如果语句不适用于android studio

我在发送短信时有一个简单的应用程序，当用户没有在字段中输入内容时我想显示一条错误消息，但是当我运行我的应用程序并尝试在字段中输入内容时，if语句不起作用并说出何时字段是空的...

这是Java代码：

package com.george.getoutgr;

import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import android.Manifest;
import android.content.pm.PackageManager;
import android.media.Image;
import android.os.Bundle;
import android.telephony.SmsManager;
import android.view.View;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.Toast;

public class MainActivity extends AppCompatActivity {

final int SMS_PERMISSION_REQUEST_CODE = 1;

public void sendSms(int num,String name,String address){
      // send SMS \\
      String phone_number = "13033";
      SmsManager smsManager = SmsManager.getDefault();
      smsManager.sendTextMessage(phone_number, null,num + "" + name + "" + address,null,null);

}

public boolean checkPermission(String permission){
    int check = ContextCompat.checkSelfPermission(this, permission);
    return (check == PackageManager.PERMISSION_GRANTED);
}

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

    final ImageButton hospital = findViewById(R.id.hospital_btn);
    final ImageButton store = findViewById(R.id.store_btn);
    final ImageButton bank = findViewById(R.id.bank_btn);
    final ImageButton help = findViewById(R.id.help_btn);
    final ImageButton cerenomy = findViewById(R.id.cerenomy_btn);
    final ImageButton gym = findViewById(R.id.gym_btn);

    hospital.setOnClickListener(new View.OnClickListener() {
        final EditText username = findViewById(R.id.username);
        final EditText address = findViewById(R.id.address);
        final String username_input = username.getText().toString();
        final String address_input = address.getText().toString();

        @Override
        public void onClick(View v) {
            if(username_input.isEmpty() || address_input.isEmpty()){
                Toast.makeText(MainActivity.this, "Παρακαλω συμπληρωσε το ονομα αι την διευθηνση σου!", Toast.LENGTH_LONG).show();
            }else {
                if (checkPermission(Manifest.permission.SEND_SMS)) {
                    // send sms \\
                    sendSms(1, username_input, address_input);
                } else {
                    ActivityCompat.requestPermissions(MainActivity.this,
                            new String[]{Manifest.permission.SEND_SMS}, SMS_PERMISSION_REQUEST_CODE);
                }
            }
        }
    });
}
}