You would call myBigDecimal.intValueExact() (or just intValue()) and it will even throw an exception if you would lose information. That returns an int but autoboxing takes care of that.

To multiply an integer (or byte/short/float/double) with a BigInteger (or BigDecimal), you must convert the native number to BigInteger/BigDecimal first.

// int parameter can be int or Integer
public static BigInteger multiply ( int a, BigInteger b ) {
   return BigInteger.valueOf( a ).multiply( b );
}

// BigInteger <> BigDecimal
public static BigDecimal multiply ( int a, BigDecimal b ) {
   return BigDecimal.valueOf( a ).multiply( b );
}

// same for add, subtract, divide, mod etc.

Note: valueOf is not the same as new, and for different reasons on BigDecimal and BigInteger. In both cases, I recommend valueOf over new.

I see that you added your code, nice. It doesn't work because Integer is mixed with BigDecimal, and also * does not work with BigDecimal. If you compare it with my code, the fix should be obvious:

public BigDecimal methCal ( int quantite, BigDecimal prixUnit ) {
    return BigDecimal.valueOf( quantite ).multiply( prixUnit );
}