public class Main {

private final String INVALID_FILE_NAME_LENGTH = "INVALID FILE NAME LENGTH";
private final String INVALID_FILE_EXTENSION = "INVALID FILE EXTENSION";
private final String INVALID_FILE_SIZE = "INVALID FILE SIZE";

public static void main(String... args) {
    File file = new File("text.pdf");
    Main main = new Main();
    main.validateFileFunctionally(file);
    main.validateFileRegularly(file);
}

private void validateFileFunctionally(File file) {
    BiPredicate<File, Integer> validateFileName = (f, maxLength) -> f.getName().length() < maxLength;
    BiPredicate<File, String> validateExtension = (f, type) -> f.getName().endsWith(type);
    BiPredicate<File, Integer> validateSize = (f, maxSize) -> f.length() <= maxSize;

    BiConsumer<Boolean, String> throwExceptionIfInvalid = (isValid, errorMessage) -> {
        if(!isValid) {
            throw new InvalidFileException(errorMessage);
        }
    };

    throwExceptionIfInvalid.accept(validateFileName.test(file, 20), INVALID_FILE_NAME_LENGTH);
    throwExceptionIfInvalid.accept(validateExtension.test(file, ".pdf") || validateExtension.test(file, ".csv"), INVALID_FILE_EXTENSION);
    throwExceptionIfInvalid.accept(validateSize.test(file, 20), INVALID_FILE_SIZE);
}

private void validateFileRegularly(File file) {
    if (file.getName().length() > 20) {
        throw new InvalidFileException("INVALID FILE NAME LENGTH");
    } else if (!file.getName().endsWith(".pdf") && !file.getName().endsWith(".csv")) {
        throw new InvalidFileException("INVALID FILE NAME LENGTH");
    } else if (file.length() > 20) {
        throw new InvalidFileException("INVALID FILE NAME LENGTH");
    }
}

class InvalidFileException extends RuntimeException {
    public InvalidFileException(String message) {
        super(message);
    }
}