为什么单击菜单项时活动无法打开？

Question

我想做一个简单的菜单(和我做菜单的方式完全一样(。当我点击菜单项时，我会打开一个新的活动。问题是，我可以毫无错误地从主活动移动到任何其他活动，但当我尝试单击菜单时，其他活动会出现以下错误。我还将所有活动添加到清单文件中

我做错了什么？请告诉我。我是个初学者。

主要活动：

public class MainActivity extends AppCompatActivity{
DrawerLayout drawerLayout;
RecyclerView recyclerView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
drawerLayout = findViewById(R.id.drawer_layout);
recyclerView = findViewById(R.id.rv2);
recyclerView.setLayoutManager(new LinearLayoutManager(this, RecyclerView.VERTICAL, false));
TextView next = (TextView) findViewById(R.id.cards);
next.setOnClickListener(new View.OnClickListener() {
public void onClick(View view) {
Intent myIntent = new Intent(view.getContext(), Cards.class);
startActivityForResult(myIntent, 0);
}
});
ImageView hamburger = (ImageView) findViewById(R.id.menu_ham);
hamburger.setOnClickListener(new View.OnClickListener() {
public void onClick(View view) {
drawerLayout.openDrawer(Gravity.START);
}
});
}
}

卡片：

public class Cards extends AppCompatActivity {
DrawerLayout drawerLayout;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_cards);
drawerLayout = findViewById(R.id.drawer_layout);
ImageView hamburger = (ImageView) findViewById(R.id.menu_ham);
hamburger.setOnClickListener(new View.OnClickListener() {
public void onClick(View view) {
drawerLayout.openDrawer(Gravity.START);
}
});
TextView home = (TextView) findViewById(R.id.home);
home.setOnClickListener(new View.OnClickListener() {
public void onClick(View view) {
Intent myIntent = new Intent(view.getContext(), MainActivity.class);
startActivityForResult(myIntent, 0);
}
});
}
}

Activity_cards.xml：

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/drawer_layout"
tools:context=".Cards">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<include layout="@layout/main_toolbar" />
</LinearLayout>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:layout_marginTop="50dp"
android:background="@color/white" >
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Cards" />
</androidx.constraintlayout.widget.ConstraintLayout>
</androidx.drawerlayout.widget.DrawerLayout>

activity_main.xml：

<?xml version="1.0" encoding="utf-8"?>
<androidx.drawerlayout.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/drawer_layout"
tools:context=".MainActivity">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<include
layout="@layout/main_toolbar" />
<androidx.recyclerview.widget.RecyclerView
android:padding="15dp"
android:id="@+id/rv2"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</LinearLayout>
<RelativeLayout
android:layout_width="300dp"
android:layout_height="match_parent"
android:layout_gravity="start"
android:background="@color/white">
<include layout="@layout/main_nav_drawer" />
</RelativeLayout>
</androidx.drawerlayout.widget.DrawerLayout>

main_nav_drawer：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
android:gravity="center_vertical">
<ImageView
android:layout_width="80px"
android:layout_height="80px"
android:layout_marginLeft="15dp"/>
<TextView
android:id="@+id/home"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginLeft="27dp"
android:layout_weight="1"
android:padding="12dp"
android:text="Home"
android:textSize="18dp"
android:textStyle="bold"
android:textColor="#000"/>

</LinearLayout>
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
android:gravity="center_vertical">
<ImageView
android:layout_width="80px"
android:layout_height="80px"
android:layout_marginLeft="15dp"/>
<TextView
android:id="@+id/cards"
android:textColor="#000"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="Cards"
android:padding="12dp"
android:textStyle="bold"
android:textSize="18dp"
android:layout_marginLeft="27dp"/>
</LinearLayout>
</LinearLayout>

main_toolbar.xml：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
android:padding="12dp"
android:gravity="center_horizontal">
<ImageView
android:id="@+id/menu_ham"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:src="@drawable/ic_baseline_menu_24"/>
<TextView
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight="1"
android:text="android coding"
android:textSize="20sp"
android:textStyle="bold"
android:textAlignment="center"
android:textColor="#636363"
android:gravity="center_horizontal" />
</LinearLayout>

错误看起来像：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myapplication0000, PID: 24658
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myapplication0000/com.example.myapplication0000.Cards}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.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$MethodAndArgsCaller.run(RuntimeInit.java:592)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.example.myapplication0000.Cards.onCreate(Cards.java:31)
at android.app.Activity.performCreate(Activity.java:8000)
at android.app.Activity.performCreate(Activity.java:7984)
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main_nav_drawer父线性布局中添加SlideMain id。

并从要切换抽屉的位置调用此方法onclick evenet。

fun toggleDrawer() {
if (drawer_layout.isDrawerOpen(SlideMain)) {
drawer_layout.closeDrawer(SlideMain)
} else {
drawer_layout.openDrawer(SlideMain)
}
}

Java：

private void toggleDrawer() {
if (drawer_layout.isDrawerOpen(SlideMain)) {
drawer_layout.closeDrawer(SlideMain)
} else {
drawer_layout.openDrawer(SlideMain)
}
}

调用方法如下：

btn_toggle_drawer.onClicklistener{
toggleDrawer();
}

监听器语法可能是来自移动的错误编辑答案