DecimalFormat
DecimalFormat format = new DecimalFormat("00"); //两位输出低于两位补零
DecimalFormat format2 = new DecimalFormat("##"); //两位输出低于两位不补零
double pi = Math.PI;
double a = 100.123;
System.out.println("00format"+format.format(pi));
System.out.println("##format"+format2.format(pi));
System.out.println("00format"+format.format(a));