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My app crashes on launch with getActionBar() [duplicate]

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What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 5 years ago.
I am making an app with view pager and fragments. But it crashes on launch so can you please help me finding the error in my code. I am providing both my code and Logcat. I am learning the tutorial from this website : https://www.androidhive.info/2013/10/android-tab-layout-with-swipeable-views-1/
package com.example.android.resume;
import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import tabsswiper.TabsPagerAdapter;
public class MainActivity extends FragmentActivity implements
ActionBar.TabListener {
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
String[] tabs = {"About Me", "Sample Photos", "Book Me"};
// Initilization
viewPager = findViewById(R.id.pager);
actionBar = getActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
// Adding Tabs
for (String tab_name : tabs) {
actionBar.addTab(actionBar.newTab().setText(tab_name)
.setTabListener(this));
}
/*
* on swiping the viewpager make respective tab selected
* */
viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageSelected(int position) {
// on changing the page
// make respected tab selected
actionBar.setSelectedNavigationItem(position);
}
#Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
#Override
public void onPageScrollStateChanged(int arg0) {
}
});
}
#Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
}
#Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
// on tab selected
// show respected fragment view
viewPager.setCurrentItem(tab.getPosition());
}
#Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
}
}
My Logcat :
FATAL EXCEPTION: main
Process: com.example.android.resume, PID: 32723
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.android.resume/com.example.android.resume.MainActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2381)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2433)
at android.app.ActivityThread.access$800(ActivityThread.java:155)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1346)
at android.os.Handler.dispatchMessage(Handler.java:110)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:5341)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:515)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:830)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:646)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.example.android.resume.MainActivity.onCreate(MainActivity.java:30)
at android.app.Activity.performCreate(Activity.java:5343)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1088)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2335)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2433) 
at android.app.ActivityThread.access$800(ActivityThread.java:155) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1346) 
at android.os.Handler.dispatchMessage(Handler.java:110) 
at android.os.Looper.loop(Looper.java:193) 
at android.app.ActivityThread.main(ActivityThread.java:5341) 
at java.lang.reflect.Method.invokeNative(Native Method) 
at java.lang.reflect.Method.invoke(Method.java:515) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:830) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:646) 
at dalvik.system.NativeStart.main(Native Method) 

Use AppCompactActivity in place of FragmentActivity and also use Toolbar
as
toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);

you can use getSupportActionBar() instead of getActionBar() method.

You should use getSupportActionBar() instead of getActionBar().
Firstly remove your import of
import android.app.ActionBar;
and then check if you declared the supported of v7 in dependence as below
compile 'com.android.support:appcompat-v7:+'
good luck.

Try
getContext().getActionBar()
or
getContext().getSupportActionBar()

Try this one
package com.demotest;
import android.os.Bundle;
import android.support.v4.app.FragmentTransaction;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBar;
import android.support.v7.app.AppCompatActivity;
public class MainActivity extends AppCompatActivity implements
ActionBar.TabListener {
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
// Tab titles
private String[] tabs = {"Top Rated", "Games", "Movies"};
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Initilization
viewPager = (ViewPager) findViewById(R.id.pager);
actionBar = getSupportActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
// Adding Tabs
for (String tab_name : tabs) {
actionBar.addTab(actionBar.newTab().setText(tab_name)
.setTabListener(MainActivity.this));
}
viewPager.addOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageScrolled(int position, float positionOffset, int
positionOffsetPixels) {
actionBar.setSelectedNavigationItem(position);
}
#Override
public void onPageSelected(int position) {
}
#Override
public void onPageScrollStateChanged(int state) {
}
});
}
#Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
viewPager.setCurrentItem(tab.getPosition());
}
#Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {
}
#Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {
}
}

Fragment creates an Activity object but it is null

In my fragment class, I create an object of my Activity class:
MainActivity mainActivity;
and in my fragment's OnCreateView(), I initialize it:
mainActivity = new MainActivity();
However, when I debug my app, mainActivity is always null. Why is this?
My Activity class:
package com.example.brettrosen.atls;
import android.annotation.TargetApi;
import android.app.ActionBar;
import android.app.FragmentTransaction;
import android.os.Build;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
#TargetApi(Build.VERSION_CODES.HONEYCOMB)
public class MainActivity extends FragmentActivity implements ActionBar.TabListener {
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
public ActionBar actionBar;
// Tab titles
private String[] tabs = {"Pre-arrival Plan", "Primary Survey", "Secondary Survey"};
#TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Initialization
viewPager = (ViewPager) findViewById(R.id.pager);
actionBar = getActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
// Adding Tabs
for (String tab_name : tabs) {
actionBar.addTab(actionBar.newTab().setText(tab_name).setTabListener(this));
}
/**
* on swiping the viewpager make respective tab selected
* */
viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageSelected(int position) {
// on changing the page
// make respected tab selected
actionBar.setSelectedNavigationItem(position);
}
#Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
#Override
public void onPageScrollStateChanged(int arg0) {
}
});
updateTabTitles(0, 12);
updateTabTitles(1, 2);
updateTabTitles(2, 14);
}
public void updateTabTitles(int pageNum, int remaining) {
String text = tabs[pageNum] + " (" + remaining + ")";
System.out.println(actionBar);
actionBar.getTabAt(pageNum).setText(text);
}
#Override
public void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft) {
viewPager.setCurrentItem(tab.getPosition());
}
#Override
public void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft) {
}
#Override
public void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft) {
}
};

I see your problem is that you want to access MainActivity from within a fragment that it hosts.
To do that, you could just call this:
getActivity();
And it will return your MainAcitvity (or any other Activity that is currently hosting your Fragment).
To access a method from it, you'd have to cast the return of that method.
MainActivity yourMain = (MainActivity) getActivity();
yourMain.method1();
Remember to do this inside of your fragment.

Changing the starting tab of multiple swipe views with tabs

So I am "playing" around with some code i got from a tutorial on Tab layout with swipeable views..
My Problem is that I cannot figure out how to change the default tab it opens, when the app is launched.. Right now it starts in the 1st tab(Top Rated) but I want to have it start the second tab called Games?
I've browsed around and looked for solutions but doesn't seem to find one that fits my code..
Here is my main acitivity:
package info.androidhive.tabsswipe;
import info.androidhive.tabsswipe.adapter.TabsPagerAdapter;
import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
public class MainActivity extends FragmentActivity implements
ActionBar.TabListener {
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
// Tab titles
private String[] tabs = { "Top Rated", "Games", "Movies" };
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Initilization
viewPager = (ViewPager) findViewById(R.id.pager);
actionBar = getActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
// Adding Tabs
for (String tab_name : tabs) {
actionBar.addTab(actionBar.newTab().setText(tab_name)
.setTabListener(this));
}
/**
* on swiping the viewpager make respective tab selected
* */
viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageSelected(int position) {
// on changing the page
// make respected tab selected
actionBar.setSelectedNavigationItem(position);
}
#Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
#Override
public void onPageScrollStateChanged(int arg0) {
}
});
}
#Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
}
#Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
// on tab selected
// show respected fragment view
viewPager.setCurrentItem(tab.getPosition());
}
#Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
}
}

In your addTab call, add the one you want set first followed by the rest.
So change
// Adding Tabs
for (String tab_name : tabs) {
actionBar.addTab(actionBar.newTab().setText(tab_name)
.setTabListener(this));
}
to
actionBar.addTab(actionBar.newTab().setText(tabs[1])
.setTabListener(this), 1, true);
actionBar.addTab(actionBar.newTab().setText(tabs[0])
.setTabListener(this), 0, false);
actionBar.addTab(actionBar.newTab().setText(tabs[2])
.setTabListener(this), 2, false);
See Android ActionBar tabs set initially selected tab

Android: method does not override or implement a method from a supertype

I am trying to build a simple three (3) tabs app. I am supposed to be implementing three public methods of ActionBar.TabListener.
I believe that is exactly what I am doing in the below code. The compiler however believes that I do not. All three come with the error: "method does not override or implement a method from supertype"
I checked methods' signatures, do not even know where to look, now.
Please advise.
import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.support.v4.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.view.ViewPager;
import android.support.v4.app.FragmentActivity;
public class InSync extends FragmentActivity implements ActionBar.TabListener {
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
// Tab titles
private String[] tabs = { "Backup", "Restore", "Settings"};
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_in_sync);
// Initilization
viewPager = (ViewPager) findViewById(R.id.pager);
actionBar = getActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
// Adding Tabs
for (String tab_name : tabs) {
actionBar.addTab(actionBar.newTab().setText(tab_name)
.setTabListener(this));
}
/**
* on swiping the viewpager make respective tab selected
* */
viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageSelected(int position) {
// on changing the page
// make respected tab selected
actionBar.setSelectedNavigationItem(position);
}
#Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
#Override
public void onPageScrollStateChanged(int arg0) {
}
});
}
#Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
// on tab selected
// show respected fragment view
viewPager.setCurrentItem(tab.getPosition());
}
#Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
}
#Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
}
}
Thank you.

I believe, you messed up with support libraries. Try
import android.app.FragmentTransaction;
instead of
import android.support.v4.app.FragmentTransaction;
Upd.
Or better use only support libraries:
import android.support.v4.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.view.ViewPager;
import android.support.v4.app.FragmentActivity;
import android.support.v7.app.ActionBar;
import android.support.v7.app.ActionBar.Tab;

The methods you try to implement are
void onTabReselected(ActionBar.Tab tab, FragmentTransaction ft)
void onTabSelected(ActionBar.Tab tab, FragmentTransaction ft)
and
void onTabUnselected(ActionBar.Tab tab, FragmentTransaction ft)
I guess the Tab class you are using is not the "right" Tab class.
EDIT:
On this link you can see that there is a certain android.app.FragmentTransaction. However, you have imported android.support.v4.app.FragmentTransaction. Isn't it possible that this is the problem, as the links to FragmentTransaction from here point to the page of android.app.FragmentTransaction. I believe you should have this code:
import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.view.ViewPager;
import android.support.v4.app.FragmentActivity;
public class InSync extends FragmentActivity implements ActionBar.TabListener {
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
// Tab titles
private String[] tabs = { "Backup", "Restore", "Settings"};
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_in_sync);
// Initilization
viewPager = (ViewPager) findViewById(R.id.pager);
actionBar = getActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
// Adding Tabs
for (String tab_name : tabs) {
actionBar.addTab(actionBar.newTab().setText(tab_name)
.setTabListener(this));
}
/**
* on swiping the viewpager make respective tab selected
* */
viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageSelected(int position) {
// on changing the page
// make respected tab selected
actionBar.setSelectedNavigationItem(position);
}
#Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
#Override
public void onPageScrollStateChanged(int arg0) {
}
});
}
#Override
public void onTabSelected(ActionBar.Tab tab, android.app.FragmentTransaction ft) {
// on tab selected
// show respected fragment view
viewPager.setCurrentItem(tab.getPosition());
}
#Override
public void onTabUnselected(ActionBar.Tab tab, android.app.FragmentTransaction ft) {
}
#Override
public void onTabReselected(ActionBar.Tab tab, android.app.FragmentTransaction ft) {
}
}
and if it works you can test even without namespaces.

Android ActionBar custom view and title change with tab nevigation.

I am trying to developing Tab Navigation app in Android. I have three tab. Every tab is swipable.
MainClass extends SherlockFragmentActivity from where tab are created. Every class is extends SherlockFragment.
If i swipe tab then i need to change ActionBar custom view and title . But i can not access getSupportActionBar()
method from SherlockFragment class. How can i do that.
Can anyone please help me.
Thanks in Advance

MainActivity.java
import info.androidhive.tabsswipe.adapter.TabsPagerAdapter;
import info.androidhive.tabsswipe.R;
import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.app.FragmentTransaction;
import android.os.Bundle;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import android.view.Menu;
public class MainActivity extends FragmentActivity implements
ActionBar.TabListener {
private ViewPager viewPager;
private TabsPagerAdapter mAdapter;
private ActionBar actionBar;
// Tab titles
private String[] tabs = { "Top Rated", "Games", "Movies" };
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// Initilization
viewPager = (ViewPager) findViewById(R.id.pager);
actionBar = getActionBar();
mAdapter = new TabsPagerAdapter(getSupportFragmentManager());
viewPager.setAdapter(mAdapter);
actionBar.setHomeButtonEnabled(false);
actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
// Adding Tabs
for (String tab_name : tabs) {
actionBar.addTab(actionBar.newTab().setText(tab_name)
.setTabListener(this));
}
/**
* on swiping the viewpager make respective tab selected
* */
viewPager.setOnPageChangeListener(new ViewPager.OnPageChangeListener() {
#Override
public void onPageSelected(int position) {
// on changing the page
// make respected tab selected
actionBar.setSelectedNavigationItem(position);
}
#Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
#Override
public void onPageScrollStateChanged(int arg0) {
}
});
}
#Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
}
#Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
// on tab selected
// show respected fragment view
viewPager.setCurrentItem(tab.getPosition());
}
#Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
}
}
Use the above code and try it with the following sample link.. This link has the source..
Source link click here: Android Tab Layout with Swipeable Views.