Java 8 - How to use a predicate that has a function with a 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 correct way to convert it to use Java 8 Stream s?

When I try to use map() , I get an error because getAttribute not Function .

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

Answer 1

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 passes a call to getAttribute("style") :

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

This will allow you to use the ref method:

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

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

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

This will 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 terribly overwhelming)

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

Then you need to call toAttributeExtractor to get Function and pass it to map :

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

Although realistic, just 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(); 

Answer 2

You cannot pass a parameter to a method reference. You can use the lambda expression instead:

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