When you receive a call, send an SMS to that number

What to add to AndroidManifest.xml

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

As it is written,

• READ_PHONE_STATE is about the phone
• SEND_SMS is about SMS Privileges. Give this one.

Java

Description of the phone

TelephonyManager and PhoneStateListener do something about it.

TelephonyManager telephonyManager = (TelephonyManager) getSystemService(TELEPHONY_SERVICE);
telephonyManager.listen(new PhoneStateListener(){
	@Override
	public void onCallStateChanged(int state, String number) {
		//The state is the number that indicates the state, and the number is the number that was received when the call was received.
		switch(state) {
			//Incoming event
			case TelephonyManager.CALL_STATE_RINGING:
				System.out.println(number + "It's an incoming call from Mr.");
				break;
			//Event at the start of a call
			case TelephonyManager.CALL_STATE_OFFHOOK:
				System.out.println("I started calling");
				break;
			//When changing to the standby state(When you hang up)Event
			case TelephonyManager.CALL_STATE_IDLE:
				System.out.println("I hung up the phone");
				break;
		}
	}
}, PhoneStateListener.LISTEN_CALL_STATE);

Description of SMS

SmsManager does a good job.

sendTextMessage method

Send text-based SMS. All I have to do is send an SMS, so I'll look into the details at another time.

argument

1. String destinationAddress --Destination address
2. String scAddress --Service center address to use the default SMSC ――Honestly, I don't really understand, so null
3. String text --Message content
4. PendingIntent sentIntent --Broadcast when a message is successfully sent or fails if not null --How to specify when you want detailed control
5. PendingIntent deliveryIntent --Broadcast when the message is delivered to the recipient, if not null --Honestly, I don't know the difference from sentIntent, so I need to investigate.

String number = "09000000000";	//Destination phone number

SmsManager smsManager = SmsManager.getDefault();
smsManager.sendTextMessage(
	number,
	null,
    "Hello",
    null,
    null
);

Final description

Send a simple message to the person who called you, just saying "Hey !!".

MainActivity.java

package example.com.sample003;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.telephony.PhoneStateListener;
import android.telephony.SmsManager;
import android.telephony.TelephonyManager;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // -----The important thing is ↓ from here ↓----- //
        TelephonyManager telephonyManager = (TelephonyManager) getSystemService(TELEPHONY_SERVICE);
        telephonyManager.listen(new PhoneStateListener() {
            @Override
            public void onCallStateChanged(int state, String number) {
                //The state is the number that indicates the state, and the number is the number that was received when the call was received.
                switch (state) {
                    //Incoming event
                    case TelephonyManager.CALL_STATE_RINGING:
                        SmsManager smsManager = SmsManager.getDefault();
                        smsManager.sendTextMessage(
                                number,
                                null,
                                "Hey!!",
                                null,
                                null
                        );
                        break;
                    //Event at the start of a call
                    case TelephonyManager.CALL_STATE_OFFHOOK:
                        break;
                    //When changing to the standby state(When you hang up)Event
                    case TelephonyManager.CALL_STATE_IDLE:
                        break;
                }
            }
        }, PhoneStateListener.LISTEN_CALL_STATE);
        // -----↑ Up to here ↑----- //
    }
}

bonus

Sample