Android studio defining objects(TabItem) in other java files

Question

As a beginner java and AndroidStudio user, I've created a java file (in same package) called objects and decided to define the items(such as buttons,views etc) there instead of defining them into void createOn().

package com.example.turref;

import androidx.appcompat.app.AppCompatActivity;
import com.google.android.material.tabs.TabItem;

public class Object extends AppCompatActivity {

Tabitem settingsPanel = findViewById(R.id.settingsPanel);

}

    

However, I also want to use triggers in Mainactivity and make sth visible/invisible once they triggered.

package com.example.turref;

import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import com.google.android.material.tabs.TabItem;

public class MainActivity extends AppCompatActivity {

    public class Main2 extends Object {

        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
            
            /* some kind of trigger (can be button)

            calls shower() when triggered        */
        }

        void shower() {
            settingsPanel.setVisibility(View.VISIBLE);
        }
    }
}

When I do sth like that it kinda works but I get a black screen once I run the emulator. Can someone help me solve this?

(I`m open to all kind of ways, the important point is to be able to access settingsPanel in show() method )

Answer 1

First of all, you getting a black screen could be because you didnt turn the emulator on;) There's a power button you need to press before you can use it.

I really like your idea to extract all the view elements into their own file. However, as of now, your app would crash. Your settings panel will be null , because your Object Activity can't access the view items, resulting in a NullPointerException . There just isnt a computed layout for Object to use, because the view items don't exist yet! setContentView() needs to be called before accessing view.

So, either you place setContentView() before the super -call OR in the onCreate() of the Object class itself. This is also where you would place the findView -calls:

// Possibility 1
public class MainActivity extends Object{
    protected void onCreate(Bundle savedInstanceState) {
        setContentView(R.layout.activity_main); // <---
        super.onCreate(savedInstanceState);
    }
}
        
// Possibility 2
public class Object extends AppCompatActivity {
    Tabitem settingsPanel;

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        settingsPanel= findViewById(R.id.SettingsPanel);
    }
}

Also, don't nest Activities into one another. MainActivity can also directly extend Object

Still, I wouldn't recommend doing it like this. If you are really bothered by all the findViewById calls, you can try ViewBinding , it's the recommended way by google.

Concerning your trigger, why not use a button?

findViewById(R.id.Button).setOnClickListener(new View.OnClickListener() {
    @Override
    public void onClick(View v) {
        shower()
    }
});       

(A little side note to the end: Since you are new to android and java and you say you are open for different ways, if this is gonna be a big project with lots of dynamic view changes, don't use java. Kotlin, the other official android language with long-time support from google, has a feature called jetpack compose. It's basically declarative view building -> less code, more dynamic. You can read about it here . If it's a small project, stick with java.

Also, to learn android, here are two good tutorial resources/sites: developers.android andfree code camp .)