вопросы

@Override
public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
    // размер файла: partOfContent.length
    byte[] content = Files.readAllBytes(file);
    if ((minSize > 0 && content.length < minSize) || (maxSize > 0 && content.length > maxSize)) {
        return FileVisitResult.CONTINUE;
    }
    if (partOfContent != null && !partOfContent.isEmpty()) {
        String hasCont = new String(content);
        if (!hasCont.contains(partOfContent)) {
            return FileVisitResult.CONTINUE;
        }
    }
    if (partOfName != null && !file.toString().contains(partOfName)) {
            return FileVisitResult.CONTINUE;
    }
    foundFiles.add(file);
    return super.visitFile(file, attrs);
}

if (
        attrs.isRegularFile()
                && file.getFileName().toString().contains(partOfName)
                && isPartOfContent(file, partOfContent)
                && attrs.size() >= minSize
                && attrs.size() <= maxSize
) {

@Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
        // размер файла: partOfContent.length
        byte[] content = Files.readAllBytes(file);

        if (
                attrs.isRegularFile()
                        && file.getFileName().toString().contains(partOfName)
                        && IsPartOfContent(file, partOfContent) //что это такое? подчеркивается
                        && attrs.size() >= minSize
                        && attrs.size() <= maxSize
        ) {
            foundFiles.add(file);
        }
        return super.visitFile(file, attrs);
    }

public class SearchFileVisitor extends SimpleFileVisitor<Path> {

    private String partOfName;
    private String partOfContent;
    private int minSize;
    private int maxSize;
    private List<Path> foundFiles = new ArrayList<>();

    @Override
    public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
        if (
                file.getFileName().toString().contains(partOfName)
                        && isPartOfContent(file, partOfContent)
                        && attrs.size() >= minSize
                        && attrs.size() <= maxSize
        )
            foundFiles.add(file);
        return super.visitFile(file, attrs);
    }

    public boolean isPartOfContent(Path path,String partOfContent) throws IOException {
        byte[] content = Files.readAllBytes(path);
        String hasCont = new String(content);
        return hasCont.contains(partOfContent);
    }

    public void setPartOfName(String partOfName) {
        this.partOfName = partOfName;
    }

    public void setMaxSize(int maxSize) {
        this.maxSize = maxSize;
    }

    public void setMinSize(int minSize) {
        this.minSize = minSize;
    }

    public void setPartOfContent(String partOfContent) {
        this.partOfContent = partOfContent;
    }

    public List<Path> getFoundFiles() {
        return foundFiles;
    }
}

private boolean partOfContent(Path file, String partOfContent) {
    if (!partOfContent.equals("")) {
        try {
            return Files.readAllLines(file).stream()
                    .anyMatch(line -> line.contains(partOfContent));
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }
    return true;
}

private boolean partOfContent(Path file, String partOfContent)  throws IOException {
    return partOfContent.equals("")
            || Files.readAllLines(file).stream()
            .anyMatch(line -> line.contains(partOfContent));
}

public class Solution {

    public static void main(String[] args) throws IOException {
        SearchFileVisitor searchFileVisitor = new SearchFileVisitor();

        searchFileVisitor.setPartOfName("amigo");
        searchFileVisitor.setPartOfContent("programmer");
        searchFileVisitor.setMinSize(500);
        searchFileVisitor.setMaxSize(10000);

        Files.walkFileTree(Paths.get("O:/test"), searchFileVisitor);

        List<Path> foundFiles = searchFileVisitor.getFoundFiles();
        for (Path file : foundFiles) {
            System.out.println(file);
        }
    }
}

catch (IOException e) {
            e.printStackTrace();
        }

e.printStackTrace();

return super.visitFile(file, attrs);