Android 应用程序在模拟器中崩溃，代码为 java

Question

我的 android 应用程序在模拟器中崩溃。该应用程序未在模拟器中打开。我正在与 java 合作。在我将底部导航添加到我的应用程序之前，它运行良好。请解决我的问题，因为我是这个领域的新手。下面是我的主要 activity 的代码。

package com.example.bottomnavigation;
import android.content.DialogInterface;
import android.content.Intent;
import android.os.Bundle;
import android.view.MenuItem;
import android.view.View;
import android.widget.ImageView;
import androidx.annotation.NonNull;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;
import androidx.fragment.app.FragmentTransaction;
import com.example.bottomnavigation.databinding.ActivityMainBinding;
import com.google.android.material.bottomnavigation.BottomNavigationView;
public class MainActivity extends AppCompatActivity {
ActivityMainBinding binding;
ImageView picture, video, message, mail;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    binding = ActivityMainBinding.inflate(getLayoutInflater());
    setContentView(binding.getRoot());
    getSupportActionBar().hide();
    binding.bottomNav.setOnNavigationItemSelectedListener(new BottomNavigationView.OnNavigationItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            switch (item.getItemId()){
                case R.id.first:
                    FragmentTransaction firsttrans = getSupportFragmentManager().beginTransaction();
                    firsttrans.replace(R.id.frame, new HomeFragment());
                    firsttrans.commit();
                    break;
                case R.id.search:
                    FragmentTransaction searchtrans = getSupportFragmentManager().beginTransaction();
                    searchtrans.replace(R.id.frame, new SearchFragment());
                    searchtrans.commit();
                    break;
                case R.id.exit:
                    new AlertDialog.Builder(MainActivity.this)
                            .setTitle("Exit")
                            .setIcon(R.drawable.exit)
                            .setMessage("Are you sure you want to exit")
                            .setPositiveButton("Yes", new DialogInterface.OnClickListener() {
                                @Override
                                public void onClick(DialogInterface dialog, int which) {
                                    finishAffinity();
                                }
                            }).show();

            }
            return true;
        }
    });
    picture = findViewById(R.id.arham);
    picture.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent = new Intent(MainActivity.this, Pictures.class);
            startActivity(intent);
        }
    });
}

}

这是logcat

2021-04-05 12:35:38.662 19375-19375/? E/ottomnavigatio: Unknown bits set in runtime_flags: 0x8000
2021-04-05 12:35:40.544 19375-19375/? E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.bottomnavigation, PID: 19375
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.bottomnavigation/com.example.bottomnavigation.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.hide()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3270)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.appcompat.app.ActionBar.hide()' on a null object reference
        at com.example.bottomnavigation.MainActivity.onCreate(MainActivity.java:27)
        at android.app.Activity.performCreate(Activity.java:7802)
        at android.app.Activity.performCreate(Activity.java:7791)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1299)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3245)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)

Answer 1

抛出异常是因为您在对 ActionBar 的 null 引用上调用 hide()。假设这是相关代码，它发生在这一行：

    getSupportActionBar().hide();

如果您阅读 getSupportActionBar() 的 javadocs (here and here)，您将看到它 returns null 如果 activity 没有行动栏。这一定发生在这里。

（那么，为什么您的应用在没有 activity 栏的情况下试图隐藏它？？）

看来解决方案是删除违规行。或者，如果在某些情况下您的应用可能具有操作栏，请将其更改为：

    ActionBar bar = getSupportActionBar();
    if (bar != null) {
        bar.hide();
    }

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