Develop Reference

Android back button - how to preserve parent state when navigating to and back from next activity

Activity A has a list of categories (RecyclerView) in a fragment. When clicking on an item, a new Activity B is launched, which also contains a list of items (RecyclerView) in a fragment, via an adapter:
`
myViewHolder.itemView.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
Intent dynamicCableView = new Intent(view.getContext(), CableOverview.class);
dynamicItemView.putExtra("systemModel", systemModelList.get(position));
view.getContext().startActivity(dynamicItemView);
}`
Clicking on one of those, Activity C, containing data pertaining to that item, is launched, basically like this:
public void goToAnActivity(View view) {
Intent Intent = new Intent(this, AnActivity.class);
startActivity(Intent);
}
When returning from Activity C via the back button to its parent Activity B, the previous state is not stored, so the list of items is gone whereas in my understanding, it should be retrieved from the stack. Instead onDestroy of B (NOT only of C) is called. I do NOT want to preserve data from Activity C but go back to its parent which should maintain its state. It basically should remember which category was selected to display its subset of items. I am using API 25.
I tried: overriding onSaveInstanceState in the fragment of B or in the activity. Doing away with the fragments and putting the lists in the activity itself. However, savedInstanceState is always null when navigating back.
How do I ensure my list can be recreated?
EDIT: Manifest contains:
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity
android:name=".MainActivity"
android:label="#string/app_name"
android:theme="#style/AppTheme.Launcher">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>

I found the solution when realizing that the problem actually was OnDestroy always getting called when navigating back. Thanks to #gokhan, I knew it was related to the launch mode. According to this answer, Activity B requires this line in the manifest:
android:launchMode="singleTop"

Do i have to define all function in mainActivity.java

When I made an new activity .xml file in android studio, it created a corresponding .java file.
I set an onClick method call from new activity, but it still is looking for that method in mainActivity.java.
I want to define activities method in corrosopnding .java file and link .xml with .java

If I am understanding you correctly, you are trying to set an action onClick() for a button or some other view, however when you click it nothing happens. Is that right? It seems to me like your activity is tied to the wrong .xml file. Double check that in your .java files have the correct layout defined in setContentView();
For example...
In MainActivity.java:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main); // pay attention to this
Button button = (Button) findViewById(R.id.button); // or else it won't know where to find the button in the layout
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Intent intent = new Intent(MainActivity.this, Main2Activity.class);
tartActivity(intent);
}
});
}
In Main2Activity.java:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2); // pay attention to this too
}

Whenever you create a new activity on Android it will not come to life unless you tell it to by code. Now, if you look into your manifest file you will be able to see which one is the activity that is started with the program, the initial is the MainActivity.java file(It does not have to be named MainActivity but it is the default in Android)
<application
android:allowBackup="true"
android:icon="#mipmap/ic_launcher"
android:label="#string/app_name"
android:roundIcon="#mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="#style/AppTheme">
<activity android:name=".MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
</application>
If you look into your manifest file after you have added the second activity you will see a new <activity> tag with the name of the second activity that you added right before the end of the application tag:
<activity
android:name=".Main2Activity"
android:label="#string/title_activity_main2"
android:theme="#style/AppTheme">
</activity>
This means that your activities are properly registered on your manifest file(as they should)
In order for you to use it you would require something to call it, like a button click that activates an Intent just like Michael said above me. Try to go through the basic Android tutorial to see this happening and try and implement it yourself. Every view that you have in android corresponds to an activity file(for the most part, they can be fragments as well but that is a topic for another day) and most of the code corresponding the activity events will definitely require you to code your logic inside the activity file. Think of the java file as the controller, the xml layouts as the view, your controller file manipulates the view.

make an android app in which the launch icon is working as a link

I was wondering if it is possible to make an android app and when the user hits the launch icon, the browser opens with a specific URL. I know that you can make shortcuts on your phone home screen with the URL you want but I was wondering if it is possible as an android app. So the app should don almost nothing just when i tap on it the browser should open..
is this possible? and is yes how should I think it?

Do this in the main activity's onCreate:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse("https://stackoverflow.com"));
startActivity(intent);
finish();
}
in the manifest add noDisplay theme:
<activity
android:name=".MainActivity"
android:label="#string/app_name"
android:theme = "#android:style/Theme.NoDisplay">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>

Unable to find explicit activity class Android

Ugh, so many posts on StackOverflow similar to my problem but none fix it :/
I am currently learning android development. In my main activity I do:
public boolean onOptionsItemSelected(MenuItem item) {
super.onOptionsItemSelected(item);
switch(item.getItemId()){
case R.id.changeName:
Intent intent = new Intent(this, PreferenceActivity.class);
startActivityForResult(intent, 1);
return true;
}
return false;
}
Preference.java is a freshly created activity with nothing in it except the automatically created onCreate method. The error is:
android.content.ActivityNotFoundException: Unable to find explicit activity class {com.package.chat.oneway.client/android.preference.PreferenceActivity}; have you declared this activity in your AndroidManifest.xml?
My android manifest has:
<application
android:allowBackup="false"
android:icon="#drawable/ic_launcher"
android:label="#string/app_name">
<activity
android:name=".ChatClient"
android:label="#string/app_name" />
<activity android:name=".Parent">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity android:name=".PreferenceActivity"></activity>
</application>
As you can see, .PreferenceActivity is in the manifest. The packages for the ChatClient and PreferenceActivity activities are the same com.package.chat.oneway.client package.It's been driving me nuts for the past 4 hours.

As you can see, .PreferenceActivity is in the manifest.
Yes, it is. However, that is not what your Intent is using. If you look more closely at the error message, you will see that your Intent is using android.preference.PreferenceActivity.
This comes when you reuse existing class names. There are countless possible class names. Rename yours to something else, and you can avoid this sort of problem in the future.
Or, if you are completely set on having your class named PreferenceActivity, then remove the import android.preference.PreferenceActivity statement from the class where you are creating your Intent. Or, change your Intent to:
Intent intent = new Intent(this, com.package.chat.oneway.client.PreferenceActivity.class);

Launching Intent cannot find Activity

My program has a list of tabs that each contain a list of people. Clicking a person should edit their details, but instead, clicking a person throws an ActivityNotFoundException. My code that starts the edit activity:
Intent intent = new Intent("my.package.EDIT"); // Person is both
intent.putExtra("my.package.PERSON", (Parcelable) myPerson); // Parcelable and
GroupArrayAdapter.this.getContext().startActivity(intent); // Serializable
Relevant part of AndroidManifest.xml:
<activity android:name="EditActivity">
<intent-filter>
<action android:name="my.package.EDIT" />
</intent-filter>
</activity>
What's going on?

There's two ways to start an Activity, either explicitly or implicitly. Explicitly means that you use the class name directly; usually if you're calling an Activity that is within your own application, what you mean to use is the explicit method because it's far easier:
Intent intent = new Intent(context, EditActivity.class);
context.startActivity(intent);
However, if you really do mean to use implicit filtering, then your intent filter is missing something. Every implicit intent filter requires the category android.intent.category.DEFAULT (with few exceptions). Try this and see if it works:
<activity android:name="EditActivity">
<intent-filter>
<category android:name="android.intent.category.DEFAULT" />
<action android:name="my.package.EDIT" />
</intent-filter>
</activity>
Suggested reading: intent filters.

When you instantiate your Adapter, pass the context from the calling activity. Then in your adapter use context.startActivity(...);