[Solved] why this lambda expression do not work when in statement ,but work in method?

Question

In your second code version:

Map<Object, Boolean> seen = new ConcurrentHashMap<>();

The map Map<> seen is initialized for every element x, therefore your filter always return true which will let all elements passthrough.

Move the Map<> seen declaration to outside the lambda will correct your usage.

In the first code version, the Map<> seen is only initialized one. What it returns is just a predicate:

return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;

When you pass this predicate to your stream, it does not create a new Map but uses the reference Map<> seen declared before.

Accepted Answer

In your second code version:

Map<Object, Boolean> seen = new ConcurrentHashMap<>();

The map Map<> seen is initialized for every element x, therefore your filter always return true which will let all elements passthrough.

Move the Map<> seen declaration to outside the lambda will correct your usage.

In the first code version, the Map<> seen is only initialized one. What it returns is just a predicate:

return t -> seen.putIfAbsent(keyExtractor.apply(t), Boolean.TRUE) == null;

When you pass this predicate to your stream, it does not create a new Map but uses the reference Map<> seen declared before.