How to call a method in MainActivity from another class

Question

How do I call the method startChronometer in another class when the method is declared inside the main activity    Inside MainActivity    public void startChronometer         mChronometer start        showElapsedTime        Inside another class  I tried to do this   MainActivity mActivity  mActivity startChronometer      But an error occurred which said    java lang NullPointerException     May I know what more I need to add to the code

User · Answer

You have to pass instance of MainActivity into another class, then you can call everything public (in MainActivity) from 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();
    }
}

User · Answer

But an error occurred which says java lang NullPointerException    Thats because  you never initialized your MainActivity  you should 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

User · Answer

You can make this method static   public static void startChronometer            mChronometer start            showElapsedTime             you can call this function in other class as below   MainActivity startChronometer      OR  You can make an object of the main class in second class like   MainActivity mActivity   new MainActivity    mActivity startChronometer

User · Answer

I would suggest  one should not make object of an Activity type class   MainActivity mActivity   new MainActivity        BIG NO TO THIS    All Activities in Android must go through the Activity lifecycle so that they have a valid context attached to them   By treating an Activity as a normal Java class  you end up with a null context  As most methods in an Activity are called on its Context  you will get a null pointer exception  which is why your app crashes   Instead  move all such methods which need to be called from other classes into a Utility class which accepts a valid context in its constructor  and then use that context in the methods to do the work

User · Answer

You can easily call a method from any Fragment inside your Activity by doing a cast like this   Java    MainActivity getActivity    startChronometer      Kotlin   activity as MainActivity  startChronometer     Just remember to make sure this Fragment s activity is in fact MainActivity before you do it   Hope this helps

User · Answer

Simply  You can make this method static as below   public static void startChronometer        mChronometer start        showElapsedTime         you can call this function in other class as below   MainActivity startChronometer

User · Answer

Initialize it first    MainActivity mActivity  new MainActivity      Then you can continue   mActivity startChronometer

User · Answer

What i have done and it works is create an instance in the MainActivity and getter for that instance    public class MainActivity extends AbstractMainActivity       private static MainActivity mInstanceActivity      public static MainActivity getmInstanceActivity         return mInstanceActivity          And the in the onCreate method just point to that 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 whatever class you want   MainActivity getmInstanceActivity   yourMethod

User · Answer

Use this code in sub Fragment of MainActivity to call the method on it      MainActivity  getActivity    startChronometer

User · Answer

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

User · Answer

What I have done with no memory leaks or lint warnings is to use  f trajkovski s method  but instead of using MainActivity  use WeakReference lt MainActivity gt  instead   public class MainActivity extends AppCompatActivity   public static WeakReference lt MainActivity gt  weakActivity     etc    public static MainActivity getmInstanceActivity         return weakActivity get          Then in MainActivity OnCreate    weakActivity   new WeakReference lt  gt  MainActivity this     Then in another class  MainActivity getmInstanceActivity   yourMethod      Works like a charm

[java] How to call a method in MainActivity from another class?

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