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

111231.56
111231.56
111231.56
111,231.56

public static void main(String[] args) { 
  double finalMoney = 27.989; 
  System.out.println(finalMoney); 
  DecimalFormat formater = new DecimalFormat(); 
  formater.setMaximumFractionDigits(2); 
  formater.setGroupingSize(0); 
  formater.setRoundingMode(RoundingMode.FLOOR); 
  System.out.println(formater.format(finalMoney)); 
} 

27.989
27.98