添加Android Studio的setOnClickListener时发生错误

Question

so for a school project I'm making an app, but I"m stuck. I've got a button on my main class. XML code of this class:

 <?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" tools:context="com.example.larsb.csvvg.Home" android:background="@drawable/home"> <Button android:id="@+id/Lariks" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_centerHorizontal="true" android:layout_centerVertical="true" android:text="Button" android:visibility="visible" /> <Button android:id="@+id/Salland" android:layout_width="wrap_content" android:layout_height="wrap_content" android:layout_marginTop="45dp" android:text="Button" android:visibility="visible" android:layout_alignTop="@+id/Lariks" android:layout_alignLeft="@+id/Lariks" android:layout_alignStart="@+id/Lariks" /> <Button android:id="@+id/CSG" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Button" android:visibility="visible" android:layout_below="@+id/Salland" android:layout_alignLeft="@+id/Salland" android:layout_alignStart="@+id/Salland" /> </RelativeLayout> 

So what I want to do is when I button click Lariks I switch to a new activity. I try to do this with the following code:

 public class MainActivity extends AppCompatActivity { private static int WELCOME=4000; Button lariks; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); lariks = (Button)findViewById(R.id.Lariks); lariks.setOnClickListener(new View.OnClickListener() { public void onClick(View v) { } }); new Handler().postDelayed(new Runnable() { @Override public void run() { Intent intent=new Intent(MainActivity.this,Home.class ); startActivity(intent); finish(); } },WELCOME); } } 

In the public void onClick(view) thing I got the code for switching activity, that's not the problem. The problem is that as soon as I add the:

 lariks.setOnClickListener(new View.OnClickListener() { public void onClick(View v) { } }); 

the app can't launch anymore. It starts the emulator but it keeps saying the app has stopped. Anyone has any idea why this is? If I remove that part of the code the app actually does run.

Answer 1

Your Button with android:id="@+id/Lariks" is not in the activity_main layout (as evidenced by tools:context="com.example.larsb.csvvg.Home" and your crash). Therefore findViewById(R.id.Lariks) returns null, and attempting to call setOnClickListener() on this null reference causes a crash.

Your findViewById() - setOnClickListener() pair should likely be in the Home activity, or the button should be in the activity_main layout.