Question

嗨，我是Android开发的初学者，所以我真的不太了解我在做什么。我一直得到nullpointerexecption，并且每当调用getActivity（）时我的程序都会崩溃。这包括Toast消息以及当我尝试以意图启动另一个活动时。我不完全确定如何使用意图，如果可以解释那将是很好的。

以下是具有getActivity（）调用以及intent的类：

import android.app.AlertDialog;
import android.app.Dialog;
import android.app.DialogFragment;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.widget.Toast;

public class loginDialog extends DialogFragment {
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState){
        AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
        builder.setTitle(R.string.loginw)
                .setItems(R.array.account_types, new DialogInterface.OnClickListener(){
                    @Override
                    public void onClick(DialogInterface dialog, int which){
                        if(which==0){
                            Toast.makeText(getActivity(),"todo",Toast.LENGTH_SHORT).show();
                        }else if(which==1){
                            Toast.makeText(getActivity(),"todo",Toast.LENGTH_SHORT).show();
                        }else if(which==2){
                            AlertDialog.Builder builder1=new AlertDialog.Builder(getActivity());
                            LayoutInflater inflater=getActivity().getLayoutInflater();
                            View view = inflater.inflate(R.layout.login_dialog,null);
                            builder1.setView(view);
                            builder1.setTitle("Login with Email Address");
                            builder1.setNegativeButton(R.string.cancel, new DialogInterface.OnClickListener() {
                                @Override
                                public void onClick(DialogInterface dialog1, int which) {
                                    //Toast.makeText(getActivity(), "Cancel", Toast.LENGTH_SHORT).show(); //Force Closes. Nullpointer Exception? Probably because of getActivity()
                                    //dialog1.dismiss();
                                }
                            });
                            builder1.setNeutralButton("Forgot Password",new DialogInterface.OnClickListener(){
                                @Override
                                public void onClick(DialogInterface dialog1, int which){
                                    dialog1.dismiss();
                                    AlertDialog.Builder builder2=new AlertDialog.Builder(getActivity());
                                    builder2.setTitle("Forgot Password");
                                    builder2.setMessage("Enter your Email Address");

                                    builder2.setPositiveButton("OK",new DialogInterface.OnClickListener() {
                                        @Override
                                        public void onClick(DialogInterface dialog, int which) {
                                            //TODO
                                        }
                                    });
                                    builder2.setNegativeButton("Cancel",new DialogInterface.OnClickListener() {
                                        @Override
                                        public void onClick(DialogInterface dialog, int which) {
                                            //TODO
                                        }
                                    });
                                    builder2.show();
                                }
                            });
                            builder1.setPositiveButton(R.string.login, new DialogInterface.OnClickListener() {
                                @Override
                                public void onClick(DialogInterface dialog1, int which) {
                                    //Toast.makeText(getActivity(),"Login complete",Toast.LENGTH_SHORT).show(); //Force Closes. Nullpointer Exception? Probably because of getActivity()
                                    dialog1.dismiss();

                                    Intent intent = new Intent(getActivity(),Groups.class);
                                    startActivity(intent);
                                }
                            });
                            Dialog dialog1 = builder1.create();
                            dialog1.show();
                        }
                    }
                });
        Dialog dialog=builder.create();
        return dialog;
    }
}

这是logcat（请注意，这仅适用于intent调用，因为其他getActivity（）被注释掉了）：

java.lang.NullPointerException
        at android.content.ComponentName.<init>(ComponentName.java:77)
        at android.content.Intent.<init>(Intent.java:3813)
        at com.app.u.loginDialog$1$3.onClick(loginDialog.java:67)
        at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:166)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:136)
        at android.app.ActivityThread.main(ActivityThread.java:5017)
        at java.lang.reflect.Method.invokeNative(Native Method)
        at java.lang.reflect.Method.invoke(Method.java:515)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
        at dalvik.system.NativeStart.main(Native Method)

Answer 1

在Overriden dialogfragment类中返回onActivityCreated方法后，应该调用getActivity。在你的案例loginDialog类。这是调用getActivity的安全的地方

public class LoginDialog extends DialogFragment {
    Activity mAct; 

    @Override
    public void onAttach(Activity act) {
        super.onAttach(act);
        mAct = act; // use this mAct instead of getActivity() function
    }

Answer 2

你不能在对话弹出消息中创建Intent所以你应该像这样使用或在你的对话中声明这样它会帮助你绝对......一切顺利

Context context = activity.getApplicationContext();                         
Intent intent = new Intent(context, LoginActivity.class);
intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);     
context.startActivity(intent);

Answer 3

尝试使用getApplicationContext()代替

Intent intent = new Intent(getApplicationContext(), Groups.class);
startActivity(intent);

请记得在AndroidManifest文件中声明您的新活动！

<activity android:name=".Groups"/>

NullPointerException与getActivity（）并不确定如何进行意图

3 个答案: