我有一个TabAcitivity和一些标签。其中一个选项卡（选项卡的名称为HomeTab）是一个Button。我希望Button能够做点什么。所以，我在SectionsPagerAdapter -> getItem(int position)方法中设置了Button的EventListener。但它返回NullPointerException。我应该在哪里设置EventListener，以便findViewById()不会返回null？
MainActivity.java：
package com.whatsyouridea.myapp;
import android.content.Intent;
import android.support.design.widget.TabLayout;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.RadioGroup;
import android.widget.TimePicker;
import java.util.TimerTask;
public class MainActivity extends AppCompatActivity {
/**
* The {@link android.support.v4.view.PagerAdapter} that will provide
* fragments for each of the sections. We use a
* {@link FragmentPagerAdapter} derivative, which will keep every
* loaded fragment in memory. If this becomes too memory intensive, it
* may be best to switch to a
* {@link android.support.v4.app.FragmentStatePagerAdapter}.
*/
private SectionsPagerAdapter mSectionsPagerAdapter;
/**
* The {@link ViewPager} that will host the section contents.
*/
private ViewPager mViewPager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
// Create the adapter that will return a fragment for each of the three
// primary sections of the activity.
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
// Set up the ViewPager with the sections adapter.
mViewPager = (ViewPager) findViewById(R.id.container);
mViewPager.setAdapter(mSectionsPagerAdapter);
TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
tabLayout.setupWithViewPager(mViewPager);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
public void start_timer_for_timer(int hours, int minutes, int seconds) {
}
public void start_timer_for_clock(TimePicker time_picker) {
}
public void start_btn_clicked(View view) {
RadioGroup radioGroup = (RadioGroup)view.findViewById(R.id.selection_buttons);
int selectedIndex = ComponentTools.getSelectedRadioButtonIndex(view, radioGroup);
}
public void stop_btn_clicked() {
}
/**
* A {@link FragmentPagerAdapter} that returns a fragment corresponding to
* one of the sections/tabs/pages.
*/
public class SectionsPagerAdapter extends FragmentPagerAdapter {
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
switch (position) {
case 0:
HomeTab homeTab = new HomeTab();
/**************Here is where the error occured***************/
// Set up start button EventListener
Button start_button = (Button)homeTab.getView().findViewById(R.id.start_btn);
start_button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Button start_button = (Button)view.findViewById(R.id.start_btn);
start_button.setEnabled(false);
Button stop_button = (Button)view.findViewById(R.id.stop_btn);
stop_button.setEnabled(true);
start_btn_clicked(view);
}
});
// Set up stop button EventListener
Button stop_button = (Button)homeTab.getView().findViewById(R.id.stop_btn);
stop_button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Button start_button = (Button)view.findViewById(R.id.start_btn);
start_button.setEnabled(true);
Button stop_button = (Button)view.findViewById(R.id.stop_btn);
stop_button.setEnabled(false);
stop_btn_clicked();
}
});
return homeTab;
case 1:
TimerSettingsTab timerSettingsTab = new TimerSettingsTab();
return timerSettingsTab;
case 2:
ClockSettingsTab clockSettingsTab = new ClockSettingsTab();
return clockSettingsTab;
default:
return null;
}
}
@Override
public int getCount() {
// Show 3 total pages.
return 3;
}
@Override
public CharSequence getPageTitle(int position) {
switch (position) {
case 0:
return "HOME";
case 1:
return "TIMER SETTINGS";
case 2:
return "CLOCK SETTINGS";
}
return null;
}
}
}
HomeTab.java（Fragment java class）
package com.whatsyouridea.myapp;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
public class HomeTab extends Fragment {
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.tab_home, container, false);
return rootView;
}
}
完全错误：
09-14 07:56:20.852 8483-8483/com.whatsyouridea.myapp E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.whatsyouridea.myapp, PID: 8483
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.whatsyouridea.myapp/com.whatsyouridea.myapp.MainActivity}: 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:2325)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.whatsyouridea.myapp.MainActivity.onCreate(MainActivity.java:66)
at android.app.Activity.performCreate(Activity.java:5990)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2387)
at android.app.ActivityThread.access$800(ActivityThread.java:151)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
答案 0 :(得分：0)
执行此操作
// inside the onCreateView method of the fragment
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_home, container, false);
Button myButton= (Button) rootView.findViewById(R.id.button);
myButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// do something..
}});
return rootView;
}
或者......
// implements OnClickListener interface
public class HomeFragment extends BaseFragment implements OnClickListener {
private Button myButton;
@Override
public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_home, container, false);
myButton= (Button) rootView.findViewById(R.id.button);
// set the listen to the contex
myButton.setOnClickListener(this);
return rootView;
}
// override onClick and check using switch
@Override
public void onClick(View view) {
switch (view.getId()) {
case R.id.button:
// do something
break;
}
}
}