Java 8 - How to use predicate that has a function with parameter?

Question

I have the following code:

public boolean isImageSrcExists(String imageSrc) {
    int resultsNum = 0;
    List<WebElement> blogImagesList = driver.findElements(blogImageLocator);

    for (WebElement thisImage : blogImagesList) {
        if (thisImage.getAttribute("style").contains(imageSrc)) {
            resultsNum++;
        }
    }

    if (resultsNum == 2) {
        return true;
    } else {
        return false;
    }
}

What is the proper way of converting it to use Java 8 Streams?

When I'm trying to use map(), I get an error since getAttribute isn't a Function.

int a = (int) blogImagesList.stream()
                            .map(WebElement::getAttribute("style"))
                            .filter(s -> s.contains(imageSrc))
                            .count();

we could make some "curry" thingy, but it's probably not worth it - `curry(::getAttribute, "style")` — ZhongYu, Sep 01 '15 at 14:26

Boris the Spider · Accepted Answer · 2015-09-01T15:27:53.847

You cannot do exactly what you want - explicit parameters are not allowed in method references.

But you could...

...create a method that returns a boolean and harcoded the call to getAttribute("style"):

public boolean getAttribute(final T t) {
    return t.getAttribute("style");
}

This would allow you to use the method ref:

int a = (int) blogImagesList.stream()
              .map(this::getAttribute)
              .filter(s -> s.contains(imageSrc))
              .count();

...or you could define a variable to hold the function:

final Function<T, R> mapper = t -> t.getAttribute("style");

This would allow you to simply pass the variable

int a = (int) blogImagesList.stream()
              .map(mapper)
              .filter(s -> s.contains(imageSrc))
              .count();

...or you could curry and combine the above two approaches (this is certainly horribly overkill)

public Function<T,R> toAttributeExtractor(String attrName) {
    return t -> t.getAttribute(attrName);
}

Then you would need to call toAttributeExtractor to get a Function and pass that into the map:

final Function<T, R> mapper = toAttributeExtractor("style");
int a = (int) blogImagesList.stream()
              .map(mapper)
              .filter(s -> s.contains(imageSrc))
              .count();

Although, realistically, simply using a lambda would be easier (as you do on the next line):

int a = (int) blogImagesList.stream()
              .map(t -> t.getAttribute("style"))
              .filter(s -> s.contains(imageSrc))
              .count();

Why do you have all these awkward solutions in the middle instead of going directly to the solution at the end? — Louis Wasserman, Sep 01 '15 at 20:42
@LouisWasserman because the OP wanted a workaround - I wanted to illustrate that the simple solution the OP was already using is the best one. — Boris the Spider, Sep 01 '15 at 21:23

score 7 · Answer 2 · edited Sep 01 '15 at 13:25

7

You can't pass a parameter to a method reference. You can use a lambda expression instead :

int a = (int) blogImagesList.stream()
                            .map(w -> w.getAttribute("style"))
                            .filter(s -> s.contains(imageSrc))
                            .count();

edited Sep 01 '15 at 13:25

Peter Lawrey

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answered Sep 01 '15 at 13:05

Eran

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3

I took the liberty of formatting the code so you could read it without scrolling. – Peter Lawrey Sep 01 '15 at 13:26
1

@PeterLawrey Thanks :) – Eran Sep 01 '15 at 13:28

Java 8 - How to use predicate that has a function with parameter?

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