BigDecimal adouble =new BigDecimal(1.22);
System.out.println("construct with a double value: " + adouble);
BigDecimal astring = new BigDecimal("1.22");
System.out.println("construct with a String value: " + astring);

construct with a doublevalue:1.2199999999999999733546474089962430298328399658203125
construct with a String value: 1.22

public static BigDecimal valueOf(double val) {
// Reminder: a zero double returns '0.0', so we cannotfastpath
// to use the constant ZERO. This might be important enough to
// justify a factory approach, a cache, or a few private
// constants, later.
return new BigDecimal(Double.toString(val));
}

BigDecimal a =new BigDecimal("1.22");
System.out.println("construct with a String value: " + a);
BigDecimal b =new BigDecimal("2.22");
a.add(b);
System.out.println("aplus b is : " + a);

construct with a Stringvalue: 1.22
a plus b is :3.44

public BigDecimal add(BigDecimal augend) {
          long xs =this.intCompact; //整型數(shù)字表示的BigDecimal,例a的intCompact值為122
          long ys = augend.intCompact;//同上
          BigInteger fst = (this.intCompact !=INFLATED) ?null :this.intVal;//初始化BigInteger的值，intVal為BigDecimal的一個(gè)BigInteger類型的屬性
          BigInteger snd =(augend.intCompact !=INFLATED) ?null : augend.intVal;
          int rscale =this.scale;//小數(shù)位數(shù)
          long sdiff = (long)rscale - augend.scale;//小數(shù)位數(shù)之差
          if (sdiff != 0) {//取小數(shù)位數(shù)多的為結(jié)果的小數(shù)位數(shù)
              if (sdiff < 0) {
                 int raise =checkScale(-sdiff);
                 rscale =augend.scale;
                 if (xs ==INFLATED ||
                     (xs = longMultiplyPowerTen(xs,raise)) ==INFLATED)
                     fst =bigMultiplyPowerTen(raise);
                }else {
                   int raise =augend.checkScale(sdiff);
                   if (ys ==INFLATED ||(ys =longMultiplyPowerTen(ys,raise)) ==INFLATED)
                       snd = augend.bigMultiplyPowerTen(raise);
               }
          }
          if (xs !=INFLATED && ys !=INFLATED) {
              long sum = xs + ys;
              if ( (((sum ^ xs) &(sum ^ ys))) >= 0L)//判斷有無(wú)溢出
                 return BigDecimal.valueOf(sum,rscale);//返回使用BigDecimal的靜態(tài)工廠方法得到的BigDecimal實(shí)例
           }
           if (fst ==null)
               fst =BigInteger.valueOf(xs);//BigInteger的靜態(tài)工廠方法
           if (snd ==null)
               snd =BigInteger.valueOf(ys);
           BigInteger sum =fst.add(snd);
           return (fst.signum == snd.signum) ?new BigDecimal(sum,INFLATED, rscale, 0) :
              new BigDecimal(sum,compactValFor(sum),rscale, 0);//返回通過(guò)其他構(gòu)造方法得到的BigDecimal對(duì)象
       }

BigDecimal b1 = BigDecimal.ONE;
BigDecimal b2 = new BigDecimal("3");
System.out.println(b1.divide(b2));

System.out.println(b1.divide(b2,3,BigDecimal.ROUND_HALF_UP));

MathContext mc = new MathContext(5, RoundingMode.HALF_UP);System.out.println(b1.divide(b2,mc));

public static void main(String[] args) {
double i = 3.856;
舍掉小數(shù)取整
System.out.println("舍掉小數(shù)取整:Math.floor(3.856)=" + (int) Math.floor(i));
四舍五入取整
System.out.println("四舍五入取整:(3.856)="+ new BigDecimal(i).setScale(0, BigDecimal.ROUND_HALF_UP));
四舍五入保留兩位小數(shù)
System.out.println("四舍五入取整:(3.856)="+ new BigDecimal(i).setScale(2, BigDecimal.ROUND_HALF_UP));
湊整，取上限
System.out.println("湊整:Math.ceil(3.856)=" + (int) Math.ceil(i));
舍掉小數(shù)取整
System.out.println("舍掉小數(shù)取整:Math.floor(-3.856)=" + (int) Math.floor(-i));
四舍五入取整
System.out.println("四舍五入取整:(-3.856)=" + new BigDecimal(-i).setScale(0, BigDecimal.ROUND_HALF_UP));
四舍五入保留兩位小數(shù)
System.out.println("四舍五入取整:(-3.856)="+ new BigDecimal(-i).setScale(2, BigDecimal.ROUND_HALF_UP));
湊整，取上限
System.out.println("湊整(-3.856)=" + (int) Math.ceil(-i));