I am guessing you need the button to be created when the activity pop_cart.java is in the foreground.
In that case why don’t you directly execute the button code in the onCreate() of pop_cart.java?

You can pass a boolean from pop_accompaniments.java to pop_cart.java via an intent if you want something in pop_accompaniments.java to decide whether the button will be created or not!

In pop_accompaniments.java:

Intent intent = new Intent(this, pop_cart.class);
intent.putExtra("someBool", true);

In pop_cart.java:

@Override
protected void onCreate(Bundle savedInstanceState) {
Boolean bool = getIntent().getExtras().getBoolean("someBool");

//Now you can check if the bool is true or false
if(bool){
    //Execute button code here
}
}

I want to add a button from 1 activity – “pop_accompaniments.xml” to another activity – “pop_cart.xml“, using java code. For this I created an instance of the class “pop_cart.java” in “pop_accompaniments.java“, to call the func – “cart_button_creator” which is in “pop_cart.java“.
But my app just crashes as soon as I open it.

pop_accompaniments.java

package com.example.myapplication;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

import androidx.appcompat.app.AppCompatActivity;

import java.util.Random;

public class pop_accompaniments extends AppCompatActivity implements View.OnClickListener {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.pop_accompaniments);

        Button assorted_condiments = findViewById(R.id.assorted_condiments);
    }

    @Override
    public void onClick(View v) {
        setContentView(R.layout.pop_cart);
        pop_cart main = new pop_cart();
        int id;
        String name;
        Button myButton;
        switch (v.getId()) {
            case R.id.assorted_condiments:
                myButton = new Button(this);
                name = "Assorted Condiments";
                id = generateButtonId();
                main.cart_content_keeper(id, name, myButton);
        }
    }
}

pop_cart.java

package com.example.myapplication;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.LinearLayout;

import androidx.annotation.Nullable;

public class pop_cart extends Activity {
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        //setContentView(R.layout.pop_cart);
    }

    public void cart_button_creator(String name, int id, Button myButton) {
        //setContentView(R.layout.pop_cart);
        LinearLayout ll = findViewById(R.id.cart_items);
        myButton.setText(name);
        myButton.setLayoutParams(new LinearLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT));
        myButton.setId(id);
        ll.addView(myButton);
    }


    public void cart_content_keeper(int id, String name, final Button myButton) {
        int n = 0;
        int[] id_arr = {};
        String[] name_arr = {};
        //int price = 0;

        name_arr = add_item(n, name_arr, name);
        id_arr = add_id(n, id_arr, id);
        n = n + 1;

        Button refresh = findViewById(R.id.refresh);
        final int finalN = n;
        final String[] finalName_arr = name_arr;
        final int[] finalId_arr = id_arr;
        refresh.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (v.getId() == R.id.refresh) {
                    for (int i = 0; i < finalN; i++) {
                        cart_button_creator(finalName_arr[i], finalId_arr[i], myButton);
                    }
                }
            }
        });
    }


    public static String[] add_item(int n, String[] arr, String x)
    {
        int i;

        String[] newarr = new String[n + 1];

        for (i = 0; i < n; i++)
            newarr[i] = arr[i];

        newarr[n] = x;

        return newarr;
    }


    public static int[] add_id(int n, int[] arr, int x)
    {
        int i;

        int[] newarr = new int[n + 1];

        for (i = 0; i < n; i++)
            newarr[i] = arr[i];

        newarr[n] = x;

        return newarr;
    }
}

Sorry for such a long code but I have tried almost all the possible ways of calling a func from an instance of another class, passing the variables in a different manner, trying to call the whole variable generator as a function and only passing the view v as an argument in pop_accompaniments.java but the app keeps in crashing either when I click on the button “ASSORTED CONDIMENTS” at the very first step or if I just start the app it crashes.

Also, the buttons are created just fine if I am creating them in the same activity that is “pop_accompaniments.xml” while calling the button_creator method from “pop_accompaniments.java“.

But I want them to show in the CART activity not the ACCOMPANIMENTS activity.