Here's the complete solution if you want to implement the wifi hotspot feature programmatically in your android app.

SOLUTION FOR API < 26:

For devices < API 26. There is no public API by Android for this purpose. So, in order to work with those APIs you've to access private APIs through reflection. It is not recommended but if you've no other options left, then here's a trick.

First of all, you need to have this permission in your manifest,

  <uses-permission  
  android:name="android.permission.WRITE_SETTINGS"  
  tools:ignore="ProtectedPermissions"/>

  <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>
  <uses-permission android:name="android.permission.CHANGE_WIFI_STATE"/>
  <uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>

Here's how you can ask it on run-time:

 private boolean showWritePermissionSettings() {    
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M  
    && Build.VERSION.SDK_INT < Build.VERSION_CODES.O) { 
  if (!Settings.System.canWrite(this)) {    
    Log.v("DANG", " " + !Settings.System.canWrite(this));   
    Intent intent = new Intent(android.provider.Settings.ACTION_MANAGE_WRITE_SETTINGS); 
    intent.setData(Uri.parse("package:" + this.getPackageName()));  
    intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); 
    this.startActivity(intent); 
    return false;   
  } 
}   
return true; //Permission already given 
}

You can then access the setWifiEnabled method through reflection. This returns true if the action you asked for is being process correctly i.e. enabling/disabling hotspot.

     public boolean setWifiEnabled(WifiConfiguration wifiConfig, boolean enabled) { 
 WifiManager wifiManager;
try {   
  if (enabled) { //disables wifi hotspot if it's already enabled    
    wifiManager.setWifiEnabled(false);  
  } 

   Method method = wifiManager.getClass()   
      .getMethod("setWifiApEnabled", WifiConfiguration.class, boolean.class);   
  return (Boolean) method.invoke(wifiManager, wifiConfig, enabled); 
} catch (Exception e) { 
  Log.e(this.getClass().toString(), "", e); 
  return false; 
}   
}

You can also get the wificonfiguration of your hotspot through reflection. I've answered that method for this question on StackOverflow.

P.S: If you don't want to turn on hotspot programmatically, you can start this intent and open the wifi settings screen for user to turn it on manually.

SOLUTION FOR API >= 26:

Finally, android released an official API for versions >= Oreo. You can just use the public exposed API by android i.e. startLocalOnlyHotspot

It turns on a local hotspot without internet access. Which thus can be used to host a server or transfer files.

It requires Manifest.permission.CHANGE_WIFI_STATE and ACCESS_FINE_LOCATION permissions.

Here's a simple example of how you can turn on hotspot using this API.

private WifiManager wifiManager;
WifiConfiguration currentConfig;
WifiManager.LocalOnlyHotspotReservation hotspotReservation;

The method to turn on hotspot:

@RequiresApi(api = Build.VERSION_CODES.O)
public void turnOnHotspot() {

      wifiManager.startLocalOnlyHotspot(new WifiManager.LocalOnlyHotspotCallback() {

        @Override
        public void onStarted(WifiManager.LocalOnlyHotspotReservation reservation) {
          super.onStarted(reservation);
          hotspotReservation = reservation;
          currentConfig = hotspotReservation.getWifiConfiguration();

          Log.v("DANG", "THE PASSWORD IS: "
              + currentConfig.preSharedKey
              + " \n SSID is : "
              + currentConfig.SSID);

          hotspotDetailsDialog();

        }

        @Override
        public void onStopped() {
          super.onStopped();
          Log.v("DANG", "Local Hotspot Stopped");
        }

        @Override
        public void onFailed(int reason) {
          super.onFailed(reason);
          Log.v("DANG", "Local Hotspot failed to start");
        }
      }, new Handler());
    }
`

Here's how you can get details of the locally created hotspot

private void hotspotDetaisDialog()
{

    Log.v(TAG, context.getString(R.string.hotspot_details_message) + "\n" + context.getString(
              R.string.hotspot_ssid_label) + " " + currentConfig.SSID + "\n" + context.getString(
              R.string.hotspot_pass_label) + " " + currentConfig.preSharedKey);

}

If it throws, a security exception even after giving the required permissions then you should try enabling your location using GPS. Here's the solution.

Recently, I've developed a demo app called Spotserve. That turns on wifi hotspot for all devices with API>=15 and hosts a demo server on that hotspot. You can check that for more details. Hope this helps!

For Android 8.0, there is a new API to handle Hotspots. As far as I know, the old way using reflection doesn't work anymore. Please refer to:

Android Developers https://developer.android.com/reference/android/net/wifi/WifiManager.html#startLocalOnlyHotspot(android.net.wifi.WifiManager.LocalOnlyHotspotCallback,%20android.os.Handler)

void startLocalOnlyHotspot (WifiManager.LocalOnlyHotspotCallback callback, 
                Handler handler)

Request a local only hotspot that an application can use to communicate between co-located devices connected to the created WiFi hotspot. The network created by this method will not have Internet access.

Stack Overflow
How to turn on/off wifi hotspot programmatically in Android 8.0 (Oreo)

onStarted(WifiManager.LocalOnlyHotspotReservation reservation) method will be called if hotspot is turned on.. Using WifiManager.LocalOnlyHotspotReservation reference you call close() method to turn off hotspot.

APManager - Access Point Manager

allprojects {
    repositories {
        ...
        jcenter()
    }
}

dependencies {
    implementation 'com.vkpapps.wifimanager:APManager:1.0.0'
}

APManager apManager = APManager.getApManager(this);
apManager.turnOnHotspot(this, new APManager.OnSuccessListener() {

    @Override
    public void onSuccess(String ssid, String password) {
        //write your logic
    }

}, new APManager.OnFailureListener() {

    @Override
    public void onFailure(int failureCode, @Nullable Exception e) {
        //handle error like give access to location permission,write system setting permission,
        //disconnect wifi,turn off already created hotspot,enable GPS provider
        
        //or use DefaultFailureListener class to handle automatically
    }

});

check out source code https://github.com/vijaypatidar/AndroidWifiManager

You can use command line with root access

To turn on hotspot.

adb shell su -c service call connectivity 24 i32 0 i32 0 i32 0 s16 random

To turn off hotspot

adb shell su -c service call connectivity 25 i32 0 i32 0 i32 0 s16 random

**For Oreo & PIE ** I found below way through this

private WifiManager.LocalOnlyHotspotReservation mReservation;
private boolean isHotspotEnabled = false;
private final int REQUEST_ENABLE_LOCATION_SYSTEM_SETTINGS = 101;

private boolean isLocationPermissionEnable() {
    if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
        ActivityCompat.requestPermissions(this, new String[] {Manifest.permission.ACCESS_COARSE_LOCATION}, 2);
        return false;
    }
    return true;
}

@RequiresApi(api = Build.VERSION_CODES.O)
private void turnOnHotspot() {
    if (!isLocationPermissionEnable()) {
        return;
    }
    WifiManager manager = (WifiManager) getApplicationContext().getSystemService(Context.WIFI_SERVICE);

    if (manager != null) {
        // Don't start when it started (existed)
        manager.startLocalOnlyHotspot(new WifiManager.LocalOnlyHotspotCallback() {

            @Override
            public void onStarted(WifiManager.LocalOnlyHotspotReservation reservation) {
                super.onStarted(reservation);
                //Log.d(TAG, "Wifi Hotspot is on now");
                mReservation = reservation;
                isHotspotEnabled = true;
            }

            @Override
            public void onStopped() {
                super.onStopped();
                //Log.d(TAG, "onStopped: ");
                isHotspotEnabled = false;
            }

            @Override
            public void onFailed(int reason) {
                super.onFailed(reason);
                //Log.d(TAG, "onFailed: ");
                isHotspotEnabled = false;
            }
        }, new Handler());
    }
}

@RequiresApi(api = Build.VERSION_CODES.O)
private void turnOffHotspot() {
    if (!isLocationPermissionEnable()) {
        return;
    }
    if (mReservation != null) {
        mReservation.close();
        isHotspotEnabled = false;
    }
}

@RequiresApi(api = Build.VERSION_CODES.O)
private void toggleHotspot() {
    if (!isHotspotEnabled) {
        turnOnHotspot();
    } else {
        turnOffHotspot();
    }
}

@RequiresApi(api = Build.VERSION_CODES.O)
private void enableLocationSettings() {
    LocationRequest mLocationRequest = new LocationRequest();
    /*mLocationRequest.setInterval(10);
    mLocationRequest.setSmallestDisplacement(10);
    mLocationRequest.setFastestInterval(10);
    mLocationRequest.setPriority(LocationRequest.PRIORITY_HIGH_ACCURACY);*/
    LocationSettingsRequest.Builder builder = new LocationSettingsRequest.Builder();
    builder.addLocationRequest(mLocationRequest)
            .setAlwaysShow(false); // Show dialog

    Task<LocationSettingsResponse> task= LocationServices.getSettingsClient(this).checkLocationSettings(builder.build());

    task.addOnCompleteListener(task1 -> {
        try {
            LocationSettingsResponse response = task1.getResult(ApiException.class);
            // All location settings are satisfied. The client can initialize location
            // requests here.
            toggleHotspot();

        } catch (ApiException exception) {
            switch (exception.getStatusCode()) {
                case LocationSettingsStatusCodes.RESOLUTION_REQUIRED:
                    // Location settings are not satisfied. But could be fixed by showing the
                    // user a dialog.
                    try {
                        // Cast to a resolvable exception.
                        ResolvableApiException resolvable = (ResolvableApiException) exception;
                        // Show the dialog by calling startResolutionForResult(),
                        // and check the result in onActivityResult().
                        resolvable.startResolutionForResult(HotspotActivity.this, REQUEST_ENABLE_LOCATION_SYSTEM_SETTINGS);
                    } catch (IntentSender.SendIntentException e) {
                        // Ignore the error.
                    } catch (ClassCastException e) {
                        // Ignore, should be an impossible error.
                    }
                    break;
                case LocationSettingsStatusCodes.SETTINGS_CHANGE_UNAVAILABLE:
                    // Location settings are not satisfied. However, we have no way to fix the
                    // settings so we won't show the dialog.
                    break;
            }
        }
    });
}

@RequiresApi(api = Build.VERSION_CODES.O)
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    final LocationSettingsStates states = LocationSettingsStates.fromIntent(data);
    switch (requestCode) {
        case REQUEST_ENABLE_LOCATION_SYSTEM_SETTINGS:
            switch (resultCode) {
                case Activity.RESULT_OK:
                    // All required changes were successfully made
                    toggleHotspot();
                    Toast.makeText(HotspotActivity.this,states.isLocationPresent()+"",Toast.LENGTH_SHORT).show();
                    break;
                case Activity.RESULT_CANCELED:
                    // The user was asked to change settings, but chose not to
                    Toast.makeText(HotspotActivity.this,"Canceled",Toast.LENGTH_SHORT).show();
                    break;
                default:
                    break;
            }
            break;
    }
}

UseAge

btnHotspot.setOnClickListenr(view -> {
   if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) {
        // Step 1: Enable the location settings use Google Location Service
        // Step 2: https://stackoverflow.com/questions/29801368/how-to-show-enable-location-dialog-like-google-maps/50796199#50796199
        // Step 3: If OK then check the location permission and enable hotspot
        // Step 4: https://stackoverflow.com/questions/46843271/how-to-turn-off-wifi-hotspot-programmatically-in-android-8-0-oreo-setwifiapen
        enableLocationSettings();
        return;
    }
}

<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />

implementation 'com.google.android.gms:play-services-location:15.0.1'

We can programmatically turn on and off

setWifiApDisable.invoke(connectivityManager, TETHERING_WIFI);//Have to disable to enable
setwifiApEnabled.invoke(connectivityManager, TETHERING_WIFI, false, mSystemCallback,null);

Using callback class, to programmatically turn on hotspot in pie(9.0) u need to turn off programmatically and the switch on.

You can use the console and service for that option.

I think with this you can solve that. I made a test directly in console and enable hotspot

I found this in this article Is it possible to USB tether an android device using adb through the terminal?

And after read the interface we can use the same 24 but needs more parameters

service call connectivity 24 i32 0 i32 1 i32 0 s16 random