Don't get hooked on Java's Big Decimal

A memo because my colleague was addicted to treating Big Decimal.

What is Immutable?

~~ Variable value ~~ When manipulating an object, rewriting the value of the ~~ variable ~~ object itself is called "mutable", and not rewriting it is called "immutable". In Java, ~~ int and double ~~ Calendar are mutable, and String is immutable.

Which is BigDecimal?

BigDecimal API documentation https://docs.oracle.com/javase/jp/8/docs/api/java/math/BigDecimal.html

Let's take a look at the add method that adds.

/**
value(this+augend)Scale is max(this.scale(), augend.scale())Returns the Big Decimal that is.
Parameters: augend -The value to add to this Big Decimal.
Return value: this + augend
*/
public BigDecimal add(BigDecimal augend)

Now, what is the output of the following code?

public static void main(String args[]) {
    BigDecimal a = new BigDecimal("1");
    BigDecimal b = new BigDecimal("2");
    a.add(b);
    System.out.println(a);
}

Output result.

1

that? Isn't it 3? I thought you. You've overlooked an important sentence in the API documentation.

** Immutable **, arbitrary precision signed decimal number.

In other words, this code meant to ** create a new BigDecimal object with a value of a + b ** and return that object. The value of a itself does not change at all.

a.add(b);

It was a code that calculates money by extracting a numerical value from DB with Big Decimal, but I proceeded with processing without assigning the result of a method such as add to a variable, and it was a bug.

You can't see immutable things just by writing the code ... It was a nasty bug.