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I want to change the preferred network mode ie. gsm or wcdma or auto, programmatically, from code, on Android.

Is this possible, and if so how ?

Answer is NO

We can open directly the settings app of mobile network settings to switch between "2G" and "allow 3G" networks.A direct switch is sadly not possible.

We can develop something which will show current network and allow user short-cut from the app where they can switch network.

It is possible, I did it.

For this to work, your app must be signed with the system key or have carrier privilege. Otherwise the app will throw
java.lang.SecurityException: No modify permission or carrier privilege.

My app runs on Android 5.1 Lollipop(API 22) and is signed with the system key, so this is the only configuration I can confirm for sure that works. I can't confirm the carrier privilege approach.

AndroidManifest.xml

Add this permission to your app manifest. If you are using Android Studio, it will probably mark this line as an error because only system apps can have this permission. If you can sign your app with the system keys, don't worry.

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

Get Preferred Network

The return is defined in RILConstants.java, e.g. RILConstants.NETWORK_MODE_WCDMA_PREF

public int getPreferredNetwork() 
 Method method = getHiddenMethod("getPreferredNetworkType", TelephonyManager.class, null);
 int preferredNetwork = -1000;
 try 
 preferredNetwork = (int) method.invoke(mTelephonyManager);
 Log.i(TAG, "Preferred Network is ::: " + preferredNetwork);
 catch (IllegalAccessException e) 
 e.printStackTrace();
 catch (InvocationTargetException e) 
 e.printStackTrace();
 

 return preferredNetwork;

Set Preferred Method.

The parameter must be based on RILConstants.java, e.g.: RILConstants.NETWORK_MODE_LTE_ONLY

public void setPreferredNetwork(int networkType) 
 try 
 Method setPreferredNetwork = getHiddenMethod("setPreferredNetworkType",
 TelephonyManager.class, new Class int.class);
 Boolean success = (Boolean)setPreferredNetwork.invoke(mTelephonyManager,
 networkType);
 Log.i(TAG, "Could set Network Type ::: " + (success.booleanValue() ? "YES" : "NO"));
 catch (InvocationTargetException e) 
 e.printStackTrace();
 catch (IllegalAccessException e) 
 e.printStackTrace();
 

This is an utility method to access the hidden API methods.

/**
 * Get a hidden method instance from a class
 * @param methodName The name of the method to be taken from the class
 * @param fromClass The name of the class that has the method
 * @return A Method instance that can be invoked
 */
public Method getHiddenMethod(String methodName, Class fromClass, Class params) 
 Method method = null;
 try 
 Class clazz = Class.forName(fromClass.getName());
 method = clazz.getMethod(methodName, params);
 method.setAccessible(true);
 catch (ClassNotFoundException e) 
 e.printStackTrace();
 catch (NoSuchMethodException e) 
 e.printStackTrace();
 

 return method;

This is not an answer, but expanding on Tulio-F's answer.

The RILConstants.java contain the following:

// NETWORK_MODE_* See ril.h RIL_REQUEST_SET_PREFERRED_NETWORK_TYPE 
int NETWORK_MODE_WCDMA_PREF = 0; // GSM/WCDMA (WCDMA preferred) 
int NETWORK_MODE_GSM_ONLY = 1; // GSM only 
int NETWORK_MODE_WCDMA_ONLY = 2; // WCDMA only 
int NETWORK_MODE_GSM_UMTS = 3; // GSM/WCDMA (auto mode, according to PRL)** 
int NETWORK_MODE_CDMA = 4; // CDMA and EvDo (auto mode, according to PRL)**
int NETWORK_MODE_CDMA_NO_EVDO = 5; // CDMA only 
int NETWORK_MODE_EVDO_NO_CDMA = 6; // EvDo only 
int NETWORK_MODE_GLOBAL = 7; // GSM/WCDMA, CDMA, and EvDo (auto mode, according to PRL)**
int NETWORK_MODE_LTE_CDMA_EVDO = 8; // LTE, CDMA and EvDo 
int NETWORK_MODE_LTE_GSM_WCDMA = 9; // LTE, GSM/WCDMA 
int NETWORK_MODE_LTE_CDMA_EVDO_GSM_WCDMA = 10; // LTE, CDMA, EvDo, GSM/WCDMA 
int NETWORK_MODE_LTE_ONLY = 11; // LTE Only mode. 
int NETWORK_MODE_LTE_WCDMA = 12; // LTE/WCDMA 
int NETWORK_MODE_TDSCDMA_ONLY = 13; // TD-SCDMA only 
int NETWORK_MODE_TDSCDMA_WCDMA = 14; // TD-SCDMA and WCDMA 
int NETWORK_MODE_LTE_TDSCDMA = 15; // TD-SCDMA and LTE 
int NETWORK_MODE_TDSCDMA_GSM = 16; // TD-SCDMA and GSM 
int NETWORK_MODE_LTE_TDSCDMA_GSM = 17; // TD-SCDMA,GSM and LTE 
int NETWORK_MODE_TDSCDMA_GSM_WCDMA = 18; // TD-SCDMA, GSM/WCDMA 
int NETWORK_MODE_LTE_TDSCDMA_WCDMA = 19; // TD-SCDMA, WCDMA and LTE 
int NETWORK_MODE_LTE_TDSCDMA_GSM_WCDMA = 20; // TD-SCDMA, GSM/WCDMA and LTE 
int NETWORK_MODE_TDSCDMA_CDMA_EVDO_GSM_WCDMA = 21; // TD-SCDMA,EvDo,CDMA,GSM/WCDMA
int NETWORK_MODE_LTE_TDSCDMA_CDMA_EVDO_GSM_WCDMA = 22; // TD-SCDMA/LTE/GSM/WCDMA, CDMA, and EvDo

int PREFERRED_NETWORK_MODE = SystemProperties.getInt("ro.telephony.default_network", NETWORK_MODE_WCDMA_PREF);

Where:

** = "AVAILABLE Application Settings menu"