Develop Reference

Android: Button not showing on view

I'm completely new to Android and am trying to add a simple button to my view, but my view is completely blank. Here is my setup:
EDIT: I have now changed my code to write this programmatically since I'm used to that with iOS.
I'm now getting the following error:
Error: Attempt to invoke virtual method 'void android.widget.LinearLayout.setOrientation(int)' on a null object reference
activity_main.xml
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="#+id/linearLayout1"
android:layout_width="wrap_content"
android:layout_height="wrap_content">
</LinearLayout>
MainActivity.java
public class MainActivity extends Activity {
LinearLayout linearLayout;
public Button myButton;
#Override
protected void onCreate(Bundle savedInstanceState) {
requestWindowFeature(Window.FEATURE_NO_TITLE);
super.onCreate(savedInstanceState);
//setContentView(R.layout.activity_main);
// Create main view
linearLayout = (LinearLayout) findViewById(R.id.linearLayout1);
linearLayout.setOrientation(LinearLayout.VERTICAL);
// Create button
myButton = new Button(this);
myButton.setText("Fire Call");
// Add button to view
linearLayout.addView(myButton);
}
}

You need to setContentView which takes a View or a layout resource id.
Also, you can only call findViewById if you provide a layout resource id, and call setContentView with it. Alternatively, you could inflate it and call view.findViewById on the resulting inflated view.
Here's your code with some changes that should hopefully fix your new issue:
public class MainActivity extends Activity {
LinearLayout linearLayout;
public Button myButton;
#Override
protected void onCreate(Bundle savedInstanceState) {
requestWindowFeature(Window.FEATURE_NO_TITLE);
super.onCreate(savedInstanceState);
// Create main view
linearLayout = new LinearLayout(this);
linearLayout.setOrientation(LinearLayout.VERTICAL);
// Create button
myButton = new Button(this);
myButton.setText("Fire Call");
// Add button to view
linearLayout.addView(myButton);
setContentView(linearLayout);
}
}

I cannot pinpoint the error but maybe you can.
Try checking after every line you type and see if the button appears. Also check if the problem is with button only or does it extend to other layouts as well.
Just type and see what happens.
Follow it up with dimensions.
Don not forget to save after each step. Usually all such problems can be solved by detailed debugging.
If none of the layout appears, try and restart the software.

You didn't callsetContentView(R.layout.activity_main) , in your code there, you commented it out, so your layout wasnt inflated so you got a blank white screen. Uncomment that line and you will be fine.

linking my java layout object with xml layout in android

I have created an LinearLayout object in my .java file and tried to connect it with my .xml layout by using the following code,but it is not responding. Can you please explain to me why and what errors are in this code? I have named the id of the LinearLayout of .xml file as "root".
public class MainActivity extends Activity {
LinearLayout l;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
ImageView HelloWorldImageView = new ImageView(this);
TextView t=new TextView(this);
t.setText("hello");
HelloWorldImageView.setImageResource(R.drawable.ic_launcher);
l=new LinearLayout(this);
l=(LinearLayout)findViewById(R.id.root);
l.addView(t);
l.addView(HelloWorldImageView);
setContentView(l);
}
}

Do it like this-
setContentView(R.layout.yourxml);
ImageView HelloWorldImageView = new ImageView(this);
TextView t=new TextView(this);
t.setText("hello");
HelloWorldImageView.setImageResource(R.drawable.ic_launcher);
l=(LinearLayout)findViewById(R.id.root);
l.addView(t);
l.addView(HelloWorldImageView);
You have to set that xml file in the setContentView() which has the root linear layout in it.

First off- you don't need the new LinearLayout line. The findViewById() will return an instantiated version. Same for the the other newed Views.
Secondly- you want to do a setContentView() before making any calls to findViewById(). This way you set the layout (by passing it a layout id), Android will instantiate all the Views in your layout, and you can get references to them via findViewById().

findviewbyid another activity different ContentView in Android

this is MainActivity
//class declare
public class MainActivity extends Activity {
// a lot of code
}
//OnCreate Set
setContentView(R.layout.activity_main);
and Inside having a TableLayout with ID R.id.widget101, TableLayout having a dynamic created LinearLayout.
here is the second activity code,trying to get the MainActivity TableLayout and get all the inside LinearLayout child
//class declare
public class SecondActivity extends Activity {
// a lot of code
}
//OnCreate Set
setContentView(R.layout.second);
public void EnableButton(boolean bool){
View view = LayoutInflater.from(getApplication()).inflate(R.layout.activity_main, null);
ViewGroup layout = (ViewGroup) view.findViewById(R.id.widget101);
if (layout != null){
Log.d(null,"EnableButton getchild="+layout.getChildCount()+"");
Log.d(null,"EnableButton Function");
}
}
Problem Occurs here, When the Second Aciticity calling the MainActivity to get the TableLayout Child are return 0
===============================================================
this is how i call the SecondActivity
public class MainActivity extends Activity
implements OnClickListener {
//overrides the OnClickListener interface method
#Override
public void onClick(View arg0) {
if(arg0.getId() == R.id.btnWEmas){
//define a new Intent for the second Activity
Intent intent = new Intent(this,SecondActivity.class);
intent.putExtra("From_LocationName",global.From_LocationName);
intent.putExtra("To_LocationName",global.To_LocationName);
TextView textview = (TextView) findViewById(R.id.lblPrice);
intent.putExtra("Price",textview.getText());
//start the second Activity
this.startActivity(intent);
}
// still have a lot of code

put this in your first activity
static ViewGroup layout ; //define it globally
layout = (ViewGroup) view.findViewById(R.id.widget101); //put it inside the oncreate of main activity
and use this object to second activity
public void EnableButton(boolean bool){
View view = LayoutInflater.from(getApplication()).inflate(R.layout.activity_main, null);
if (MainActivity.layout != null){
Log.d(null,"EnableButton getchild="+MainActivity.layout.getChildCount()+"");
Log.d(null,"EnableButton Function");
}
}

You should follow the following scenario :
create a Custom class file, which contains the Table layout and its children elements. Create a default Constructor with a context parameter. Please follow below link for more clearity.
https://stackoverflow.com/a/20367698/1944782
Here I have created a custom Toast, You can customize your Table Layout according that.
One more thing, please ignore the use of static variable as much as possible. It leads to memory leak and also crashes the app.

findViewById in specific layout?

Instead of prefixing id's in xml, is it possible to specify the particular layout in code? For example, if I have 3 layouts, each with a button that has an id of "btn". Is it possible to specify which layout to findViewById(R.id.btn) in?

The basic context is defined via setContentView(R.lyaout.my_layout). If you inflate another layout using LayoutInflater.inflate() you get a layout object, lets call it buttonLayout. You can now differ between this.findViewById(R.id.button) and buttonLayout.findViewById(R.id.button) and both will give you different button references.

findViewById is a method of the View class. You can specify where the view should be searched for like that
final View container = new View(context);
container.findViewById(R.id.btn);

If your content view is a complex hierarchy that has several views with id btn, you will need to navigate to a subtree of the hierarchy and search from there. Suppose you have three LinearLayout views, each with a btn view somewhere in it. If you can first select the correct LinearLayout (by id, tag, position, or some other means), you can then find the correct btn view. If the relevant LinearLayout has id of branch1, for instance:
View parent = findViewById(R.id.branch1); // Activity method
View btn = parent.findViewById(R.id.btn); // View method

if you have your btns inside different Viewgroups, it is possible, but requires to give ViewGroups a different name! Easyiest will be for that purpose to define the Layout of the Button inside its own XML (i.e button_layout.xml)
inside your Activity you can do this:
public MyActivity extends Activity{
Button btn1, btn2, btn3;
public void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
LayoutInflater inflater = (LayoutInflater)getSystemService(LAYOUT_INFLATER_SERVICE);
LinearLayout ll = new LinearLayout(this);
setContentView(ll);
btn1 = (Button)inflater.inflate(R.layout.button_layout, ll);
btn2 = (Button)inflater.inflate(R.layout.button_layout, ll);
btn3 = (Button)inflater.inflate(R.layout.button_layout, ll);
}
}

Search Textview which is created programmatic

I'm adding TextViews programmatic using a for-loop and use setId(Int int) to set unique id for that perticular Textview.
But now I want to search textView on the basis of that id.
How can I search?
erorr occurd 'app stopped unexpectedly...' Here is my code.
public class Idtest extends Activity {
TextView tv;
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
LinearLayout ll=new LinearLayout(this);
tv=new TextView(this);
tv.setText("Hello");
ll.addView(tv);
tv=new TextView(this);
tv=(TextView)ll.getChildAt(1);
setContentView(tv);
}
}

tv is obviously null at the end (the LinearLayout only has one child, which is at index 0), so you're basically calling setContentView(null) which results in an exception. It's not clear for me what you're trying to do (your code is pretty messed up).
Supposing you are trying to show multiple TextViews in a LinearLayout, here's my suggestion:
public class Idtest extends Activity {
LinearLayout mainLayout;
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mainLayout = new LinearLayout(this);
setContentView(mainLayout);
for (int i=0; i < 10; i++) {
TextView tv = new TextView(this);
tv.setText("Hello " + i);
mainLayout.addView(tv);
}
}
}
If, at any later point, you need one of the TextViews, do:
TextView tvX = mainLayout.getChildAt(X); // where X is between 0 and 9
Also, please note that creating layout from code is evil. If you can avoid it, please do. For example, if the number of TextViews is dynamic, then it's perfectly normal to create them from code (although you could inflate them). However, it's not advisable to also create the LinearLayout from code. You should have that in an XML. If it's possible for you to also have the TextViews in an XML, that would be even better.

There is another option too...we have setTag() and getTag() methods. While adding a textview give a tag for it and whenever u want to do a search use getTag().

I suppose you are adding your TextView controls to some ViewGroup (e.g. LInearLayout)?
You can iterate through the ViewGroup child views using getChildCount() / getChildAt(index) and compare the child view id with the one you're searching for.