Send data to android from ble(nrf51822)?

[ahmet]

I want to send data to my android phone from Nrf51822.My compiler is mbed I use redbearlab NRF51822 kit.For example,I want to see on my phone "hello world". Mbed Program example,

void onDataWritten(const GattCharacteristicWriteCBParams *params)
{
 DEBUG("hello world");
or
 printf("hello world");
     
 } 

---

 //Android studio side example,
 public void onCharacteristicChanged(BluetoothGatt gatt, BluetoothGattCharacteristic characteristic) {
            super.onCharacteristicChanged(gatt, characteristic);
            writeLine("Alinan Deger " + characteristic.getStringValue(0));


  }

This is the part where I experience the problem? Can anyone tell me what is the problem with this part?

[ahmet]

void onDataWritten(const GattCharacteristicWriteCBParams *params) { printf("hello world"); } //for mbed compiler pc.printf=is for serial comminacation.