Ex :
double a=1.2356;
//方法1
double b=(double)Math.round(a*100)/100; //1.23
//方法2
String s1=String.format("%.2f",a); //1.23
//方法3
DecimalFormat df=new DecimalFormat("#.##");
String s2=df.format(a); //1.23 //如果a=1.2结果为1.2
//方法4
DecimalFormat df2=new DecimalFormat("#.00");
String s3=df2.format(a); //1.23 //如果a=1.2结果为1.20
//方法5
NumberFormat nf=NumberFormat.getInstance();
nf.setMaximumFractionDigits(2);
nf.setRoundingMode(RoundingMode.DOWN );//down不四舍五入,up四舍五入
String s4=nf.format(a);
//down a=1.2356 s4 =1.23
//up a=1.2356 s4 =1.24