如何使程序等待Java中的方法

Question

I am writing a programm that returns me a ArrayList of Strings. Problem is, when I call the method the list is not filled yet so I get an empty list back . I tried it with a thread but now I get a null reference when I call the method. By the way i had to implement a async task, otherwise I get an exception when trying to use InetAddress .

private class DeviceManager extends Thread {

    private ArrayList<String> deviceList;
    private String networkIP;

    public DeviceManager(String networkIP) {
        this.networkIP = networkIP;
    }

    public void run() {
        getDeviceList();
    }

    public ArrayList<String> getDeviceList() {
        new AsyncTask<Void, Void, Void>() {

            @Override
            protected Void doInBackground(Void... params) {
                try {
                    deviceList = new ArrayList<String>();
                    InetAddress address;

                    Log.i("NetworkIPgetDeviceList", networkIP);

                    String deviceIP = networkIP;

                    for (int i = 0; i < 255; i++) {
                        address = InetAddress.getByName(deviceIP += "" + i);
                        if (address.isReachable(2000)) {    
                            Log.i("Devicefound", deviceIP);
                            deviceList.add(deviceIP);
                        }
                        deviceIP = networkIP;
                    }

                } catch (UnknownHostException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                return null;
            }
        }.execute();
        return deviceList;
    }

    public ArrayList<String> getList() {
        return this.deviceList;
    }
}

Answer 1

You are doing it completely wrong. A Thread runs in the background and so does AsyncTask , so basically you are making a background task run in background. Inception .

Try this:

public class DeviceManager {

    private ArrayList<String> deviceList;
    private String networkIP;
    private ConnectionCompleteListener listener;

    public interface ConnectionCompleteListener {
        void onSuccess();

        void onFailure();
    }

    public void setConnectionCompleteListener(ConnectionCompleteListener listener) {
        this.listener = listener;
    }

    public DeviceManager(String networkIP) {
        this.networkIP = networkIP;
    }

    public void getDeviceList() {
        new AsyncTask<Void, Void, Boolean>() {

            @Override
            protected void onPostExecute(Boolean result) {
                if(result) listener.onSuccess();
                else listener.onFailure();
            }

            @Override
            protected Boolean doInBackground(Void... params) {
                try {
                    deviceList = new ArrayList<String>();
                    InetAddress address;

                    Log.i("NetworkIPgetDeviceList", networkIP);

                    String deviceIP = networkIP;

                    for (int i = 0; i < 255; i++) {
                        address = InetAddress.getByName(deviceIP += "" + i);
                        if (address.isReachable(2000)) {
                            Log.i("Devicefound", deviceIP);
                            deviceList.add(deviceIP);
                        }
                        deviceIP = networkIP;
                    }
                    return true;
                } catch (UnknownHostException e) {
                    e.printStackTrace();
                    return false;
                } catch (IOException e) {
                    e.printStackTrace();
                    return false;
                }
                return null;
            }
        }.execute();
    }

    public ArrayList<String> getList() {
        return this.deviceList;
    }
}

Then in your other class:

private class classname{
    DeviceManager manager=new DeviceMnager(networkIp);
    manger.setConnectionCompleteListener(new DeviceManager.ConnectionCompleteListener() {
        @Override
        public void onSuccess() {
            // get your list here
            manager.getList();
        }

        @Override
        public void onFailure() {
            // connection failed show error
        }
    });
}

Answer 2

Artur what you are doing in your code is starting a thread to retrieve device list and then another thread(AsyncTask) to actually creates the device list. So you have three threads running here simultaneously (assuming you are using DeviceManager class in UIThread). The reason getDeviceList() is returning null is because AsyncTasks doInBackground hasn't run yet to collect your device list it might be waiting for its chance to get scheduled. so to conclude that, you just need one thread(other than UIThread), it can either be a Thread or AsyncTask (more preferable as it gives better control) as rusted brain has used in his answer. I prefer to make DeviceManager as AsyncTask (just a bit cleaner and if device managers only task is to retrieve device list) as code below.

in AsyncTask doInBackground runs in a background thread(as name suggests) and onPostExecute runs on the UI thread after doInBackground

class DeviceManager extends AsyncTask<String, Void, List<String>> {

    private ConnectionCompleteListener listener;

    public interface ConnectionCompleteListener {
        void onSuccess(List<String> deviceList);

        // if you need to know reason for failure you can add
        // parameter to onFailure
        void onFailure();
    }

    public DeviceManager(ConnectionCompleteListener listener) {
        this.listener = listener;
    }

    @Override
    protected List<String> doInBackground(String... params) {
        List<String> deviceList = new ArrayList<>();
        String networkIP = params[0];
        try {
            InetAddress address;
            Log.i("NetworkIPgetDeviceList", networkIP);

            String deviceIP = networkIP;
            for (int i = 0; i < 255; i++) {
                address = InetAddress.getByName(deviceIP += "" + i);
                if (address.isReachable(2000)) {
                    Log.i("Devicefound", deviceIP);
                    deviceList.add(deviceIP);
                }
                deviceIP = networkIP;
            }
        } catch (IOException e) {
            deviceList = null;
            e.printStackTrace();
        }
        return deviceList;
    }

    @Override
    protected void onPostExecute(List<String> deviceList) {
        if (deviceList == null) {
            this.listener.onFailure();
        } else {
            this.listener.onSuccess(deviceList);
        }
    }
}

so in your activity you can call

new DeviceManager(new DeviceManager.ConnectionCompleteListener
            () {
        @Override
        public void onSuccess(List<String> deviceList) {

        }

        @Override
        public void onFailure() {

        }
    }).execute("YOUR_NETWORK_IP");

Answer 3

You are getting empty array list because as you are using async task for getting array list and async task doINBackground method runs on different thread(means not on main thread). So when your program runs then your program doesn't wait for async task response.

You can solve this like that... Use onPostExecute method in async task class and return the arraylist

@Override
protected void onPostExecute(Void result) {
 //return array list here
getList();
}

Hope this will help you

Answer 4

First of All you don't need to make DeviceManager a thread as the task which you are running in getDeviceList will start in another new thread. Second You shouldn't wait on main(UI) thread so instead of waiting callback is a better mechanism.

If you insist on the same code try this..

public class DeviceManager extends Thread {
private ArrayList<String> deviceList;

private String networkIP;
private boolean dataAvailable;

public DeviceManager(String networkIP) {
    this.networkIP = networkIP;
}

public void run() {
    getDeviceList();
}

public ArrayList<String> getDeviceList() {

    new AsyncTask<Void, Void, Void>() {
        @Override
        protected Void doInBackground(Void... params) {
            try {
                deviceList = new ArrayList<String>();
                InetAddress address;

                 Log.i("NetworkIPgetDeviceList", networkIP);

                String deviceIP = networkIP;

                for (int i = 0; i < 255; i++) {
                    System.out.println("checking " + i);
                    address = InetAddress.getByName(deviceIP += "" + i);
                    if (address.isReachable(2000)) {

                         Log.i("Devicefound", deviceIP);

                        deviceList.add(deviceIP);
                    }
                    deviceIP = networkIP;
                }

            } catch (UnknownHostException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
            dataAvailable = true;
            synchronized (DeviceManager.this) {
                DeviceManager.this.notify();
            }
            return null;
        }

    }.execute();

    return deviceList;
}

synchronized public ArrayList<String> getList() {

    while (!dataAvailable) {
        try {
            wait();
        } catch (InterruptedException e) {
        }

    }
    return this.deviceList;
}



}