public class MainActivity extends AppCompatActivity {

private static final int MY_PERMISSIONS_REQUEST_SEND_SMS =0 ;
Button send;
EditText message;
EditText phoneno;
String number;
String txt;

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

    send=(Button)findViewById(R.id.sendButton);
    message=(EditText)findViewById(R.id.textMessage);
    phoneno=(EditText)findViewById(R.id.phone);

    send.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            sendSMSMessage();
        }
    });
}

public void sendSMSMessage(){
    number=phoneno.getText().toString();
    txt=message.getText().toString();

    if (ContextCompat.checkSelfPermission(this,
            Manifest.permission.SEND_SMS)
            != PackageManager.PERMISSION_GRANTED) {
        if (ActivityCompat.shouldShowRequestPermissionRationale(this,
                Manifest.permission.SEND_SMS)) {
        } else {
            ActivityCompat.requestPermissions(this,
                    new String[]{Manifest.permission.SEND_SMS},
                    MY_PERMISSIONS_REQUEST_SEND_SMS);
        }
    }

}

@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
    //super.onRequestPermissionsResult(requestCode, permissions, grantResults);
    switch (requestCode) {
        case MY_PERMISSIONS_REQUEST_SEND_SMS: {
            if (grantResults.length > 0
                    && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                SmsManager smsManager = SmsManager.getDefault();
                smsManager.sendTextMessage(number, null, txt, null, null);
                Toast.makeText(getApplicationContext(), "SMS sent.",
                        Toast.LENGTH_LONG).show();
            } else {
                Toast.makeText(getApplicationContext(),
                        "SMS faild, please try again.", Toast.LENGTH_LONG).show();
                return;
            }
        }
    }

}

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.shaloin.sample852.MainActivity">

<EditText
    android:id="@+id/textMessage"
    android:textSize="20sp"
    android:layout_marginTop="20dp"
    android:hint="Text Message"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />
<EditText
    android:id="@+id/phone"
    android:layout_below="@+id/textMessage"
    android:textSize="20sp"

    android:layout_marginTop="20dp"
    android:hint="Phone Number"
    android:layout_width="match_parent"
    android:layout_height="wrap_content" />
<Button
    android:id="@+id/sendButton"
    android:layout_below="@+id/phone"
    android:layout_centerHorizontal="true"
    android:text="Send"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.shaloin.sample852">

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

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:roundIcon="@mipmap/ic_launcher_round"
    android:supportsRtl="true"
    android:theme="@style/AppTheme">
    <activity android:name=".MainActivity">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>