Working the android hello world, I next added some strings to the strings.xml resource file. I then tried setting a member variable of my main activity class to the value of one of the strings:
public class MyActivity extends Activity {
/** Called when the activity is first created. */
public String myString = getString(R.string.MY_STRING); // compiles, but crashes
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
TextView tv = new TextView(this);
tv.setText(myString);
setContentView(tv);
}
}
When it crashes, I get this in the logcat:
Unable to instantiate activity ComponentInfo{com.myclass}: java.lang.NullPointerException
So, am I doing it wrong, or is this expected behavior? Looking over the documentation,
I don't see anything to lead me to think that the resources wouldn't be available while the main activity is being constructed.
http://developer.android.com/guide/topics/resources/accessing-resources.html
However, I am pretty sure this will work in other classes--just not the main activity class.
I don't think you can call getString on the Activity class as it may not have been properly initialised yet. You may need to split the declaration (keep it as a public String) and then the assignment (move that to onCreate).
public class MyActivity extends Activity {
/** Called when the activity is first created. */
public String myString;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
myString = getString(R.string.MY_STRING); // compiles, but crashes
TextView tv = new TextView(this);
tv.setText(myString);
setContentView(tv);
}
}
Related
I have my code defined the way below. There are two crucial activities. Activity (1) shows some images in a ViewFlipper. It uses methods to load desired image directly. The onOptionsItemSelected() method fetches data from a menu defined within linked XML layout R.layout.browse. The other method, displaySelectedFlag(), gets a tag parameter passed from a different activity, let's call it activity (2).
Activity (1):
public class BrowserActivity extends AppCompatActivity implements SimpleGestureListener, View.OnClickListener {
public ViewFlipper vFlipper;
(...)
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.browse);
vFlipper = (ViewFlipper) findViewById(R.id.viewFlipperBrowser);
(...)
} // onCreate() ends here
// this method below works fine:
#Override
public boolean onOptionsItemSelected(MenuItem item) {
vFlipper.setDisplayedChild(item.getOrder());
return true;
}
// and this one doesn't:
public void displaySelectedFlag(int orderTag) {
vFlipper.setDisplayedChild(orderTag); // crashes here
}
}
Activity (2):
public class ListActivity extends Activity implements View.OnClickListener {
private BrowserActivity browserActivity = new BrowserActivity();
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_list);
ImageButton imageA = (ImageButton) findViewById(R.id.img_a);
imageA.setOnClickListener(this);
ImageButton imageB = (ImageButton) findViewById(R.id.img_b);
imageB.setOnClickListener(this);
}
public void displayImageInfo(View view) {
String tagValue = (String) view.getTag();
int tagId = Integer.parseInt(tagValue);
Intent intent = new Intent(this, BrowserActivity.class);
startActivity(intent);
browserActivity.displaySelectedImage(imageId);
}
#Override
public void onClick(View view) {
displayImageInfo(view);
}
}
As I checked, the method onClick() called in activity (2) fetches an ID of an ImageButton and passes it to activity (1). Unfortunately, I get a NullPointerException when calling the ViewFlipper (the line is marked in the code above, activity (1)).
Any idea why it happens?
You cannot reference one Activity from another activity. You must let the Android OS create the Activity object via the call to "startActivity". Allocating a local variable as an instance of an Activity doesn't actually mean anything (like your instantiation of the BrowserActivity). Apoorv's comment links to a decent article on the subject.
If you want to pass data from one Activity to another, you need to pass extras within the Intent's bundle. This post goes into detail: https://stackoverflow.com/a/819427/504252
I was trying out coding android layout on Java. Then, I noticed that following code is incorrect (as in not working):
public class MainActivity extends Activity {
LinearLayout topLayout = new LinearLayout(this);
Button button1 = new Button(this);
protected void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
... setText, layoutParam and etc ...
topLayout.addView(button1, layoutParam);
setContentView(topLayout);
}
}
while following code is correct (working):
public class MainActivity extends Activity {
protected void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
LinearLayout topLayout = new LinearLayout(this);
Button button = new Button(this);
... setText, layoutParm and etc ...
topLayout.addView(button1, layoutParam);
setContentView(topLayout);
}
}
Why is that?
The context of an Activity is created once onCreate() is called.
In the first case, you don't have context set.
The comment made by #Blundell explains it, but if you want to go a little bit deeper, you need to understand the object lifecycle and order of things.
An Activity is a Java Object in the end. And so it must go through the Java Object creation process.
public class MainActivity extends Activity {
LinearLayout topLayout = new LinearLayout(this);
Button button1 = new Button(this);
Member variables (or Fields) are created right after the constructor. If you don't have a constructor, one is created for you (even tho you don't see it) you can try to create one:
public class MainActivity extends Activity {
public MainActivity() {
super();
Log.d("MAINACTIVITY", "Constructor!");
}
Now add log to your onCreate()…
protected void onCreate(Bundle savedInstanceState){
super.onCreate(savedInstanceState);
Log.d("MAINACTIVITY", "onCreate!");
}
Start your Activity and look at the log.
You should see:
Constructor!
onCreate!
With that being said, Since an Activity implements a Context interface, you cannot use them until they are not constructed, something that can be considered done after the constructor is called and ends.
Field members which are initialized inline, must be initialized during object Construction too, because that's the idea of their initialization being there…
So in short, you're trying to initialize Field Members that require something (a context) that is not yet available because it hasn't been created (this).
;)
I do my android app , but I have a bug I don't know how to fix it.
My code is below :
This is my Main Activity :
public class MainActivity extends Activity {
private ImageView imgHot;
public final static String EXTRA_MESSAGE="com.cuonglm.KhoHinh.MESSAGE";
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
imgHot=(ImageView)findViewById(R.id.imageViewHot);
imgHot.setOnClickListener(toContentHot);
}
View.OnClickListener toContentHot=new View.OnClickListener() {
#Override
public void onClick(View v) {
// TODO Auto-generated method stub
Intent content=new Intent(MainActivity.this,ContentActivity.class);
String signal="1";
content.putExtra(EXTRA_MESSAGE,signal);
startActivity(content);
}
};
And this is my second Activity :
public class ContentActivity extends Activity {
private TextView viewMessage;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Intent content=getIntent();
String messageReceive=content.getStringExtra(MainActivity.EXTRA_MESSAGE);
viewMessage=(TextView)findViewById(R.id.content_receive);
viewMessage.setText(messageReceive);
setContentView(R.layout.activity_content);
}
I want to click on the image on the Main Activity , string "1" or number "1" will send to the Second Activity via Intent and view on the TextView.
But my app will be close "Unfortunately..."
Thanks
Change to
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_content);
viewMessage=(TextView)findViewById(R.id.content_receive);
viewMessage.setText(messageReceive);
In your ContentActivity
findViewById looks for a view with the id for the current infalted layout. SO you need to set the content of your layout to the activity first and then initialize your views.
You are probably gettting NullPointerException coz your initialization fails.
You need to call setContentView() in your second Activity before trying to access any of the Views in that layout. Change it to
public class ContentActivity extends Activity {
private TextView viewMessage;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_content);
Intent content=getIntent();
String messageReceive=content.getStringExtra(MainActivity.EXTRA_MESSAGE);
viewMessage=(TextView)findViewById(R.id.content_receive);
viewMessage.setText(messageReceive);
}
If this doesn't fix your problem then please post your logcat so we can see the error. Also always post logcat in the future when your app crashes. They aren't always this easy to see.
Also, I'm not sure you understand how putExtra() works. It is a key, value pair so when you put EXTRA_MESSAGE as the key then that is what you would use to retrieve the value added in the second param. So the way you are doing it may work if the Activity gets destroyed but it looks really strange to me and probably not realy safe or efficient. I would change it to something like
content.putExtra("message",signal);
in your first Activity then get it with
String messageReceive = content.getStringExtra("message");
in your second Activity
You need to set the layout before trying to reference the Views associated with it.
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Intent content=getIntent();
String messageReceive=content.getStringExtra(MainActivity.EXTRA_MESSAGE);
setContentView(R.layout.activity_content);
viewMessage=(TextView)findViewById(R.id.content_receive);
viewMessage.setText(messageReceive);
}
Ok I am a complete newb when it comes to java classes. I have a public method that dynamically displays some Linearlayouts with some stuff in them. For instance this method (public void methodA)is in ClassA.java, then I want to call methodA from inside ClassB.java. Both of the classes extend Activity and the methodA is being called in the OnCreate method.
ClassA.java
public class ClassA extends Activity{
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
methodA();
}
public void methodA() {
//Do Stuff
/* This uses:
* Package Manager
* Buttons using(this)
* Linear Layouts using(this)
* TextViews using(this)
* findViewById()
* startActivity
*/
}
}
ClassB.java
public class ClassB extends Activity{
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
methodA(); //How do I do this
}
}
It is clear to me that the method is specific to the first activity, which means that it shouldn't be called in the second one. You can either reimplement the method in the second activity or, if these activities are similar (don't do this if they aren't!), you have two options:
Inherit the second activity from the first one.
Merge these two activities into one and use different intents to launch them and act accordingly.
The second method is easier to maintain, so I would prefer it over the first one in simpler cases.
Craete instance of class A in class B and then you can invoke MethodA from class B
public class ClassB extends Activity{
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
//MethodA(); //How do I do this
ClassA a=new ClassA ();
a.MethodA();
}
}
In my Android application I have to use common string value for all activities. "commonValue" is the common string value that I want to use in all activities. Relevant code of the main activity like this :
public class TestActivity extends Activity {
public String commonValue;//THE COMMON STRING FOR ALL ACTIVITIES
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
commonValue = "DemoValue";
}
}
In my next activity I created an object of "TestActivity" class and tried to assign "testValue" string to another string named "str"
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.testlist);
TestActivity obj = new TestActivity();//OBJECT OF MAIN ACTIVITY
String str = obj.commonValue;
}
but the "str" value in second activity does not equal to the value assigned in my first activity. Why is that & How can I do this?
Thanks!
Put your value in string.xml
<string name="common_value">DemoValue</string>
and use in any activity like this..
String common_value = getApplicationContext().getString(R.string.common_value);
Start using SharedPreferences in your app.
In your first activity you would do
SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0);
SharedPreferences.Editor editor = settings.edit();
editor.putString("commonValue", "DemoValue");
editor.commit();
In your second activity
SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0);
String str = settings.getString("commonValue", null);
Try this -
TestActivity.java
public class TestActivity extends Activity {
public static String commonValue;//THE COMMON STRING FOR ALL ACTIVITIES
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
commonValue = "DemoValue";
}
}
another activity
/** Called when the activity is first created. */
#Override
public void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.testlist);
String str = TestActivity.commonValue;
}
If the value is always the same you can create a public static final variable and access it via TestActivity.COMMON_VALUE.
If you want to pass around a value between to Activities you should use Intents and add an extra with the value you want to pass.
As Sana has suggested, use SharedPreferences.
Alternatively, use a global constant class. If you want to stick with what you have, then you could try:
String str = TestActivity.this.commonValue;
Your existing code is creating a new instance of the activity, so it's not going to have the value you had set.
To pass data between activities use Bundle. and methods,
intent.putExtra()
and If you want to set data to be global to your app, then create an application class, and save the data there.
We have an Application file for each app you can declare the variable there and as the Application file can get from any activity so using the public getter setter and can get/set that
there are vaious oter metjod you can sue as mention on developer.android http://developer.android.com/resources/faq/framework.html
Singleton class
A public static field/method
A HashMap of WeakReferences to Objects (almost same as my above solution )
Persistent Objects
take a look on them as well
The reason why commonValue doesn't equal what you set in TestActivity onCreate method is because that function hasn't been called yet.
The solution for this is already mentioned by others. Like putting the value in a bundle.