Android: ViewPager

A little bit confused about the viewpager from the official document. Created this simple test for better understanding.

SimpeViewPager.java

package com.example.viewpager;

import android.app.Activity;
import android.os.Bundle;
import android.support.v4.view.PagerAdapter;
import android.support.v4.view.ViewPager;
import android.util.TypedValue;
import android.view.Gravity;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;

import java.util.Random;

// This app simplify the demo app in Android document for understanding ViewPager 
// without involving unrelated factors.

public class SimpleViewPager extends Activity {

    PagerAdapter mAdapter;
    ViewPager mPager;

    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        mPager = (ViewPager) findViewById(R.id.pager);
        mAdapter = new TextAdapter();
        mPager.setAdapter(mAdapter);

        Button button = (Button) findViewById(R.id.goto_first);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mPager.setCurrentItem(0);
            }
        });

        button = (Button) findViewById(R.id.go_last);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                mPager.setCurrentItem(mAdapter.getCount() - 1);
            }
        });
    }

    public static class TextAdapter extends PagerAdapter {
        static final int TOTAL_PAGES = 5;

        @Override
        public int getCount() {
            return TOTAL_PAGES;
        }

        @Override
        public Object instantiateItem(ViewGroup container, int position) {

            TextView itemView = new TextView(container.getContext());
            int viewId = new Random().nextInt(Integer.MAX_VALUE) + 1;
            itemView.setId(viewId);

            itemView.setGravity(Gravity.CENTER_HORIZONTAL | Gravity.CENTER_VERTICAL);
            itemView.setTextSize(TypedValue.COMPLEX_UNIT_PX, 180);
            itemView.setText(String.valueOf(position));

            ViewGroup.LayoutParams layoutParams = new ViewPager.LayoutParams();
            layoutParams.height = ViewGroup.LayoutParams.MATCH_PARENT;
            layoutParams.width = ViewGroup.LayoutParams.MATCH_PARENT;
            container.addView(itemView, layoutParams);

            // The return object is the key object associated with the item view just added.
            // There are many ways to set up the mapping between the item view and its key
            // Some examples use the view itself as the key, Here we generate an Integer 
            // which equals to the view id for testing purpose.

            Integer itemKey = new Integer(viewId);
            return itemKey;
        }

        @Override
        public boolean isViewFromObject(View view, Object o) {

            // We know how to check if object o is the key object of view.
            // The relationship between two is set in initantiateItem()

            Integer itemKey = (Integer) o;
            int viewId = itemKey.intValue();
            return view.getId() == viewId;
        }

        @Override
        public void destroyItem(ViewGroup container, int position, Object object) {
            Integer itemKey = (Integer) object;
            int viewId = itemKey.intValue();
            View itemView = container.findViewById(viewId);
            container.removeView(itemView);
        }
    }
}

The layout file

<?xml version="1.0" encoding="utf-8"?>

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:layout_width="match_parent"
              android:layout_height="match_parent">

    <android.support.v4.view.ViewPager
            android:id="@+id/pager"
            android:layout_width="match_parent"
            android:layout_height="0px"
            android:layout_weight="1"
            />
    <LinearLayout android:orientation="horizontal"
                  android:gravity="center"
                  android:measureWithLargestChild="true"
                  android:layout_width="match_parent"
                  android:layout_height="wrap_content"
                  android:layout_weight="0">
        <Button android:id="@+id/goto_first"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="First"
                />
        <Button android:id="@+id/go_last"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:text="Last"
                />
    </LinearLayout>
</LinearLayout>