Should an object know which 'arguments' it takes

Question

Consider you got a class with a constructor like the follwing

public class Clazz {

    protected BigDecimal bigDecimal;

    public Clazz(BigDecimal bigDecimal) {
        this.bigDecimal= bigDecimal;
    }

    public BigDecimal getBigDecimal() {
        return bigDecimal;
    }

}

Clazz takes only BigDecimals which are at least zero, that is

new Clazz(BigDecimalUtils.minZero(someBigDecimal));

Who is responsible to ensure that the class only gets instantiated with BigDecimals greater than zero? Is it the class itself (e.g. in the constructor)?

Answer 1

It's whomever you want to be, generally I'd have something akin to:

public void setBigDecimal(BigDecimal aBigDecimal) {
     if (aBigDecimal.longValue() > 0l) {
            this.bigDecimal = aBigDecimal;
     } else { throw new IllegalArgumentException("positive numbers only!");
}

And then have your constructor leverage the above method:

public Clazz(BigDecimal b) {
    this.setBigDecimal(b);
}

I hope this is clear, do leave a comment if you have further questions.