Android应用程序崩溃时更改活动在Android Studio[重复]

Question

我是新的Android Studio。我想开始一个新的活动时，一个按钮被点击，我的应用程序不断停止。

这是我的代码：这是第一个活动（我有所有的导入）

    RecyclerView recyclerView;
    FloatingActionButton add_button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.dispozitive);

        recyclerView=findViewById(R.id.recyclerView);
        add_button=findViewById(R.id.add_button1);
        add_button.setOnClickListener(v -> {
                Intent intent=new Intent(Dispozitive.this, AddActivity.class);
                startActivity(intent);

        });
    }
}

第二类：

public class AddActivity extends AppCompatActivity {

    EditText nume,nr_disp,watts,nr_zile,nr_ore;
    Button add_button;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_add);

        nume=findViewById(R.id.nume);
        nr_disp=findViewById(R.id.nr_disp);
        watts=findViewById(R.id.nr_watts);
        nr_zile=findViewById(R.id.nr_zile);
        nr_ore=findViewById(R.id.nr_ore);

        add_button.setOnClickListener(v -> {
            MyDatabase myDB=new MyDatabase(AddActivity.this);
            myDB.adaugareDispozitiv(nume.getText().toString().trim(),
                    Integer.valueOf(nr_disp.getText().toString().trim()),
                    Integer.valueOf(watts.getText().toString().trim()),
                    Integer.valueOf(nr_zile.getText().toString().trim()),
                    Integer.valueOf(nr_ore.getText().toString().trim()));
        });

    }
}

清单文件：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.pa8">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/Theme.PA8">
        <activity
            android:name=".AddActivity"
            android:exported="false"
            android:label="Adăugare dispozitiv"
            android:parentActivityName=".Dispozitive"
            />
        <activity
            android:name=".Dispozitive"
            android:exported="false"
            android:label="Dispozitive"
            android:parentActivityName=".MainActivity"
            />
        <activity
            android:name=".MainActivity"
            android:exported="true">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />
                <action android:name="android.intent.action.VIEW" />


                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

这是按钮：

<com.google.android.material.floatingactionbutton.FloatingActionButton
        android:id="@+id/add_button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginEnd="24dp"
        android:layout_marginBottom="24dp"
        android:clickable="true"
        android:focusable="true"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:srcCompat="@drawable/ic_add_white" />

从主活动到离散的过渡工作得很好。我不明白为什么这两个不工作。谢谢帮忙！新年快乐！（如果你今天看到这个：））

logcat：

2021-12-31 15:52:31.032 10328-10328/com.example.pa8 E/RecyclerView: No adapter attached; skipping layout
2021-12-31 15:52:33.304 10328-10328/com.example.pa8 E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.pa8, PID: 10328
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.pa8/com.example.pa8.AddActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
        at com.example.pa8.AddActivity.onCreate(AddActivity.java:26)
        at android.app.Activity.performCreate(Activity.java:7994)
        at android.app.Activity.performCreate(Activity.java:7978)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 

Answer 1

在您的AddActivity中，您没有初始化add_按钮，因此您得到了NullPointerException