Wie zeichne ich eine Linie mit Canvas ?

package com.example.heinz.testgraphic;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Layout;
import android.util.AttributeSet;
import android.util.DisplayMetrics;
import android.view.View;
import android.widget.LinearLayout;

import static android.graphics.Color.BLACK;

public class MainActivity extends AppCompatActivity {
    private LinearLayout mLayout;
    private DrawLine mDrawLine;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mLayout =  (LinearLayout)findViewById(R.id.activity_main);
        mDrawLine = new DrawLine(this, null);
        mLayout.addView(mDrawLine);     // <-- wird das eine ansprechbare View ?
    }           //end onCreate -------------------------------------------

    /**
     * Reaktion auf Click Button
     * @param Button
     */
    public void onStartPaint(View Button) {
        mDrawLine.draw();
    }           //end onStartPaint ---------------------------------------

    /**
     * Diese Class zeichnet (hoffentlich)
     */
    protected  class DrawLine extends View {
        private Canvas mCanvas;     // <-- notwendig ?, wie mit onDraw verbinden ?
        /**
         * Constructor
         * @param context
         */
        public DrawLine(Context context, AttributeSet attrs) {  // <-- attr ist null, notwendig ?
            super(context, attrs);
        }           //end Constructor

        public void draw() {
            invalidate();           // <-- bis hierher kommt der Ablauf. und dann ?
        }           //end draw ----------------------------------------------

        @Override
        protected void onDraw(Canvas canvas) {
            Paint mPaint = new Paint();
            mPaint.setColor(BLACK);
            mPaint.setStrokeWidth(4);
            canvas.drawLine(10, 50, 100, 50, mPaint);
        }           //end onDraw ---------------------------------------------
    }           //end C DrawClass --------------------------------------------

}           //end C MainActivity ---------------------------------------------

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:orientation="vertical"
    tools:context="com.example.heinz.testgraphic.MainActivity">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="onStartPaint"
        android:text="@string/start_paint" />
</LinearLayout>

public class MyView extends View
{
  Paint paint = new Paint();

  @Override
  protected void onDraw(Canvas canvas)
  {
   paint.setAntiAlias(true);
   paint.setStrokeWidth(6f);
   paint.setColor(Color.BLACK);
   paint.setStyle(Paint.Style.STROKE);
   paint.setStrokeJoin(Paint.Join.ROUND);
   canvas.drawLine(startX, startY, stopX, stopY, paint);
  }
}

package com.example.heinz.testgraphic;
import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Paint;
import android.support.v4.view.ViewPager;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Layout;
import android.util.AttributeSet;
import android.util.DisplayMetrics;
import android.view.View;
import android.widget.LinearLayout;

import static android.graphics.Color.BLACK;
import static android.graphics.Color.RED;

public class MainActivity extends AppCompatActivity {
    private LinearLayout mLayout;       //Instanzvariable des Layout
    private DrawView mDrawView;         //Instanzvariable des View zum zeichnen
    private boolean isHorizontal;       //Merker für die Ausrichtung der Linie
    /**
     * Einstiegspunkt der App
     * @param savedInstanceState
     */
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        isHorizontal = false;
            // View zum Zeichnen in das Layout einbinden
        mLayout =  (LinearLayout)findViewById(R.id.activity_main);
        mDrawView = new DrawView(this);
        mLayout.addView(mDrawView);
    }           //end onCreate -------------------------------------------
    /**
     * Der Button wurde geklickt, das Zeichnen der Linie wird eingeleitet
     * @param Button
     */
    public void onStartPaint(View Button) {
        mDrawView.invalidate();     //löst das onDraw in der Class DrawView aus
    }           //end onStartPaint ---------------------------------------
    /**
     * Diese Class definiert einen View in dem gezeichnet werden kann.
     * Sie muss in das Layout eingebunden werden (siehe onCreate)
     */
    public   class DrawView extends View {
        public DrawView(Context context) {
            super(context);
        }           //end Constructor
        /**
         * Diese überschriebene Methode ermöglicht das Zeichnen
         * Sie kann mit .invalidate() angestoßen werden
         * @param canvas
         */
        @Override
        protected void onDraw(Canvas canvas) {
            Paint mPaint = new Paint();     //das Zeichenwerkzeug
            mPaint.setAntiAlias(true);
            mPaint.setColor(RED);
            mPaint.setStrokeWidth(10f);
            int xs = 50;
            int ys = 50;
            int xe = 150;
            int ye = 50;
            if(isHorizontal) {
                xe = 50;
                ye = 150;
                isHorizontal = false;
            }
            else {
                xe = 150;
                ye = 50;
                isHorizontal = true;
            }
            canvas.drawLine(xs, ys, xe, ye, mPaint);
        }           //end onDraw ---------------------------------------------
    }           //end C DrawView ---------------------------------------------
}           //end C MainActivity ---------------------------------------------