android - Android中的倒数计时器错误

Question

我搜索了它，但是任何解决方案都无法解决我的问题。
我正在片段中实现计时器，但我的应用程序停止工作并显示错误。

这是我的应用程序的工作。
当用户单击“开始”按钮时，会将用户带到下一个 Activity ，在该 Activity 中在容器中显示片段(RelativeLayout)。我已经在片段中实现了Count CountDownTimer函数。片段在CountDownTimer函数之前运行良好。

有帮助吗？
这是代码。

启动.java

package com.example.sheikhspc.timerexample;

import android.app.Fragment;
import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

public class Start extends AppCompatActivity {

Fragment frag = null;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_start);
    FragmentManager fm = getFragmentManager();
    FragmentTransaction fragmentTransaction = fm.beginTransaction();

    frag = new BlankFragment1();
    fragmentTransaction.replace(R.id.container,frag);

    fragmentTransaction.commit();





    }
}

MainActivity.java

package com.example.sheikhspc.timerexample;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {

Button btn;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    btn = (Button)findViewById(R.id.startbtn);
}

public void start(View view)
{
    Intent intent = new Intent(this,Start.class);
    startActivity(intent);
}
}

BlankFragment1.java

package com.example.sheikhspc.timerexample;


import android.os.Bundle;
import android.app.Fragment;
import android.os.CountDownTimer;
import android.provider.DocumentsContract;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
import android.widget.Toast;


public class BlankFragment1 extends Fragment {


public BlankFragment1() {
    // Required empty public constructor
}


Fragment frag = null;
TextView tv;
View root;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    tv = (TextView)root.findViewById(R.id.timer1);

    new CountDownTimer(30000, 1000) {

        public void onTick(long millisUntilFinished) {
            tv.setText("seconds remaining: " + millisUntilFinished / 1000);
            //here you can have your logic to set text to edittext
        }

        public void onFinish() {
            tv.setText("done!");
        }

    }.start();



    return inflater.inflate(R.layout.fragment_blank_fragment1, container,    false);
}

}

错误记录

     E/AndroidRuntime: FATAL EXCEPTION: main
     Process: com.example.sheikhspc.timerexample, PID: 28970
     java.lang.RuntimeException: Unable to start activity    ComponentInfo{com.example.sheikhspc.timerexample/com.example.sheikhspc.timerexamp le.Start}: 
     java.lang.NullPointerException: Attempt to invoke virtual method  'android.view.View android.view.View.findViewById(int)' on a null object  reference

  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2339)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2413)
                  at android.app.ActivityThread.access$800(ActivityThread.java:155)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1317)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:135)
                  at android.app.ActivityThread.main(ActivityThread.java:5343)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at java.lang.reflect.Method.invoke(Method.java:372)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:905)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:700)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.View.findViewById(int)' on a null object reference
                  at com.example.sheikhspc.timerexample.BlankFragment1.onCreateView(BlankFragment1.java:46)
                  at android.app.Fragment.performCreateView(Fragment.java:2053)
                  at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:894)
                  at android.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
                  at android.app.BackStackRecord.run(BackStackRecord.java:834)
                  at android.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1454)
                  at android.app.Activity.performStart(Activity.java:6025)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2302)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2413) 
                  at android.app.ActivityThread.access$800(ActivityThread.java:155) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1317) 
                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                  at android.os.Looper.loop(Looper.java:135) 
                  at android.app.ActivityThread.main(ActivityThread.java:5343) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at java.lang.reflect.Method.invoke(Method.java:372) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:905) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:700) 
Application terminated.

Answer 1

在OnCreateView上

View v = inflater.inflate(R.layout.fragment_blank_fragment1, null);

然后按 v.findViewById查找项目并返回膨胀的 View

return v;

或者您可以通过以下方式将布局分配给root

root = inflater.inflate(R.layout.fragment_blank_fragment1, null);

tv = (TextView) root.findViewById(...);
// other implementations 

return root;