Java lambda修改本地变量

Use a wrapper Any kind of wrapper is good.

With Java 8+, use either an AtomicInteger:

AtomicInteger ordinal = new AtomicInteger(0);
list.forEach(s -> {
  s.setOrdinal(ordinal.getAndIncrement());
});

… or an array:

int[] ordinal = { 0 };
list.forEach(s -> {
  s.setOrdinal(ordinal[0]++);
});

With Java 10+:

var wrapper = new Object(){ int ordinal = 0; };
list.forEach(s -> {
  s.setOrdinal(wrapper.ordinal++);
});

Note: be very careful if you use a parallel stream. You might not end up with the expected result. Other solutions like Stuart’s might be more adapted for those cases.

For types other than int

Of course, this is still valid for types other than int. You only need to change the wrapping type to an AtomicReference or an array of that type. For instance, if you use a String, just do the following:

AtomicReference<String> value = new AtomicReference<>();
list.forEach(s -> {
  value.set("blah");
});

Use an array:

String[] value = { null };
list.forEach(s-> {
  value[0] = "blah";
});

Or with Java 10+:

var wrapper = new Object(){ String value; }
list.forEach(s->{
  wrapper.value = "blah";
});