JAVA中保留指定小数位方法

import java.math.BigDecimal;
    import java.text.DecimalFormat;
    import java.text.NumberFormat;
    public class format {
        double f = 111231.5585;
　　　/**
         * BigDecimal
         */
        public void m1() {
            BigDecimal bg = new BigDecimal(f);
            double f1 = bg.setScale(2, BigDecimal.ROUND_HALF_UP).doubleValue();
            System.out.println(f1);
        }
        /**
         * DecimalForma
         */
        public void m2() {
            DecimalFormat df = new DecimalFormat("#.00");
            System.out.println(df.format(f));
        }
        /**
         * String.format
         */
        public void m3() {
            System.out.println(String.format("%.2f", f));
        }

　　/**
         *NumberFormat
         */
        public void m4() {
            NumberFormat nf = NumberFormat.getNumberInstance();
            nf.setMaximumFractionDigits(2);
            System.out.println(nf.format(f));
        }
        public static void main(String[] args) {
            format f = new format();
            f.m1();
            f.m2();
            f.m3();
            f.m4();
        }
    }