Can't instantiate class.Tabs; no empty constructor, actionBar.Tab's in Android

Question 1

Since you have a paramterized Tabs constructor, the default constructor is not available to the system to create an instance of it. To overcome this issue, you need to explicitly define a default constructor.

But there is another issue with the current implementation:

ActionBar.Tab tab = actionBar.newTab().setText(R.string.LabelSearchTabTitle).setTabListener((ActionBar.TabListener)new Tabs(this, LandingSearch.class, TAG ));

The current approach results in creating a instance of Activity for each tab added. Instead current instance i.e. this need to be passed to setTabListener, which is not possible.

Solution : The cleaner approach would be instead of Tabs implementing ActionBar.TabListener, define a inner class in Tabs which will implement ActionBar.TabListener and explicitly set an instance of it on each tab that will be created.

Here is the code:

import android.app.ActionBar.Tab;
import android.app.ActionBar;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.app.FragmentTransaction;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Toast;


public class MainActivity extends FragmentActivity {

    protected static final String TAG = MainActivity.class.toString();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        final ActionBar bar = getActionBar();
        bar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);
        bar.setDisplayOptions(0, ActionBar.DISPLAY_SHOW_TITLE);

        bar.addTab(bar.newTab()
                .setText("Search")
                .setTabListener(new TabListener<LandingSearch>(
                        this, "search", LandingSearch.class)));

        bar.addTab(bar.newTab()
                .setText("Book")
                .setTabListener(new TabListener<LandingBook>(
                        this, "book", LandingBook.class)));
    }


    public static class TabListener<T extends Fragment> implements ActionBar.TabListener {
        private final FragmentActivity mActivity;
        private final String mTag;
        private final Class<T> mClass;
        private final Bundle mArgs;
        private Fragment mFragment;
        public TabListener(FragmentActivity activity, String tag, Class<T> clz) {
            this(activity, tag, clz, null);
        }


        public TabListener(FragmentActivity activity, String tag, Class<T> clz, Bundle args) {
            mActivity = activity;
            mTag = tag;
            mClass = clz;
            mArgs = args;
            // Check to see if we already have a fragment for this tab, probably
            // from a previously saved state.  If so, deactivate it, because our
            // initial state is that a tab isn't shown.
            mFragment = mActivity.getSupportFragmentManager().findFragmentByTag(mTag);
            if (mFragment != null && !mFragment.isDetached()) {
                FragmentTransaction ft = mActivity.getSupportFragmentManager().beginTransaction();
                ft.detach(mFragment);
                ft.commit();
            }
        }

        public void onTabSelected(Tab tab, android.app.FragmentTransaction ft) {
            // Since ActionBar.TabListener needs android.app.FragmentTransaction, define the method signature with it, but don't use it
            // Instead use the support fragment manager
            FragmentTransaction fft = mActivity.getSupportFragmentManager().beginTransaction();

            if (mFragment == null) {
                mFragment = Fragment.instantiate(mActivity, mClass.getName(), mArgs);
                fft.add(android.R.id.content, mFragment, mTag);
            } else {
                fft.attach(mFragment);
            }

            // Don't forget to call commit, as we are not using FragmentTransaction passed by ActionBar.
            fft.commit();
        }

        public void onTabUnselected(Tab tab, android.app.FragmentTransaction ft) {
            FragmentTransaction fft = mActivity.getSupportFragmentManager().beginTransaction();

            if (mFragment != null) {
                fft.detach(mFragment);
            }

            fft.commit();
        }

        public void onTabReselected(Tab tab, android.app.FragmentTransaction ft) {
            Toast.makeText(mActivity, "Reselected!", Toast.LENGTH_SHORT).show();
            }
        }

    public static class LandingSearch extends Fragment
    {
        protected static final String TAG = LandingSearch.class.toString();

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            View v = inflater.inflate(R.layout.landing_search, container, false);
            setHasOptionsMenu(true);

            // Other code.

            return v;
        }
    }

    public static class LandingBook extends Fragment
    {
        protected static final String TAG = LandingBook.class.toString();

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
            View v = inflater.inflate(R.layout.landing_book, container, false);
            setHasOptionsMenu(true);

            // Other code

            return v;
        }
    }

}

Look at this Android example.

Question 2

I cannot see this specific problem in your code but if you created the actionbar using the Eclipse wizard your problem may be solved by this: How can we retain a simple State Array across Configuration changes using Headless Fragments?