How to guarantee an NFC does not crash when writing to a tag (when no tag is there)-CodePudding

I am trying to guarantee that my Android application will not crash when I try to write to an NFC tag. The edge case I am working on is: What if I try to write but there is nothing to write to?

I created this write() method, which checks the NFC status in checkNFCStatus()... It prevents some cases (like when a user had a card against the device but removed it before writing). But it does not prevent the case when the user never had a tag to begin with.

@Throws(IOException::class, FormatException::class)
fun write(text: String, tag: Tag?, context: Context): Boolean {
    val records = arrayOf(createRecord(text))
    val message = NdefMessage(records)
    **val ndef = Ndef.get(tag)**
    if (checkNFCStatus(tag)) {
        ndef.connect()
        val ndefMessage = ndef.ndefMessage
        Log.d(TAG, "ndef is : "   ndef.type)
        Log.d(TAG, "ndef message is : $ndefMessage")
        if (ndef.isWritable) {
            Log.d("MAIN", "NDEF is writable.")
        }
        if (!ndef.isWritable) {
            Log.d("MAIN", "NDEF is read-only.")
        }
        if (!ndef.isConnected) {
            Log.d("MAIN", "NDEF is not connected.")
        }
        if (ndef.isConnected) {
            ndef.writeNdefMessage(message)
            ndef.close()
            return true
        }
        ndef.close()
    }
    DebugUtils().showToast(
        context,
        "Try repositioning the card/chip. It is not close enough to the device.")
    return false
}

private fun checkNFCStatus(myTag: Tag?): Boolean {
    try {
        if (myTag != null) {
            val ndefTag = Ndef.get(myTag)
            ndefTag.connect()
            return if (ndefTag.isConnected) {
                Log.d("network", "NFC connected")
                ndefTag.close()
                true
            } else {
                Log.d("network", "NFC disconnected")
                false
            }
        }
    } catch (e: IOException) {
        e.printStackTrace()
        Log.d("network", "NFC disconnected")
    }
    return false
}

The problematic line of code is: val ndef = Ndef.get(tag) Basically if there never was a tag to begin with, this throws an NPE:

java.lang.NullPointerException: Attempt to invoke virtual method 'boolean android.nfc.Tag.hasTech(int)' on a null object reference

CodePudding user response：

Kotlin has a null safety feature, so whenever you know there is a chance for a variable to be null, you should use safe calls :

val a : Boolean?  // now this variable can be null

so replace this :

val ndef = Ndef.get(tag)

with :

val ndef : Boolean? = Ndef.get(tag)

now you will not get this NullPointer error and whenever you want to check the value of ndef you should check it is null or not.

CodePudding user response：

Okay.... so I first went the approach of null safety, but it wasn't working. The answer should have been more obvious.

First check if the tag is null!

if (tag != null) {
        var ndef = Ndef.get(tag)
        if (checkNFCStatus(tag)) {....}