public class Hellojava{
    static final int rows=79;
    public static void main(String []arge) {
        int row;
        for(row=1;row<rows/2+1;row++) {
            for(int i=1;i<=(rows-2*row-1)/2;i++) 
                System.out.print(" ");
            for(int j=0;j<2*row-1;j++)
                System.out.print("*");
            System.out.print("\n");
        }
        for(row-=2;row>=1;row--) {
            for(int i=1;i<=(rows-2*row-1)/2;i++) 
                System.out.print(" ");
            for(int j=0;j<2*row-1;j++)
                System.out.print("*");
            System.out.print("\n");
        }
        
    }
}

总行数rows 行数row 和 “*”数有关系 number=2row+1；和“ ”有关系number=（rows-2row-1）/2