检查字符串数组是否包含不带循环的子字符串

Question

I want to find a substring in an array of strings, without using loop. I'm using:

import java.util.Arrays;

public class MyClass {
    public static void main(String args[]) {

        String[] files = new String[]{"Audit_20190204_061439.csv","anotherFile"};
        String substring= ".csv";

        if(!Arrays.stream(files).anyMatch(substring::contains)) {
            System.out.println("Not found:" + substring);
        }
    }
}

I'm always getting Not found. What is wrong with the approach?

Answer 1

You are checking whether the String ".csv" does not contain any of the elements of your Stream , which is the opposite of what you wanted.

It should be:

if (!Arrays.stream(files).anyMatch(s -> s.contains(substring))) {
    System.out.println("Not found:" + substring);
}

PS As commented, you can use noneMatch instead of anyMatch , which will save the need to negate the condition:

if (Arrays.stream(files).noneMatch(s -> s.contains(substring))) {
    System.out.println("Not found:" + substring);
}

and if the ".csv" substring should only be searched for in the end of the String (ie treated as a suffix), you should use endsWith instead of contains .

Answer 2

You possibly need to check the file extension and can use endsWith for it instead and improve your condition to:

if (Arrays.stream(files).noneMatch(a -> a.endsWith(substring))) {
    System.out.println("Not found:" + substring);
}

Answer 3

I am not a streams guru, but I believe that you want something like this:

String[] files = new String[] { "Audit_20190204_061439.csv", "anotherFile" };

for (String file : files) {
    if (file.endsWith(".csv")) {
        System.out.println("found a CSV file");
    }
}

I use String#endsWith here because presumably .csv refers to a file extension, and should only register a hit if occur at the end of the filename.

We could also use String#matches here:

Pattern pattern = Pattern.compile(".*\\.csv$");
for (String file : files) {
    Matcher matcher = pattern.matcher(file);
    if (matcher.find()) {
        System.out.println("found a CSV file");
    }
}