java - 尝试向 firebase 服务器发送消息时出现异常

Question

请问这段代码有什么问题吗？

public class NewMessageActivity extends AppCompatActivity {

private DatabaseReference mDatabaseReference;
String mDisplayName = "John";
EditText mNewMessageField;


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    final EditText mNewMessageField = (EditText) findViewById(R.id.newMessageText);
    setContentView(R.layout.activity_new_message);
    ImageButton mSendButton = (ImageButton) findViewById(R.id.sendButton);
    mSendButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            sendMessage();
            finish();
        }
    });
}

private void sendMessage(){
    mDatabaseReference= FirebaseDatabase.getInstance().getReference();
    String input = mNewMessageField.getText().toString();
    SingleMessage singleMessage = new SingleMessage(input, mDisplayName);
    mDatabaseReference.child("messages").push().setValue(singleMessage);
}

}

按下发送按钮后，应用程序立即停止工作，并收到此错误消息:

FATAL EXCEPTION: main java.lang.NullPointerException at com..NewMessageActivity.sendMessage(NewMessageActivity.java:42) at com..NewMessageActivity.access$000(NewMessageActivity.java:14) at com.***.NewMessageActivity$1.onClick(NewMessageActivity.java:33) at android.view.View.performClick(View.java:4209) at android.view.View$PerformClick.run(View.java:17457) at android.os.Handler.handleCallback(Handler.java:725) at android.os.Handler.dispatchMessage(Handler.java:92) at android.os.Looper.loop(Looper.java:153) at android.app.ActivityThread.main(ActivityThread.java:5341) at java.lang.reflect.Method.invokeNative(Native Method) at java.lang.reflect.Method.invoke(Method.java:511) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:929) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:696) at dalvik.system.NativeStart.main(Native Method) 11-28 20:05:51.566 508-527/? E/AppErrorDialog: Failed to get ILowStorageHandle instance

Answer 1

在搜索 EditText 小部件之前，您需要调用 setContentView()。您还必须使用在包级别声明的 mNewMessageField 实例。不要在 onCreate() 中声明新实例。

setContentView(R.layout.activity_new_message);
mNewMessageField = (EditText) findViewById(R.id.newMessageText);  // <= CHANGED
if (mNewMessageField == null) {
    System.out.println("mNewMessageField is NULL");
}

要获得更多线索，请添加此调试输出:

private void sendMessage(){
    mDatabaseReference= FirebaseDatabase.getInstance().getReference();
    if (mNewMessageField == null) {
        System.out.println("mNewMessageField is NULL");
    }
    Editable ed = mNewMessageField.getText();

    if (ed == null) {
        System.out.println("Editable is NULL");
    }
    String input = mNewMessageField.getText().toString();
    System.out.println("input=" + input);

    SingleMessage singleMessage = new SingleMessage(input, mDisplayName);
    mDatabaseReference.child("messages").push().setValue(singleMessage);
}