andorid 计算器

<?xml version="1.0" encoding="utf-8"?>
<GridLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:rowCount="6"
android:columnCount="4"
>
<!--rowCount   行数
columnCount   列数
layout_columnSpan    跨列
layout_rowSpan      跨行
fill_horizontal   水平填充
fill_vertical    垂直填充
layout_columnWeight   权重列
layout_rowWeight   权重行
background  @drawable/  设置图片
layout_row 指定行号
layout_column  指定列好  从0开始
-->
<EditText
android:layout_columnSpan="4"
android:layout_gravity="fill_horizontal"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:editable="false"
android:gravity="right|center_vertical"
android:id="@+id/et"/>

<Button
android:text="清除"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"
android:id="@+id/clear"
/>
<Button
android:text="后退"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"
android:id="@+id/goback"/>

<Button
android:text="/"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"/>
<Button
android:text="X"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"/>
<Button
android:text="7"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"
android:id="@+id/bt_7"/>
<Button
android:text="8"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"
android:id="@+id/bt_8"/>
<Button
android:text="9"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"
android:id="@+id/bt_9"/>
<Button
android:text="-"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"/>
<Button
android:text="4"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"/>
<Button
android:text="5"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"/>
<Button
android:text="6"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"/>
<Button
android:text="+"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"/>
<Button
android:text="1"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"/>
<Button
android:text="2"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"/>
<Button
android:text="3"
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"/>
<Button
android:text="="
android:layout_rowSpan="2"
android:layout_gravity="fill_vertical"
android:layout_columnWeight="1"
android:layout_rowWeight="2"
android:textSize="25sp"
android:background="#24f"
/>
<Button
android:text="0"
android:layout_columnSpan="2"
android:layout_gravity="fill_horizontal"
android:layout_columnWeight="2"
android:layout_rowWeight="1"
android:textSize="25sp"
/>
<Button
android:text="."
android:layout_columnWeight="1"
android:layout_rowWeight="1"
android:textSize="25sp"
/>

</GridLayout>

package com.hanqi.application3;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

/**
* Created by Administrator on 2016/3/28.
*/
public class CalculatorActivity extends Activity implements View.OnClickListener {
EditText et;
Button  bt_clear;
Button  bt_goback;
Button  bt_7;
Button  bt_8;
Button  bt_9;
//构建字符串StringBuffer
//存储显示的内容
private StringBuffer str_show= new StringBuffer();

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);

setContentView(R.layout.avtivity_main);

et=(EditText)findViewById(R.id.et);
bt_clear=(Button)findViewById(R.id.clear);
bt_7=(Button)findViewById(R.id.bt_7);
bt_8=(Button)findViewById(R.id.bt_8);
bt_9=(Button)findViewById(R.id.bt_9);

bt_clear.setOnClickListener(this);
bt_goback.setOnClickListener(this);
bt_7.setOnClickListener(this);
bt_8.setOnClickListener(this);
bt_9.setOnClickListener(this);

}

public void onClick(View v)
{
Button bt = (Button)v;

int id =bt.getId();
switch (id)
{
case R.id.clear:
str_show= new StringBuffer();
et.setText(str_show);
break;
case R.id.goback:
str_show.codePointAt(str_show.length()-1);
et.setText(str_show);
break;
case R.id.bt_7:
case R.id.bt_8:
case R.id.bt_9:
str_show.append(bt.getText());
et.setText(str_show);
break;

}
}
}