BigDecimal commonly used addition, subtraction, multiplication and division algorithms, compare sizes, and save two decimal points

The project involves the addition, subtraction, multiplication, comparison size, and accuracy of BigDecimal. So I summarize it here for future review.

 1 //加法
 2 
 3 BigDecimal coins = new BigDecimal("0");
 4 
 5 BigDecimal one = new BigDecimal("1");
 6 
 7 BigDecimal two = new BigDecimal("2");
 8 
 9 coins =  coins.add( two  );  //coins 加 2
10 System.out.println(  coins.doubleValue());  //输出为2.0
11 
12 coins =  coins.subtract( one  );  //coins 减 1
13 System.out.println(  coins.doubleValue());  //输出为1.0
14  
15 coins = coins.subtract( two );   // coins are multiplied by 2 
16 System.out.println( coins.doubleValue());   // output is 2.0 
17  
18  int   i = coins .compareTo(BigDecimal.ZERO);   // check if coins are greater than 0   
19  int   k = coins .compareTo( one ) ; // if i = 0 means equal; i < 0 means coins are less than one; i > 0 means coins > one 
20 coins = coins.setScale(2 ,BigDecimal.ROUND_HALF_UP) // Save two decimal places