从零开始学android小示例程序

<relativelayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    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/showword"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignleft="@+id/greenbtn"
        android:layout_alignparenttop="true"
        android:layout_alignright="@+id/bluebtn"
        android:layout_margintop="17dp"
        android:text="@string/showwordtext"
        android:textstyle="bold" />

    <button
        android:id="@+id/greenbtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignparentleft="true"
        android:layout_below="@+id/showword"
        android:layout_marginleft="22dp"
        android:layout_margintop="28dp"
        android:gravity="left|center_vertical|center_horizontal"
        android:text="@string/greenbtntext" />

    <button
        android:id="@+id/bluebtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignbaseline="@+id/yellowbtn"
        android:layout_alignbottom="@+id/yellowbtn"
        android:layout_torightof="@+id/yellowbtn"
        android:gravity="left|center_vertical|center_horizontal"
        android:text="@string/bluebtntext" />

    <button
        android:id="@+id/yellowbtn"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignbaseline="@+id/greenbtn"
        android:layout_alignbottom="@+id/greenbtn"
        android:layout_torightof="@+id/greenbtn"
        android:gravity="left|center_vertical|center_horizontal"
        android:text="@string/yellowbtntext" />

</relativelayout>

package com.example.demo1;
import android.os.bundle;
import android.view.view;
import android.view.view.onclicklistener;
import android.widget.button;
import android.widget.textview;
import android.app.activity;
import android.graphics.color;

 
public class mainactivity extends activity implements onclicklistener{

    textview tv_show=null;
    button greenbtn=null;
    button bluebtn=null;
    button yellowbtn=null;
    @override
    protected void oncreate(bundle savedinstancestate) {
        super.oncreate(savedinstancestate);
        setcontentview(r.layout.activity_main);

        tv_show=(textview)findviewbyid(r.id.showword);
        greenbtn=(button)findviewbyid(r.id.greenbtn);
        bluebtn=(button)findviewbyid(r.id.bluebtn);
        yellowbtn=(button)findviewbyid(r.id.yellowbtn);

        greenbtn.setonclicklistener(this);
        bluebtn.setonclicklistener(this);
        yellowbtn.setonclicklistener(this);
    }
    @override
    public void onclick(view v) {
    switch (v.getid()) {
    case r.id.greenbtn:
        tv_show.settextcolor(color.green);       
        break;
    case r.id.bluebtn:
        tv_show.settextcolor(color.blue);       
        break;
    case r.id.yellowbtn:
        tv_show.settextcolor(color.yellow);       
        break;
    default:
        break;
    }

    }
}