How to Use Canvas API in Android Apps?

import android.annotation.SuppressLint;
import android.app.Activity;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.util.DisplayMetrics;
import android.view.View;

public class PaintView extends View {
    
    // below we are creating variables for our paint
    Paint otherPaint, outerPaint, textPaint;
   
    // and a floating variable for our left arc.
    float arcLeft;

    @SuppressLint("ResourceAsColor")
    public PaintView(Context context) {
        super(context);
        
        // on below line we are initializing our paint variable for our text
        textPaint = new Paint(Paint.LINEAR_TEXT_FLAG | Paint.ANTI_ALIAS_FLAG);
        
        // on below line we are setting color to it.
        textPaint.setColor(Color.WHITE);
       
        // on below line we are setting text size to it.
        // In Paint we have to add text size using px so
        // we have created a method where we are converting dp to pixels.
        textPaint.setTextSize(pxFromDp(context, 24));
        
        // on below line we are initializing our outer paint
        outerPaint = new Paint();
        
        // on below line we are setting style to our paint.
        outerPaint.setStyle(Paint.Style.FILL);
        
        // on below line we are setting color to it.
        outerPaint.setColor(getResources().getColor(R.color.purple_200));
        
        // on below line we are creating a display metrics
        DisplayMetrics displayMetrics = new DisplayMetrics();
        
        // on below line we are getting display metrics.
        ((Activity) getContext()).getWindowManager()
                .getDefaultDisplay()
                .getMetrics(displayMetrics);
        
        // on below line we are assigning
        // the value to the arc left.
        arcLeft = pxFromDp(context, 20);
        
        // on below line we are creating 
        // a new variable for our paint
        otherPaint = new Paint();
    }

    // below method is use to generate px from DP.
    public static float pxFromDp(final Context context, final float dp) {
        return dp * context.getResources().getDisplayMetrics().density;
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        
        // below four lines of code is use to add
        // back color to our screen which is green
        canvas.drawPaint(outerPaint);
        
        // on below line we are setting color to our paint.
        otherPaint.setColor(Color.WHITE);
        
        // on below line we are setting style to out paint.
        otherPaint.setStyle(Paint.Style.FILL);
        
        // below 4 lines of code is use to
        // create white rectangle of screen
        canvas.drawRect(
                getLeft() + (getRight() - getLeft()) / 3,
                getTop() + (getBottom() - getTop()) / 3,
                getRight() - (getRight() - getLeft()) / 3,
                getBottom() - (getBottom() - getTop()) / 3, otherPaint);
        
        // on below line we are changing the color for our paint.
        otherPaint.setColor(getResources().getColor(R.color.purple_200));
        
        // on below line we are drawing a circle and passing 
        // width, height, left arc and paint to add color.
        canvas.drawCircle(getWidth() / 2, getHeight() / 2, arcLeft, otherPaint);
        
        // on below line we are adding text using paint in our canvas.
        canvas.drawText("Geeks for Geeks", (float) (getWidth() * 0.3), (float) (getHeight() * 0.8), textPaint);
    }
}

import android.os.Bundle;
import android.widget.RelativeLayout;

import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    
    // creating a variable for our relative layout
    private RelativeLayout relativeLayout;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        // initializing our view.
        relativeLayout = findViewById(R.id.idRLView);
        
        // calling our  paint view class and adding 
        // its view to our relative layout.
        PaintView paintView = new PaintView(this);
        relativeLayout.addView(paintView);
    }
}