在我的Android应用程序中集成PagerSlidingTabStrip时出错

Question

I am trying to integrate this PagerSlidingTabStrip ( https://github.com/astuetz/PagerSlidingTabStrip ) into my Android application, but I get the following error:

at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
   at android.support.v7.app.AppCompatDelegateImplBase.<init>(AppCompatDelegateImplBase.java:67)
   at android.support.v7.app.AppCompatDelegateImplV7.<init>(AppCompatDelegateImplV7.java:144)
   at android.support.v7.app.AppCompatDelegateImplV11.<init>(AppCompatDelegateImplV11.java:28)
   at android.support.v7.app.AppCompatDelegateImplV14.<init>(AppCompatDelegateImplV14.java:29)
   at android.support.v7.app.AppCompatDelegateImplV23.<init>(AppCompatDelegateImplV23.java:26)
   at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:133)
   at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:117)
   at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:456)
   at android.support.v7.app.AppCompatActivity.getSupportActionBar(AppCompatActivity.java:79)
   at com.sch.x.Welcome.<init>(Welcome.java:40)

Here is my code from the Welcome.java file:

package com.sch.x;

import com.astuetz.PagerSlidingTabStrip;
import com.parse.ParseUser;

import android.app.Activity;
import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.graphics.drawable.Drawable;
import android.graphics.drawable.LayerDrawable;
import android.graphics.drawable.TransitionDrawable;
import android.os.Build;
import android.os.Bundle;
import android.os.Handler;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.support.v7.app.ActionBar;
import android.support.v7.app.ActionBarActivity;
import android.util.TypedValue;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class Welcome extends ActionBarActivity {

    private final Handler handler = new Handler();

    private PagerSlidingTabStrip tabs;


    private Drawable oldBackground = null;
    private int currentColor = 0xFF666666;

    private ActionBar mActionBar = getSupportActionBar();


    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        // Get the view from singleitemview.xml
        setContentView(R.layout.welcome);

        final ViewPager pager;
        final MyPagerAdapter adapter;

        tabs = (PagerSlidingTabStrip) findViewById(R.id.tabs);
        pager = (ViewPager) findViewById(R.id.pager);
        adapter = new MyPagerAdapter(getSupportFragmentManager());

        pager.setAdapter(adapter);

        final int pageMargin = (int) TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP, 4, getResources()
                .getDisplayMetrics());
        pager.setPageMargin(pageMargin);

        tabs.setViewPager(pager);

        changeColor(currentColor);
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.menu_welcome, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {

        switch (item.getItemId()) {

            case R.id.action_contact:
                QuickContactFragment dialog = new QuickContactFragment();
                dialog.show(getSupportFragmentManager(), "QuickContactFragment");
                return true;

        }

        return super.onOptionsItemSelected(item);
    }

    private void changeColor(int newColor) {

        tabs.setIndicatorColor(newColor);

        // change ActionBar color just if an ActionBar is available
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB) {

            Drawable colorDrawable = new ColorDrawable(newColor);
            Drawable bottomDrawable = getResources().getDrawable(R.drawable.actionbar_bottom);
            LayerDrawable ld = new LayerDrawable(new Drawable[] { colorDrawable, bottomDrawable });

            if (oldBackground == null) {

                if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR1) {
                    ld.setCallback(drawableCallback);
                } else {
                    mActionBar.setBackgroundDrawable(ld);
                }

            } else {

                TransitionDrawable td = new TransitionDrawable(new Drawable[] { oldBackground, ld });

                // workaround for broken ActionBarContainer drawable handling on
                // pre-API 17 builds
                // https://github.com/android/platform_frameworks_base/commit/a7cc06d82e45918c37429a59b14545c6a57db4e4
                if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR1) {
                    td.setCallback(drawableCallback);
                } else {
                    mActionBar.setBackgroundDrawable(td);
                }

                td.startTransition(200);

            }

            oldBackground = ld;

            // http://stackoverflow.com/questions/11002691/actionbar-setbackgrounddrawable-nulling-background-from-thread-handler
            mActionBar.setDisplayShowTitleEnabled(false);
            mActionBar.setDisplayShowTitleEnabled(true);

        }

        currentColor = newColor;

    }

    public void onColorClicked(View v) {

        int color = Color.parseColor(v.getTag().toString());
        changeColor(color);

    }

    @Override
    protected void onSaveInstanceState(Bundle outState) {
        super.onSaveInstanceState(outState);
        outState.putInt("currentColor", currentColor);
    }

    @Override
    protected void onRestoreInstanceState(Bundle savedInstanceState) {
        super.onRestoreInstanceState(savedInstanceState);
        currentColor = savedInstanceState.getInt("currentColor");
        changeColor(currentColor);
    }

    private Drawable.Callback drawableCallback = new Drawable.Callback() {
        @Override
        public void invalidateDrawable(Drawable who) {
            mActionBar.setBackgroundDrawable(who);
        }

        @Override
        public void scheduleDrawable(Drawable who, Runnable what, long when) {
            handler.postAtTime(what, when);
        }

        @Override
        public void unscheduleDrawable(Drawable who, Runnable what) {
            handler.removeCallbacks(what);
        }
    };

    public class MyPagerAdapter extends FragmentPagerAdapter {

        private final String[] TITLES = { "Categories", "Home", "Top Paid", "Top Free", "Top Grossing", "Top New Paid",
                "Top New Free", "Trending" };

        public MyPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public CharSequence getPageTitle(int position) {
            return TITLES[position];
        }

        @Override
        public int getCount() {
            return TITLES.length;
        }

        @Override
        public Fragment getItem(int position) {
            return SuperAwesomeCardFragment.newInstance(position);
        }

    }

}

Answer 1

Isn't the issue related with this var initialiation:

private ActionBar mActionBar = getSupportActionBar();

Have you tried assigning the var in onCreate instead (preferably after the setContentView call)?