wifi_scan.java

final WifiManager wifimanager = (WifiManager)  getApplicationContext().getSystemService(WIFI_SERVICE);

wifimanager.registerScanResultsCallback(getMainExecutor(), new WifiManager.ScanResultsCallback() {
    @Override
    public void onScanResultsAvailable() {
        List<ScanResult> results = wifimanager.getScanResults();
        for (ScanResult result : results) {
            System.out.println(result.BSSID);
        }
    }
});