activity 无法启动 Fragment 且无法显示与此相关的布局。以空指针结束

Question

我正在按照书中的示例进行操作，但以下代码无法启动 CrimeFragment() 片段。屏幕上什么也没有出现。 Activity 代码如下：

CrimeActivity.java

package com.example.simerpreetjassal.criminalintent;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;

public class CrimeActivity extends FragmentActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_crime);
        FragmentManager fm = getSupportFragmentManager();
        Fragment fragment = fm.findFragmentById(R.id.framgment_container);
        if(fragment==null) {
            fragment = new CrimeFragment();
            (fm.beginTransaction().add(R.id.framgment_container, fragment)).commit();
        }

    }
}

上层布局 activity。文件名为 content_crime.

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/framgment_container"
    android:layout_height="match_parent"
    android:layout_width="match_parent" />

片段代码如下。它无法显示与此相关的布局，并在 edittext (mTittleField) 上出现空指针异常。如何修复 mTittleField 以消除空指针异常。

CrimeFragment.java

package com.example.simerpreetjassal.criminalintent;

import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.text.Editable;
import android.text.TextWatcher;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.EditText;

public class CrimeFragment extends Fragment {
    private Crime mCrime;
    private EditText mTittleField;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        mCrime = new Crime();
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View v = inflater.inflate(R.layout.fragment_crime, container, false);
        mTittleField = (EditText) v.findViewById((R.id.crime_title));
        mTittleField.addTextChangedListener(new TextWatcher() {
            @Override
            public void beforeTextChanged(CharSequence s, int start, int count, int after) {

            }

            @Override
            public void onTextChanged(CharSequence s, int start, int before, int count) {

            }

            @Override
            public void afterTextChanged(Editable s) {

            }
        });
        return v;

    }
}

与此相关的布局。文件名为 crime_fragment.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    >
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="@string/crime_title_hint"
        android:id="@+id/crime_title"
        />
</LinearLayout>

Logcat

07-28 11:27:09.056 3083-3083/com.example.simerpreetjassal.criminalintent W/System: ClassLoader referenced unknown path: /data/app/com.example.simerpreetjassal.criminalintent-1/lib/x86
07-28 11:27:09.187 3083-3083/com.example.simerpreetjassal.criminalintent W/System: ClassLoader referenced unknown path: /data/app/com.example.simerpreetjassal.criminalintent-1/lib/x86
07-28 11:27:09.309 3083-3083/com.example.simerpreetjassal.criminalintent D/AndroidRuntime: Shutting down VM
07-28 11:27:09.310 3083-3083/com.example.simerpreetjassal.criminalintent E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.simerpreetjassal.criminalintent, PID: 3083
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.simerpreetjassal.criminalintent/com.example.simerpreetjassal.criminalintent.CrimeActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.EditText.addTextChangedListener(android.text.TextWatcher)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
    at android.app.ActivityThread.-wrap11(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5417)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.EditText.addTextChangedListener(android.text.TextWatcher)' on a null object reference
    at com.example.simerpreetjassal.criminalintent.CrimeFragment.onCreateView(CrimeFragment.java:26)
    at android.support.v4.app.Fragment.performCreateView(Fragment.java:1962)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1067)
    at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1248)
    at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:738)
    at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1613)
    at android.support.v4.app.FragmentController.execPendingActions(FragmentController.java:330)
    at android.support.v4.app.FragmentActivity.onStart(FragmentActivity.java:547)
    at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1237)
    at android.app.Activity.performStart(Activity.java:6253)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2379)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
    at android.app.ActivityThread.-wrap11(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:148) 
    at android.app.ActivityThread.main(ActivityThread.java:5417) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 
    07-28 11:27:10.147 3083-3089/com.example.simerpreetjassal.criminalintent W/art: Suspending all threads took: 7.535ms

Answer 1

xml 布局文件的名称与您在片段 onCreateView() 中引用的名称不同。

您的片段布局 xml 文件是 crime_fragment.xml，您从 Java 代码中引用的布局是 fragment_crime.xml（通过 R class).

你的Activity也一样，当你布局xml文件是content_crime.xml

时，它指的是布局错误R.layout.activity_crime

将 Activity 的 onCreate() 方法的 setContentView() 行替换为：

setContentView(R.layout.content_crime);

activity 无法启动 Fragment 且无法显示与此相关的布局。以空指针结束

activity unable to start Fragment and unable to show layout related to this. ending with null pointer

xml

android

android-layout

android-fragments

android-fragmentactivity