How to perform some mathematical operation on some specific elements of a list using java 8?

list.stream()
    .map(x -> x == 0 ? x : x - 1)
    .collect(Collectors.toList());

In the example, you can use Math.max method:

list.stream()
    .map(x -> Math.max(0, x - 1))
    .collect(Collectors.toList());

In your case:

list.stream() // 1,2,0,5,0
    .filter(x -> x > 0) // 1,2,5
    .map(x -> x - 1) // 0,1,4
    .collect(Collectors.toList()); // will return list with three elements [0,1,4]

A non-stream version is using of replaceAll

list.replaceAll(x -> x != 0 ? x - 1 : x);