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;
    }

    }
}