java.lang.NullPointerException 空对象引用 viewpager

Question

我在编码时遇到困难，需要帮助。 我在我的项目中使用视图绑定。 我无法绑定并且工作正常。 除了视图绑定之外，我在我的早期项目中使用了相同的代码，并且该项目工作正常。

这是我的 XML

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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:padding="5dp"
tools:context=".Home">

<androidx.viewpager.widget.ViewPager
    android:id="@+id/pager"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_above="@id/bottom_navigation" />

<com.google.android.material.bottomnavigation.BottomNavigationView
    android:id="@+id/bottom_navigation"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentBottom="true"
    app:menu="@menu/bottom_navigation_menu" />

</RelativeLayout>

这是我的 Java 代码

package com.example.professionals;

public class Home extends AppCompatActivity {

private ActivityHomeBinding Homebinding;
protected ViewPager viewPager;
protected Mypager_Adapter mypager_adapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_home);
    Homebinding = ActivityHomeBinding.inflate(getLayoutInflater());
    View view = Homebinding.getRoot();
    setContentView(view);

    Homebinding.pager.findViewById(R.id.pager);
    mypager_adapter = new Mypager_Adapter(this);
    viewPager.setAdapter(mypager_adapter);

    Homebinding.bottomNavigation.setOnItemSelectedListener(new 
    NavigationBarView.OnItemSelectedListener() {
        @Override
        public boolean onNavigationItemSelected(@NonNull MenuItem item) {
            switch (item.getItemId()) {
                case R.id.home:
                    showMsg("Home");
                    break;
                case R.id.profile:
                    showMsg("Profile");
                    break;
            }
            return false;
        }
    });
}

private void showMsg(String msg) {
    Toast toast = Toast.makeText(this, msg, Toast.LENGTH_LONG);
    toast.show();
}
}

这是错误列表

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.professionals, PID: 17743
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.professionals/com.example.professionals.Home}: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.viewpager.widget.ViewPager.setAdapter(androidx.viewpager.widget.PagerAdapter)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2473)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2535)
    at android.app.ActivityThread.access00(ActivityThread.java:154)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1396)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:148)
    at android.app.ActivityThread.main(ActivityThread.java:5582)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void androidx.viewpager.widget.ViewPager.setAdapter(androidx.viewpager.widget.PagerAdapter)' on a null object reference
    at com.example.professionals.Home.onCreate(Home.java:32)
    at android.app.Activity.performCreate(Activity.java:6321)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1108)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2426)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2535) 
    at android.app.ActivityThread.access00(ActivityThread.java:154) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1396) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:148) 
    at android.app.ActivityThread.main(ActivityThread.java:5582) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

在此先感谢您的帮助！

Answer 1

您需要添加

viewPager = findViewById<ViewPager>(R.id.pager)

这一行之前

viewPager.setAdapter(mypager_adapter);

编辑：

实际上，您似乎在尝试使用 ViewBinding 以及常规的 findViewById，如果我是您，我会坚持使用 ViewBinding 并删除此行

protected ViewPager viewPager;

那就试试这个

Homebinding.pager.setAdapter(mypager_adapter);

Answer 2

You haven't initialize your ViewPager.

您需要初始化您的viewPager。为此你有几个选择

在设置适配器之前使用 findViewById 进行初始化。

viewPager = findViewById(R.id.pager)

因为你正在使用视图绑定是个好主意

Homebinding.pager.setAdapter(mypager_adapter)