I have the following code

private ArrayList<String> findPhotos(Date startTimestamp, Date endTimestamp, String keywords, String longitude, String latitude) {
     
     ArrayList<String> photos = new ArrayList<String>();
     File[] fList = file.listFiles();
        if (fList != null) {
            for (File f : fList) {
                if (((startTimestamp == null && endTimestamp == null) || (f.lastModified() >= startTimestamp.getTime()
                        && f.lastModified() <= endTimestamp.getTime())
                ) && (keywords == "" || f.getPath().contains(keywords))
                        && (longitude == "" || f.getPath().contains(longitude))
                        && (latitude == "" || f.getPath().contains(latitude)))
                    photos.add(f.getPath());
            }
        }
    return photos
}

This function take five parameters:

1. Startime time
2. endtime
3. keywork
4. longitude
5. latitude

I filter them out to get the only desired results. I want to transfer this file array into lambda array and then perform following code.

Filter(startTimestamp == null && endTimestamp == null)

or

Filter(f -> f.lastModified() >= startTimestamp.getTime())

and

Filter(f -> f.getPath().contains(keywords) or (keywords == ""))

and

Filter(keywords == "" || f -> f.getPath().contains(keywords))

and

Filter(latitude== "" || f -> f.getPath().contains(latitude))
Filter(keywords == "" || f -> f.getPath().contains(keywords))

and then get only paths, then

.collect(Collectors.toList());

Once I filter them out the File array and then add only file paths into Arraylist<String>. Can anyone help me to turn this one into lambda expression?

CodePudding user response：

There are several issues with your given approach:

• As others have already mentioned, the point of a lambda expression is that it should be short and easy to understand.
• To get the logical and of your multiple logical expressions you should chain the filter(f -> ...) calls. This is better than putting all the logic into one giant lambda expression.
• You need to understand the difference between == and equals. So instead of keywords == "" you probably want keywords.equals("").
• You cannot use ArrayList<String> because .collect(Colllectors.toList()) is declared to give a List<String>.

Considering all this you might end up with something like this

private List<String> findPhotos(Date startTimestamp, Date endTimestamp, String keywords, String longitude, String latitude) {
{
    File[] fList = file.listFiles();
    if (fList == null)
        return new ArrayList<String>();
    List<String> photos = Arrays.stream(fList)
        .filter(f -> f.lastModified() >= startTimestamp.getTime())
        .filter(f -> f.getPath().contains(keywords))
        .filter(f -> keywords.equals("") || f.getPath().contains(keywords))
        .filter(f -> latitude.equals("") || f.getPath().contains(latitude))
        .filter(f -> keywords.equals("") || f.getPath().contains(keywords))
        .map(f -> f.getPath())  // convert File to String
        .collect(Collectors.toList());
    return photos;
}

CodePudding user response：

I believe the following should achieve your desired result.

private List<String> findPhotos(Date startTimestamp,
                                Date endTimestamp,
                                String keywords,
                                String longitude,
                                String latitude) {
    return java.util.Arrays.stream(new file.listFiles())
                           .filter(f -> startTimestamp == null || f.lastModified() >= startTimestamp.getTime())
                           .filter(f -> endTimestamp == null || f.lastModified() <= endTimestamp.getTime())
                           .map(file -> file.getPath())
                           .filter(path -> keywords.isEmpty() || path.contains(keywords))
                           .filter(path -> longitude.isEmpty() || path.contains(longitude))
                           .filter(path -> latitude.isEmpty() || path.contains(latitude))
                           .collect(Collectors.toList());
}

• Method stream, in class java.util.Arrays, converts an array into a stream. In the above code, that stream will contain instances of class java.io.File.
• Method map, in interface java.util.stream.Stream, returns a new stream that, in the above code, contains String elements.
• Finally, all the paths that pass all the filters are collected into a java.util.List.