[SOLVED] Java dropWhile not removing item in the List

Java dropWhile not removing item in the List

I have the following code fragment and the output is not removing one last String in the list with character sequence 'c'. dropWhile should remove all the string in list with character sequence 'c'?

final List<String> threeLetters = List.of("abc", "cde", "dea", "dei", "mod", "loc", "bpa");
threeLetters.stream()
    .dropWhile(name -> name.contains("c"))
    .map(name -> name + ", ")
    .forEach(System.out::print);
System.out.println();

The output is

dea, dei, mod, loc, bpa,

I am not satisfied the output contains three letter "loc". I am using java 11.0.14 2022.01.18 LTS. Are there any explanations this could happen?

dei, dea, mod, loc, bpa,

Solution

I think filter would be more appropriate in your case. dropWhile drops items as long as a specific condition is true. Once this condition doesn't hold the first time, it will stop dropping thereafter. filter instead removes all items from the stream that do not fulfill the condition.

In your case,

abc  -> name.contains("c") is true -> drop
cde  -> name.contains("c") is true -> drop
dea  -> name.contains("c") is false -> output and stop dropping
dei  -> output
mod  -> output
loc  -> output
bpa  -> output

In contrast, filter works as follows:

abc  -> !name.contains("c") is false -> drop
cde  -> !name.contains("c") is false -> drop
dea  -> !name.contains("c") is true -> output
dei  -> !name.contains("c") is true -> output
mod  -> !name.contains("c") is true -> output
loc  -> !name.contains("c") is false -> drop
bpa  -> !name.contains("c") is true -> output

Consequently, what you probably want is:

final List<String> threeLetters = List.of("abc", "cde", "dea", "dei", "mod", "loc", "bpa");
threeLetters.stream()
    .filter(name -> !name.contains("c"))
    .map(name -> name + ", ")
    .forEach(System.out::print);
System.out.println();

See this answer for another illustrative example.