Another method to multiply two numbers without using the “*” operator [duplicate]

Answer 1

I don't know whether that has to be a strictly "programming question". But in Maths:

x * y = x / (1 / y) #divide by inverse

So:

Method 1:

public static double multiplier(double a, double b) {

    // return a / (1 / b);

    // the above may be too rough
    // Java doesn't know that "(a / (b / 0)) == 0"
    // a special case for zero should probably be added:

    return 0 == b ? 0 : a / (1 / b);
}

Method 2 (a more "programming/API" solution):

Use big decimal, big integer:

new BigDecimal("3").multiply(new BigDecimal("9"))

There are probably a few more ways.