Gson().tojson() returning {} braces

Please somebody help me to figure out the problem..I want to store BluetoothDevice object in to SharedPrefrences so that next time when I open the app not need to select the paring device again.For this I have to convert this BluetoothDevice object in to String variable and then store it in to SharedPrefrence for that I have used

Gson gson = new Gson();
String json = gson.toJson(selectedDevice,BluetoothDevice.class) ;

Where selectedDevice is an object of BluetoothDevice class. But after running the code I got the {} braces in json variable. Can anyone have any idea why this is happening.

Almost all of the properties in android.bluetooth.BluetoothDevice are static final. So, these properties do not depend on an instance.

One address property is available and depends on the instance variable. You can just save this address property to your shared-preferences using getAddress().

If bluetooth is enabled, you can, also, get device name with getName().

So, I suggest you save something like this in your shared-preferences;

class BluetoothDeviceInformation {

    String address;
    String name;

    @RequiresPermission(Manifest.permission.BLUETOOTH)
    public BluetoothDeviceInformation (
        BluetoothDevice device
    ) {
        this.address = device.getAddress();
        this.name = device.getName();
    }
}

You can create this instance using:

BluetoothDeviceInformation deviceInformation = 
    new BluetoothDeviceInformation(selectedBluetoothDevice);

There are other methods like getType(), getAlias() etc. If you need their information, just add them to your BluetoothDeviceInformation class.

Assuming you are referring https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/bluetooth/BluetoothDevice.java, it contains static fields but,

Java Serialization only serialize object's non-static and non-transient fields

Doc

But if you really need it same way you can try:

BluetoothDevice selectedDevice = new BluetoothDevice();
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.excludeFieldsWithModifiers(java.lang.reflect.Modifier.TRANSIENT);
Gson gson = gsonBuilder.create();
String json = gson.toJson(selectedDevice, BluetoothDevice.class);