单击按钮后，我希望将Fragment 1布局替换为Fragment 2布局，但其抛出:java.lang.NullPoi

Question

第一个片段的代码

class MainFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// Inflate the layout for this fragment
val mainInflater = inflater.inflate(R.layout.fragment_main, container, false)
return mainInflater
}
fun thisdata():String{
return "Hello from MainFragment"
}
}

第一片段XML

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainFragment">
<EditText
android:id="@+id/etSavedData"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="60dp"
android:hint="Enter the Text"/>
<Button
android:id="@+id/btnSaveData"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="120dp"
android:text="Save"/>
</RelativeLayout>

主要活动的代码

class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
//Set replace Main Activity content with the Fragment1 content
val mainFragment = MainFragment()
supportFragmentManager.beginTransaction().add(R.id.contain_fragment, mainFragment).commit()
val thedata = mainFragment.thisdata()
Log.e("Main Frag to Activity", thedata)
btnSaveData.setOnClickListener { 
val secondFragment = SecondFragment()
supportFragmentManager.beginTransaction().add(R.id.contain_fragment, secondFragment).commit()
}
}
}

主活动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:id="@+id/contain_fragment"
tools:context=".MainActivity">

</RelativeLayout>

第二片段的代码

class SecondFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// Inflate the layout for this fragment
val secondInflater = inflater.inflate(R.layout.fragment_second, container, false)
return secondInflater
}
}

第二片段XML

<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@android:color/holo_blue_dark"
tools:context=".SecondFragment">
<!-- TODO: Update blank fragment layout -->
<TextView
android:id="@+id/tvDisplayText"
android:layout_width="match_parent"
android:layout_height="wrap_content"
tools:text="Text displayed here." />
</FrameLayout>

Logcat中出现以下异常

由java.lang.NullPointerException引起：尝试调用虚拟方法"void"android.widget.Button.setOnClickListener(android.view.view$OnClickListener'(关于null对象引用网址：com.example.demo2.MainActivity.onCreate(MainActivity.kt:22(*

Answer 1

您需要将btnSaveData按钮移动到MainActivity中，或者将onClickListener移动到MainFragment中，同时进行一些其他更改。类似于：

class MainFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// Inflate the layout for this fragment
val mainInflater = inflater.inflate(R.layout.fragment_main, container, false)
btnSaveData.setOnClickListener { 
val secondFragment = SecondFragment()
activity.supportFragmentManager.beginTransaction().add(R.id.contain_fragment, secondFragment).commit()
}
return mainInflater
}
fun thisdata():String{
return "Hello from MainFragment"
}
}

Answer 2

**谢谢Tash，这确实有道理，不幸的是，它仍然显示出相同的异常，但我最终通过以下操作解决了问题。**

class MainFragment : Fragment() {
override fun onCreateView(
inflater: LayoutInflater, container: ViewGroup?,
savedInstanceState: Bundle?
): View? {
// Inflate the layout for this fragment
val mainInflater = inflater.inflate(R.layout.fragment_main, container, false)
mainInflater.btnSaveData.setOnClickListener {
val secondFragment = SecondFragment()
secondFragment?.let {
activity?.supportFragmentManager?.beginTransaction()?.replace(R.id.contain_fragment, secondFragment)?.commit()
}
}
return mainInflater
}
fun thisdata():String{
return "Hello from MainFragment"
}
}