Question

I know that I cannot pass parameters to the Activity constructor in android, but I would like to understand why.

What I tried to do is:

CalorieSelectorActivity csa = new CalorieSelectorActivity(userName);
                Intent i = new Intent(thisContext, csa.getClass());
                startActivity(i);

I have defined the class as follows:

public class CalorieSelectorActivity extends Activity {

public CalorieSelectorActivity(String name) {
    super();

}

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.calorieselector);

}
}

And it works perfectly when I remove the parameters from the constructor.
I finally did it using intent.putExtra, but am very curious to know why it cannot be done by passing parameters to the constructor?

LogCat:

02-04 06:46:52.257: W/dalvikvm(800): threadid=1: thread exiting with uncaught exception (group=0x4001d800)
02-04 06:46:52.277: E/AndroidRuntime(800): FATAL EXCEPTION: main
02-04 06:46:52.277: E/AndroidRuntime(800): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.manarbushnaq.calorietracker/com.manarbushnaq.calorietracker.CalorieSelectorActivity}: java.lang.InstantiationException: com.manarbushnaq.calorietracker.CalorieSelectorActivity
02-04 06:46:52.277: E/AndroidRuntime(800):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
02-04 06:46:52.277: E/AndroidRuntime(800):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
02-04 06:46:52.277: E/AndroidRuntime(800):  at android.app.ActivityThread.access$2300(ActivityThread.java:125)
02-04 06:46:52.277: E/AndroidRuntime(800):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
02-04 06:46:52.277: E/AndroidRuntime(800):  at android.os.Handler.dispatchMessage(Handler.java:99)
02-04 06:46:52.277: E/AndroidRuntime(800):  at android.os.Looper.loop(Looper.java:123)
02-04 06:46:52.277: E/AndroidRuntime(800):  at android.app.ActivityThread.main(ActivityThread.java:4627)
02-04 06:46:52.277: E/AndroidRuntime(800):  at java.lang.reflect.Method.invokeNative(Native Method)
02-04 06:46:52.277: E/AndroidRuntime(800):  at java.lang.reflect.Method.invoke(Method.java:521)
02-04 06:46:52.277: E/AndroidRuntime(800):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
02-04 06:46:52.277: E/AndroidRuntime(800):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
02-04 06:46:52.277: E/AndroidRuntime(800):  at dalvik.system.NativeStart.main(Native Method)
02-04 06:46:52.277: E/AndroidRuntime(800): Caused by: java.lang.InstantiationException: com.manarbushnaq.calorietracker.CalorieSelectorActivity
02-04 06:46:52.277: E/AndroidRuntime(800):  at java.lang.Class.newInstanceImpl(Native Method)
02-04 06:46:52.277: E/AndroidRuntime(800):  at java.lang.Class.newInstance(Class.java:1429)
02-04 06:46:52.277: E/AndroidRuntime(800):  at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
02-04 06:46:52.277: E/AndroidRuntime(800):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577)
02-04 06:46:52.277: E/AndroidRuntime(800):  ... 11 more

Answer 1

Refer to your code:

CalorieSelectorActivity csa = new CalorieSelectorActivity(userName);

Intent i = new Intent(thisContext, csa.getClass());

startActivity(i);

Even if you create an object of your activity, what you are "passing" in the Intent object is not the Activity object but just the class of your activity. In startActivity() the Android framework will try to instantiate an object of your activity. And it calls the default constructor (without parameters) when it does that. It fails when your class does not have a constructor without parameters.

Of course, you have found the correct solution, pass the parameters as part of Intent object.