How to rewrite dropWhile in java 8?

Question

As you know, dropWhile for streams was introduced in java 9. But if the target of the project is java 8, you can't use it.

Example code:

public static String getParameterValueOrDefault(String[] args, String paramName, String defaultVal) {
    return Arrays.stream(args).sequential().dropWhile(arg->!arg.equals("/"+paramName) && !arg.equals("-"+paramName)).skip(1).findFirst().orElseGet(()->defaultVal);
}

What I want is an equivalent lambda expression written in java 8.

There’s the `arg ->` missing in `dropWhile`. Further, there’s no advantage in using `orElseGet(() -> defaultVal)` over the simpler and more efficient `orElse(defaultVal)` — Holger, Jan 17 '23 at 07:45

score 2 · Answer 1 · answered Jan 17 '23 at 07:48

You can use

public static String getParameterValueOrDefault(
                     String[] args, String paramName, String defaultVal) {

    int ix = IntStream.range(0, args.length)
        .filter(i -> args[i].matches("[/-]" + Pattern.quote(paramName)))
        .map(i -> i + 1)
        .findFirst().orElse(args.length);
    return ix < args.length? args[ix]: defaultVal;
}

The matches approach is for convenience, if you prefer compact code. If you want an efficient check, you may instead use:

public static String getParameterValueOrDefault(
                     String[] args, String paramName, String defaultVal) {

    int ix = IntStream.range(0, args.length)
        .filter(i -> {
            String arg = args[i];
            return arg.length() == paramName.length() + 1 && arg.endsWith(paramName)
                && (arg.charAt(0) == '-' || arg.charAt(0) == '/');
        })
        .map(i -> i + 1)
        .findFirst().orElse(args.length);
    return ix < args.length? args[ix]: defaultVal;
}

IMHO my aswer is better (or tell me I am wrong) because it is rewriten only the dropWhile, so the rest of the expression does not change. — PatoOta, Jan 26 '23 at 16:24
@PatoOta your answer is a textbook example of how not to use streams 1.) modifying state outside the stream and relying on side effects 2.) abusing `peek`. My answer does not require `sequential()` (nor assumptions about how a sequential stream is implemented). — Holger, Jan 26 '23 at 16:50

score 0 · Answer 2 · answered Jan 16 '23 at 13:29

I found this way:

public static String getParameterValueOrDefault(String[] args, String paramName, String defaultVal) {
    MutableBoolean foundParam = new MutableBoolean(false);
    return Arrays.stream(args).sequential().peek(arg->foundParam.value = foundParam.value || arg.equals("/"+paramName) || arg.equals("-"+paramName)).filter(arg->foundParam.value).skip(1).findFirst().orElseGet(()->defaultVal);
}

Where MutableBoolean is:

private static class MutableBoolean {
    boolean value;
    public MutableBoolean(boolean value) {
        this.value = value;
    }
}

This is my first question, and my first answer. An I am not an English speaker. Sorry for any mistake abot rules and writtings. — PatoOta, Jan 16 '23 at 13:31

How to rewrite dropWhile in java 8?

2 Answers2