Android :: List Item View Order Changes When Scrolling Fast In ListView

I am new to android, and ended up (have to) ask a question here. Let's make it simple, I simply want to make my own TextView-like (MyView extends View), This is my code:

public class MyView extends View { private Paint mPaint;
private String mText; private Bitmap mBitmap1; private Bitmap mBitmap2;
public MyView(Context context) { super(context); initView(); }
public MyView(Context context, AttributeSet attrs) { super(context, attrs);
initView(); } private final void initView() { mPaint = new Paint(); }
public void setText(String text) { mText = text; }
@Override protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
int measuredWidth = measureWidth(widthMeasureSpec);
if (mBitmap1 == null) initBitmap1(measuredWidth);
int measuredHeight = measureHeight(heightMeasureSpec);
setMeasuredDimension(measuredWidth, measuredHeight);
}

private void initBitmap1(int measuredWidth) {
mBitmap1 = Bitmap.createBitmap(measuredWidth, Fonts.getHeight(), Bitmap.Config.ARGB_4444);
Canvas canvas = new Canvas(mBitmap1 ); canvas.drawText(mText, 0, 0, mPaint);
} private void initBitmap2() {
mBitmap2 = Bitmap.createBitmap(30, Fonts.getHeight(), Bitmap.Config.ARGB_4444);
Canvas canvas = new Canvas(mBitmap2); canvas.drawText(mText, 0, 0, mPaint);
} private int measureWidth(int widthMeasureSpec) {
int measuredWidth = 0; int specWidthMode = MeasureSpec.getMode(widthMeasureSpec);
int specWidthSize = MeasureSpec.getSize(widthMeasureSpec);

if (specWidthMode == MeasureSpec.EXACTLY) { measuredWidth = specWidthSize;
} else { measuredWidth = getWidth(); if (specWidthMode == MeasureSpec.AT_MOST) {
measuredWidth = Math.min(measuredWidth, specWidthSize);
} } return measuredWidth; }
private int measureHeight(int heightMeasureSpec) {
int measuredHeight = 0; int specHeightMode = MeasureSpec.getMode(heightMeasureSpec);
int specHeightSize = MeasureSpec.getSize(heightMeasureSpec);
if (specHeightMode == MeasureSpec.EXACTLY) { measuredHeight = specHeightSize;
} else { measuredHeight = 80; if (specHeightMode == MeasureSpec.AT_MOST) {
measuredHeight = Math.min(measuredHeight, specHeightSize);
} } return measuredHeight;
} @Override protected void onDraw(Canvas canvas) {
super.onDraw(canvas); canvas.drawBitmap(mBitmap1, getLeft(), 0, mPaint);
initBitmap2(); canvas.drawBitmap(mBitmap2, getLeft(), 30, mPaint);
} }
In my code, I populate some numbers of MyView (let's say 20) in a ListActivity. My question is why mBitmap1's order changes randomly while i scroll (up-down) fastly (if I scroll slowly, this problem not occur)? mBitmap2 stays where those should be.