片段替换过程中出现Nullpointerexception

Question

我已经从一个片段更改为另一个片段。但当我使用此功能时，应用程序将停止。我该怎么办？这是我的代码和Logcat。

这是Logcat。

07-11 12:45:09.787 8825-8825/? I/art: Not late-enabling -Xcheck:jni (already on)
07-11 12:45:09.855 8825-8825/com.example.yangseungchan.navigationdrawer W/System: ClassLoader referenced unknown path: /data/app/com.example.yangseungchan.navigationdrawer-2/lib/x86_64
07-11 12:45:09.875 8825-8825/com.example.yangseungchan.navigationdrawer W/art: Before Android 4.1, method android.graphics.PorterDuffColorFilter android.support.graphics.drawable.VectorDrawableCompat.updateTintFilter(android.graphics.PorterDuffColorFilter, android.content.res.ColorStateList, android.graphics.PorterDuff$Mode) would have incorrectly overridden the package-private method in android.graphics.drawable.Drawable
07-11 12:45:10.089 8825-8825/com.example.yangseungchan.navigationdrawer I/art: Rejecting re-init on previously-failed class java.lang.Class<android.support.v4.view.ViewCompat$OnUnhandledKeyEventListenerWrapper>
Rejecting re-init on previously-failed class java.lang.Class<android.support.v4.view.ViewCompat$OnUnhandledKeyEventListenerWrapper>
07-11 12:45:10.185 8825-8841/com.example.yangseungchan.navigationdrawer D/OpenGLRenderer: Use EGL_SWAP_BEHAVIOR_PRESERVED: true
07-11 12:45:10.226 8825-8841/com.example.yangseungchan.navigationdrawer I/OpenGLRenderer: Initialized EGL, version 1.4
07-11 12:45:10.227 8825-8841/com.example.yangseungchan.navigationdrawer W/OpenGLRenderer: Failed to choose config with EGL_SWAP_BEHAVIOR_PRESERVED, retrying without...
07-11 12:45:10.235 8825-8841/com.example.yangseungchan.navigationdrawer D/EGL_emulation: eglCreateContext: 0x7f18e1034b40: maj 3 min 0 rcv 3
07-11 12:45:10.241 8825-8841/com.example.yangseungchan.navigationdrawer D/EGL_emulation: eglMakeCurrent: 0x7f18e1034b40: ver 3 0 (tinfo 0x7f18e1002660)
07-11 12:45:10.242 8825-8841/com.example.yangseungchan.navigationdrawer E/eglCodecCommon: glUtilsParamSize: unknow param 0x00008cdf
glUtilsParamSize: unknow param 0x00008824
07-11 12:45:10.291 8825-8841/com.example.yangseungchan.navigationdrawer D/EGL_emulation: eglMakeCurrent: 0x7f18e1034b40: ver 3 0 (tinfo 0x7f18e1002660)
07-11 12:45:30.497 8825-8825/com.example.yangseungchan.navigationdrawer D/AndroidRuntime: Shutting down VM
07-11 12:45:30.497 8825-8825/com.example.yangseungchan.navigationdrawer E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.yangseungchan.navigationdrawer, PID: 8825
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Class java.lang.Object.getClass()' on a null object reference
at android.support.v4.app.BackStackRecord.doAddOp(BackStackRecord.java:396)
at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:444)
at android.support.v4.app.BackStackRecord.replace(BackStackRecord.java:434)
at com.example.yangseungchan.navigationdrawer.MainActivity.onFragmentChanged(MainActivity.java:82)
at com.example.yangseungchan.navigationdrawer.InformationFragment$1.onClick(InformationFragment.java:28)
at android.view.View.performClick(View.java:5198)
at android.view.View$PerformClick.run(View.java:21147)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
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)
07-11 12:45:31.824 8825-8825/? I/Process: Sending signal. PID: 8825 SIG: 9

**[MainActivity.java:82]，[InformationFragment.java:28]为蓝线。

这是MainActivity.java

1. package com.example.yangseungchan.navigationdrawer;
2.
3. import android.support.annotation.NonNull;
4. import android.support.design.widget.NavigationView;
5. import android.support.v4.view.GravityCompat;
6. import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.view.MenuItem;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener{
private DrawerLayout drawer;
InformationFragment informationFragment;
InformationFragment2 informationFragment2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
informationFragment = (InformationFragment)getSupportFragmentManager().findFragmentById(R.id.informationFragment);
informationFragment2 = new InformationFragment2();
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
drawer = findViewById(R.id.drawer_layout);
NavigationView navigationView = findViewById(R.id.nav_view);
navigationView.setNavigationItemSelectedListener(this);
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(this, drawer, toolbar,
R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawer.addDrawerListener(toggle);
toggle.syncState();
if(savedInstanceState == null){
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new InformationFragment()).commit();
navigationView.setCheckedItem(R.id.nav_information);
}
}
@Override
public boolean onNavigationItemSelected(@NonNull MenuItem menuItem) {
switch(menuItem.getItemId()){
case R.id.nav_information:
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new InformationFragment()).commit();
break;
case R.id.nav_map:
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new MapFragment()).commit();
break;
case R.id.nav_help:
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new HelpFragment()).commit();
break;
case R.id.nav_sending:
Toast.makeText(this, "Sending has been done successfully",Toast.LENGTH_SHORT).show();
break;
case R.id.nav_download:
Toast.makeText(this, "Downloading has been done successfully",Toast.LENGTH_SHORT).show();
break;
case R.id.nav_setting:
getSupportFragmentManager().beginTransaction().replace(R.id.fragment_container, new SettingFragment()).commit();
break;
}
drawer.closeDrawer(GravityCompat.START);
return true;
}
@Override
public void onBackPressed(){
if(drawer.isDrawerOpen(GravityCompat.START)){
drawer.closeDrawer(GravityCompat.START);
}
else{
super.onBackPressed();
}
}
public void onFragmentChanged(int index){
if(index == 0){
82. getSupportFragmentManager().beginTransaction().replace(R.id.container,informationFragment).commit();
}
else{
getSupportFragmentManager().beginTransaction().replace(R.id.container, informationFragment2).commit();
}
}
}

主活动82是

public void onFragmentChanged(int index){
if(index == 0){
82. getSupportFragmentManager().beginTransaction().replace(R.id.container,informationFragment).commit();
}
else{
getSupportFragmentManager().beginTransaction().replace(R.id.container, informationFragment2).commit();
}
}
}

InformationFragment.java在这里

package com.example.yangseungchan.navigationdrawer;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.widget.DrawerLayout;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;

public class InformationFragment extends Fragment {
private DrawerLayout drawer;
@Nullable
@Override

public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
ViewGroup rootView = (ViewGroup)inflater.inflate(R.layout.fragment_information,container,false);
Button nextButton = (Button)rootView.findViewById(R.id.nextButton);
nextButton.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View view) {
MainActivity activity = (MainActivity)getActivity();
activity.onFragmentChanged(0);
}
});
return rootView;
}

}

Information.java:28是

public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
ViewGroup rootView = (ViewGroup)inflater.inflate(R.layout.fragment_information,container,false);
Button nextButton = (Button)rootView.findViewById(R.id.nextButton);
nextButton.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View view) {
MainActivity activity = (MainActivity)getActivity();
28. activity.onFragmentChanged(0);
}
});
return rootView;
}

Answer 1

您在这里没有获得activity引用，即是null，因此您的应用程序正在崩溃。将您的this代码移到onAttach((方法中。

您的代码如下：

@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
mNextButton.setOnClickListener(new View.OnClickListener(){
@Override
public void onClick(View view) {
MainActivity activity = (MainActivity)getActivity();
activity.onFragmentChanged(0);
}
});
}

注意：使mNextButton类变量，即在onCreateView外部声明。

Answer 2

我认为您无法初始化要替换的片段的对象。

//instead of initialising it this way 
informationFragment = (InformationFragment)getSupportFragmentManager().findFragmentById(R.id.informationFragment);
//try using 
informationFragment = new InformationFragment();