];
			return m;
		}
	}
	public static void main(String[] args){
		double[][] a = {{1.0,2.0,1.5},{1.0,9.0,2.3}};
		double[][] b = {{6.7,3.4,2.3},{4.5,2.3,5.6}};
		Matrix ma = new Matrix(a);
		Matrix mb = new Matrix(b);
		Matrix m_plus = ma.plus(mb, 2, 3);
		Matrix m_minus = ma.minus(mb,2,3);
		Matrix m_tran = ma.transpose();
		System.out.println(m_plus);
		System.out.println("\n");
		System.out.println(m_minus);
		System.out.println("\n");
		System.out.println(m_tran);
	}
}

第一处的static如果去掉，会出现以下结果：

Exception in thread "main" java.lang.Error: Unresolved compilation problem: 
No enclosing instance of type MatrixCal is accessible. Must qualify the allocation with an enclosing instance of type MatrixCal (e.g. x.new A() where x is an instance of MatrixCal).


at MatrixCal.main(MatrixCal.java:102)

这是因为：

非静态嵌套类必须在外套类的非静态成员里构造。所以把Matrix设置成了static静态类。或者将内部类转为一个普通的类POJO。

第二处：

double值相加常常会发生错误，比如出现类似2.00000000000000000000009这样子的浮点数。

代码：

BigDecimal bigDecimal = new BigDecimal(tmp[i][j]);
					bigDecimal = bigDecimal.setScale(2,BigDecimal.ROUND_HALF_UP);
					tmp[i][j] = bigDecimal.doubleValue();

可以起到格式化的作用。

以上仅为作业思索所得，不保证理论的绝对正确。