Java JUnit: Testing a readFile method using a temporary file, FileUtils cannot be resolved

I have a problem with java, it gives me an error inside my code when I run a test for writing to a file. It isn’t finished yet but in the test I create a sort of ‘mock’ temporary file where I write a string. **as my code shouldn't be allowed to read strings

@Rule
       public TemporaryFolder tempFolder = new TemporaryFolder();
     @Test
       public void testWrite() throws IOException {
         // Create a temporary file.
         final File tempFile = tempFolder.newFile("tempFile.txt");
       
         // Write something to it.
         FileUtils.writeStringToFile(tempFile, "hello world");
       }

Unfortunately the FileUtils seems not to be recognised. Relating to that the import:

import org.apache.commons.io.FileUtils;

Isn’t recognised either.