从mainactivity开始新片段

Question

I read a lot of questions here, but I didn't get a proper solution for my problem. I have a MainActivity java class, and I have several activities in for my app. Now, I created a "Tabbed activity" automatically in Android Studio. I do have a Imagebutton which I use for opening a new fragment. This new fragment is called "Generaldiscription". I learned, that a fragment can't be started like an activity, I need to use FragmentManagener to get a fragment open. My app starts, but when I click on the imagebutton, where actually my fragment should start, the app crashes.

In the following, you can see my mainactivity.java, and the fragments as java and as xml. Thank you in anticipation.

MainActivity.java

public class MainActivity extends AppCompatActivity
    implements NavigationView.OnNavigationItemSelectedListener {



@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    final ImageButton generaldiscriptionbutton = (ImageButton) findViewById(R.id.imageButton);
    setSupportActionBar(toolbar);
    checkFirstRun();



    generaldiscriptionbutton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            FragmentManager fm = getSupportFragmentManager();
            Generaldiscription fragment = new Generaldiscription();
            fm.beginTransaction().add(R.id.main_content, ); // don't know what to write here.



        }
    });

My activity_generaldiscription.xml which was created automatically

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout 
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:id="@+id/main_content"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:fitsSystemWindows="true"
 >


<android.support.design.widget.AppBarLayout
    android:id="@+id/appbar"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:paddingTop="@dimen/appbar_padding_top"
    android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:layout_scrollFlags="scroll|enterAlways"
        app:popupTheme="@style/AppTheme.PopupOverlay">

    </android.support.v7.widget.Toolbar>


</android.support.design.widget.AppBarLayout>

<android.support.v4.view.ViewPager
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior" />

My fragment_generaldiscription.xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="@dimen/activity_vertical_margin"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
tools:context="com.example.user.myapp.Generaldiscription$PlaceholderFragment">

<TextView
    android:id="@+id/section_label"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentLeft="true"
    android:layout_alignParentStart="true"
    android:text="fffff" />

EDIT my generaldiscription.java

 import android.support.design.widget.FloatingActionButton;
 import android.support.design.widget.Snackbar;
 import android.support.v7.app.AppCompatActivity;
 import android.support.v7.widget.Toolbar;

 import android.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.LayoutInflater;
  import android.view.Menu;
 import android.view.MenuItem;
 import android.view.View;
 import android.view.ViewGroup;

import android.widget.TextView;

public class Generaldiscription 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_generaldiscription);

    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);


    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    fab.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View view) {
            Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                    .setAction("Action", null).show();
        }
    });

}


@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_generaldiscription, 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);
}

/**
 * A placeholder fragment containing a simple view.
 */
public static class PlaceholderFragment extends Fragment {
    /**
     * The fragment argument representing the section number for this
     * fragment.
     */
    private static final String ARG_SECTION_NUMBER = "section_number";

    public PlaceholderFragment() {
    }

    /**
     * Returns a new instance of this fragment for the given section
     * number.
     */
    public static PlaceholderFragment newInstance(int sectionNumber) {
        PlaceholderFragment fragment = new PlaceholderFragment();
        Bundle args = new Bundle();
        args.putInt(ARG_SECTION_NUMBER, sectionNumber);
        fragment.setArguments(args);
        return fragment;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_generaldiscription, container, false);
        TextView textView = (TextView) rootView.findViewById(R.id.section_label);
        textView.setText(getString(R.string.section_format, getArguments().getInt(ARG_SECTION_NUMBER)));
        return rootView;
    }
}

/**
 * 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) {
        // getItem is called to instantiate the fragment for the given page.
        // Return a PlaceholderFragment (defined as a static inner class below).
        return PlaceholderFragment.newInstance(position + 1);
    }

    @Override
    public int getCount() {
        // Show 3 total pages.
        return 3;
    }

    @Override
    public CharSequence getPageTitle(int position) {
        switch (position) {
            case 0:
                return "SECTION 1";
            case 1:
                return "SECTION 2";
            case 2:
                return "SECTION 3";
        }
        return null;
    }
}

}

Answer 1

Start your fragment this way:

            FragmentManager fragmentmanager = getFragmentManager();
            fragmentmanager.beginTransaction()
                .replace(R.id.main_content
                        , new Generaldiscription())
                .commit();

EDIT

In your case you should start you Generaldiscription like this:

            Intent myIntent = new Intent(MainActivity.this, Generaldiscription.class);
            MainActivity.this.startActivity(myIntent);

What is more, your GeneralDiscription should import android.support.v4.app.Fragment; instead of android.app.Fragment;

Answer 2

From the Documentation

FragmentTransaction add (int containerViewId, 
                Fragment fragment, 
                String tag)

Add a fragment to the activity state. This fragment may optionally also have its view (if Fragment. onCreateView returns non-null) inserted into a container view of the activity.

Parameters

• containerViewId int : Optional identifier of the container this fragment is to be placed in. If 0, it will not be placed in a
  container.
• fragment Fragment : The fragment to be added. This fragment must not already be added to the activity.

• tag String : Optional tag name for the fragment, to later retrieve the fragment with

  FragmentManager.findFragmentByTag(String).

so in your code write:

FragmentManager manager = getSupportFragmentManager();
Generaldiscription fragment = new Generaldiscription();
FragmentTransaction transaction = manager.beginTransaction();
transaction.add(R.id.main_content,fragment,"Tag");
transaction.addToBackStack(null);
transaction.commit();

And in your Generaldiscription Fragment:

import android.support.v4.app.Fragment;  //make sure you import the support package

public class Generaldiscription extends Fragment {

     @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
        View view = inflater.inflate(R.layout.Your_layout, container, false);
        return view;
    }
}

Answer 3

Declare in your fragment like this

public interface OnClickListener {
    void onClick();
}
private final OnClickListener listener;

In your button click listener add this

if(listener!=null){
      listener.onclick();
}

In your Activity, implements this listener and add this to the fragment than you can hide current fragment and show second fragment in onClick method in Activity