Java Utililty Methods Zip File Check

in.mark(4);
byte[] b = new byte[4];
byte[] zipSig = new byte[4];
zipSig[0] = 0x50;
zipSig[1] = 0x4b;
zipSig[2] = 0x03;
zipSig[3] = 0x04;
try {
...

return Byte.compare(bytes[0], (byte) 0x50) == 0 && Byte.compare(bytes[1], (byte) 0x4B) == 0;

return candidate.getName().toLowerCase().endsWith(".zip");

return f.getName().endsWith(".zip");

return existsWithExtension(file, ".zip");

try (FileInputStream inputStream = new FileInputStream(file)) {
    return new ZipInputStream(inputStream).getNextEntry() != null;
} catch (IOException e) {
    throw new RuntimeException("Problem to check if file " + file.getPath() + " is a zip", e);

if (file.isDirectory()) {
    return false;
if (!file.canRead()) {
    return false;
if (file.length() < 4) {
    return false;
...

if (file.exists() == false)
    return false;
try (InputStream in = new FileInputStream(file); ZipInputStream zipIn = new ZipInputStream(in)) {
    ZipEntry e = zipIn.getNextEntry();
    if (e == null)
        return false;
    int ctr = 0;
    while (e != null && ctr < 4) {
...

byte[] buffer = new byte[MAGIC_BYTES.length];
if (zip == null)
    throw new NullPointerException("Zip file is null");
assert zip != null;
if (zip.isDirectory())
    return false;
InputStream in = new DataInputStream(new FileInputStream(zip));
in.read(buffer);
...

long locatorPosition = zipEndOfCentralDirectoryPosition - ZIP64_EOCD_LOCATOR_SIZE;
if (locatorPosition < 0) {
    return false;
zip.seek(locatorPosition);
return zip.readInt() == ZIP64_EOCD_LOCATOR_SIG_REVERSE_BYTE_ORDER;