How to call a method in MainActivity from another class?

Question

How to call the startChronometer method in another class when the method is declared inside the main action?

Inside MainActivity :

 public void startChronometer() { mChronometer.start(); showElapsedTime(); } 

Inside another class, I tried to do this:

 MainActivity mActivity; mActivity.startChronometer(); 

But an error occurred that said:

 java.lang.NullPointerException. 

Can I find out what else I need to add to the code?

Answer 1

You can easily call a method from any fragment in your activity by doing a cast like this:

Java

 ((MainActivity)getActivity()).startChronometer(); 

Kotlin

 (activity as MainActivity).startChronometer() 

Just remember to make sure that this fragment action is actually MainActivity before you do it.

Hope this helps!

Answer 2

I would suggest that you should not create an object of a class of type Activity.

 MainActivity mActivity = new MainActivity(); // BIG NO TO THIS. 

All actions in Android must go through the Activity lifecycle so that they have a valid context attached to them.

Considering Activity as a regular Java class, you get a null context. Since most methods in an Activity are called in its Context, you will get a null pointer exception, so your application crashes.

Instead, move all such methods that need to be called from other classes to the Utility class, which takes a valid context in its constructor, and then use that context in the methods to do the work.

Answer 3

MainActivity.java

 public class MainActivity extends AppCompatActivity { private static MainActivity instance; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.activity_main); instance = this; } public static MainActivity getInstance() { return instance; } public void myMethod() { // do something... } ) 

AnotherClass.java

 public Class AnotherClass() { // call this method MainActivity.getInstance().myMethod(); } 

Answer 4

Initialize first

 MainActivity mActivity= new MainActivity(); 

Then you can continue

 mActivity.startChronometer(); 

Answer 5

But an error occurred that says the java.lang.NullPointerException error.

This is because you never initialized your MainActivity. you must initialize your object before you call its methods.

 MainActivity mActivity = new MainActivity();//make sure that you pass the appropriate arguments if you have an args constructor mActivity.startChronometer(); 

Answer 6

Use this code in the MainActivity to invoke the method on it.

 ((MainActivity) getActivity()).startChronometer(); 

Answer 7

You must pass the instance of MainActivity to another class, then you can call everything public (in MainActivity) everywhere.

MainActivity.java

 public class MainActivity extends AppCompatActivity { // Instance of AnotherClass for future use private AnotherClass anotherClass; @Override protected void onCreate(Bundle savedInstanceState) { // Create new instance of AnotherClass and // pass instance of MainActivity by "this" anotherClass = new AnotherClass(this); } // Method you want to call from another class public void startChronometer(){ ... } } 

AnotherClass.java

 public class AnotherClass { // Main class instance private MainActivity mainActivity; // Constructor public AnotherClass(MainActivity activity) { // Save instance of main class for future use mainActivity = activity; // Call method in MainActivity mainActivity.startChronometer(); } } 

Answer 8

What I did and it works is to create an instance in MainActivity and getter for this instance:

  public class MainActivity extends AbstractMainActivity { private static MainActivity mInstanceActivity; public static MainActivity getmInstanceActivity() { return mInstanceActivity; } 

And the onCreate method just points to this activity:

 @Override protected void onCreate(Bundle savedInstanceState) { mInstanceActivity = this; } 

And in onDestroy you should set this instance to null:

 @Override protected void onDestroy() { super.onDestroy(); mInstanceActivity = null; } 

Later you can call every method in any class you want:

 MainActivity.getmInstanceActivity().yourMethod(); 

Answer 9

Initialize the main action instance in another class where you want to use its method.

Just try it, I did it :)

 MainActivity mActivity = new MainActivity(this); mActivity.startChronometer(); 

Answer 10

You can make this method static.

 public static void startChronometer(){ mChronometer.start(); showElapsedTime(); } 

this function can be called in another class, as shown below:

 MainActivity.startChronometer(); 

OR

You can make the object of the main class in the second class, for example,

 MainActivity mActivity = new MainActivity(); mActivity.startChronometer(); 

Answer 11

You can simply make this method static, as shown below:

 public static void startChronometer(){ mChronometer.start(); showElapsedTime(); } 

this function can be called in another class, as shown below:

 MainActivity.startChronometer();