public class csdn {
    public static void main(String[] args) throws Throwable {
        int n = 5, m = 5;
        int center = n * m;
        StringBuilder buf = new StringBuilder(); //用来保存字符*
        for (int i=0; i<n; i++) { //大三角形循环
            for (int j=0; j<m; j++) { //小三角形循环
                buf.delete(0, buf.length()); //清空字符*
                for (int k=0; k<2*j+1; k++) { //计算并保存小三角形每层的字符*
                    buf.append("*");
                }
                for (int k=0, p=center-i*m+j; k<=i; k++, p=2*m) { //大三角形的每层循环
                    String format = "%" + p + "s"; //计算开始位置并设置格式化输出
                    System.out.printf(format, buf.toString()); //打印
                }
                System.out.println(); //小三角形每层换行
            }
        }
    }
}