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

 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);

 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.
    returnnew BigDecimal(Double.toString(val));//见3.1关于JDK描述的第三点
  }

public BigDecimal  add(BigDecimal augend) {
     long xs =this.intCompact; //整型数字表示的BigDecimal,例a的intCompact值为122
     long ys = augend.intCompact;//同上
     BigInteger fst = (this.intCompact !=INFLATED) ?null :this.intVal;//初始化BigInteger的值，intVal为BigDecimal的一个BigInteger类型的属性
     BigInteger snd =(augend.intCompact !=INFLATED) ?null : augend.intVal;
     int rscale =this.scale;//小数位数
     long sdiff = (long)rscale - augend.scale;//小数位数之差
     if (sdiff != 0) {//取小数位数多的为结果的小数位数
       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)//判断有无溢出
         return BigDecimal.valueOf(sum,rscale);//返回使用BigDecimal的静态工厂方法得到的BigDecimal实例
      }
      if (fst ==null)
        fst =BigInteger.valueOf(xs);//BigInteger的静态工厂方法
      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);//返回通过其他构造方法得到的BigDecimal对象
    }