>> ...stream().
       map(e -> e.getKey().getPrice().multiply(new BigDecimal(e.getValue()))).
     reduce(BigDecimal.ZERO, BigDecimal::add)

>That's very pretty to look at and easy to read. But it's a huge pain in the ass to debug when getPrice starts failing.

The trick with java and chaining calls is that you have to do the chain on each line, so the stack track can pinpoint the line it failed at.

so instead of the above, write the code as:

    ...stream().
                map(e -> e.getKey()
                          .getPrice()
                          .multiply(new BigDecimal(e.getValue())))
               .reduce(BigDecimal.ZERO, BigDecimal::add)

And the stack trace will tell you if the getKey() or getPrice() failed or the multiply(...) failed.