如代码所示：保留几位小数就在样本后面写几位0，保留时是根据四舍五入的保留的
double a = 0;
double b = 1.2;
double c = 1.23;
double d = 1.234;
double e = 1.235;
double f = 1.23567;
DecimalFormat df = new DecimalFormat("0.000");
System.out.println(df.format(a));
System.out.println(df.format(b));
System.out.println(df.format(c));
System.out.println(df.format(d));
System.out.println(df.format(e));
System.out.println(df.format(f));
输出结果如下：
0.000
1.200
1.230
1.234
1.235
1.236
