android first

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="vertical"
    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"
    tools:context=".MainActivity" >

    <TextView 
        android:id="@+id/show"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/hello_world" />
    <Button
         android:id="@+id/ok" 
        android:text="单击我"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"/>

</RelativeLayout>

package com.example.test;

import java.util.Date;

import android.os.Bundle;
import android.app.Activity;
import android.view.Menu;
import android.view.View;

import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Button bn=(Button)findViewById(R.id.ok);
        bn.setOnClickListener(new View.OnClickListener()
        {
            public void onClick(View v)
            {
                final TextView show=(TextView)findViewById(R.id.show);
                show.setText("Hello Android~"+new Date());
            }
        });
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }
    
}