Error cannot resolve symbol activity_main in Android Studio

Question

My error is: "cannot find symbol variable activity_main" How can solve it? This code is for change light of screen for android java code is I used different ways but I can't resolve it.

package com.example.lightbluered;

import androidx.appcompat.app.AppCompatActivity;
import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RelativeLayout;

public class MainActivity extends AppCompatActivity {

    Button button;
    @Override
    protected void onCreate(Bundle
        savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  RelativeLayout bgElement = (RelativeLayout) findViewById(R.id.activity_main);
  bgElement.setBackgroundColor(Color.RED);
  myButtonListenerMethod();
      }
 public void myButtonListenerMethod() {
     button = (Button)findViewById(R.id.button);
     button.setOnClickListener(new
       View.OnClickListener() {
       @Override
   public void onClick(View v) {
     RelativeLayout bgElement =
   (RelativeLayout)findViewById(R.id.activity_main);
      int color = ((ColorDrawable)
      bgElement.getBackground()).getColor();
     if (color == Color.RED) {
      bgElement.setBackgroundColor(Color.BLUE);
      }
     else {
      bgElement.setBackgroundColor(Color.RED);
                 }
              }
          });
      }
}

and activity main XML code

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.532"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.524" />
</androidx.constraintlayout.widget.ConstraintLayout>

how can solve it?

Answer 1

Change your layout according to this:

<?xml version="1.0" encoding="utf-8"?>
<Relativelayout
  xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity"
    android:id="@+id/relative"
>

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.532"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.524" />
</Relativelayout>

Then Change this line

RelativeLayout bgElement = (RelativeLayout) findViewById(R.id.activity_main);

to

RelativeLayout bgElement = (RelativeLayout) findViewById(R.id.relative);

You don't need to define relativelayout again in the onclick method

Answer 2

In Your:

RelativeLayout bgElement = (RelativeLayout) findViewById(R.id.activity_main);

You are trying to refer R.id.activity_main which is no present in your layout XML file,

Try making your layout as:

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

       <RelativeLayout
            android:id="@+id/activity_main"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
        <Button
            android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.532"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.524" />

    </RelativeLayout>

</androidx.constraintlayout.widget.ConstraintLayout>

Answer 3

Just changed the code like...

   package com.example.lightbluered;

import androidx.appcompat.app.AppCompatActivity;
import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RelativeLayout;

public class MainActivity extends AppCompatActivity {

    Button button;
    @Override
    protected void onCreate(Bundle
        savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.activity_main);
  ConstraintLayout bgElement = (ConstraintLayout ) findViewById(R.id.cl);
  bgElement.setBackgroundColor(Color.RED);
  myButtonListenerMethod();
      }
 public void myButtonListenerMethod() {
     button = (Button)findViewById(R.id.button);
     button.setOnClickListener(new
       View.OnClickListener() {
       @Override
   public void onClick(View v) {
     RelativeLayout bgElement =
   (RelativeLayout)findViewById(R.id.activity_main);
      int color = ((ColorDrawable)
      bgElement.getBackground()).getColor();
     if (color == Color.RED) {
      bgElement.setBackgroundColor(Color.BLUE);
      }
     else {
      bgElement.setBackgroundColor(Color.RED);
                 }
              }
          });
      }
}

And the xml

 <?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:id="@+id/cl"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.532"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.524" />
</androidx.constraintlayout.widget.ConstraintLayout>

Answer 4

If you need to do what you need using ConstraintLayout you can use the following layout,

You just need to add id to your current layout. I named it as constraintLayout .

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@id/constraintLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.532"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.524" />
</androidx.constraintlayout.widget.ConstraintLayout>

Then your code should be like this.

package com.example.lightbluered;

import androidx.appcompat.app.AppCompatActivity;
import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {
    Button button;
    ConstraintLayout bgElement;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        button = (Button)findViewById(R.id.button);

        bgElement = (ConstraintLayout) findViewById(R.id.constraintLayout);
        bgElement.setBackgroundColor(Color.RED);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int color = ((ColorDrawable)
                bgElement.getBackground()).getColor();

                if (color == Color.RED) {
                    bgElement.setBackgroundColor(Color.BLUE);
                } else {
                    bgElement.setBackgroundColor(Color.RED);
                }
            }
        });
    }
}

But if you need to do this with the RelativeLayout , you can do that also.

Add RelativeLayout inside the ConstraintLayout . But it is up to you to choose. You can remove the ConstraintLayout and make it as RelativeLayout or just as the follows.

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <RelativeLayout
        android:id="@+id/relativeLayout"
        android:layout_width="match_parent"
        android:layout_height="match_parent">
        <Button
            android:id="@+id/button"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintEnd_toEndOf="parent"
            app:layout_constraintHorizontal_bias="0.532"
            app:layout_constraintStart_toStartOf="parent"
            app:layout_constraintTop_toTopOf="parent"
            app:layout_constraintVertical_bias="0.524" />
    </RelativeLayout>
</androidx.constraintlayout.widget.ConstraintLayout>

Then your code should be like this.

package com.example.lightbluered;

import androidx.appcompat.app.AppCompatActivity;
import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.RelativeLayout;

public class MainActivity extends AppCompatActivity {
    Button button;
    RelativeLayout bgElement;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        button = (Button)findViewById(R.id.button);

        bgElement = (RelativeLayout) findViewById(R.id.relativeLayout);
        bgElement.setBackgroundColor(Color.RED);

        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int color = ((ColorDrawable)
                bgElement.getBackground()).getColor();

                if (color == Color.RED) {
                    bgElement.setBackgroundColor(Color.BLUE);
                } else {
                    bgElement.setBackgroundColor(Color.RED);
                }
            }
        });
    }
}

Answer 5

you are initializing a view so you should use "R.id." and then add your view id also you did not define any RelativeLayout in your activity_main.xml to solve this problem change activity_main.xml content to this:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/relative_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

</RelativeLayout>

and in MainActivity.java change RelativeLayout bgElement = (RelativeLayout) findViewById(R.id.activity_main); to RelativeLayout bgElement = (RelativeLayout) findViewById(R.id.relative_layout);

Answer 6

You have done only one mistake.. that is you are trying to use Relative layout which is not present in your xml file. now you can do 2 things, 1. change the Constraint layout to Relative layout in xml file. 2. change the Relative layout to Constraint layout in java file.