package com.king.activitytest2;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity implements View.OnClickListener{
private EditText editText;
private Button buttonSet,buttonRead;
//定义一个常量
double pai;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//获取控件
editText=(EditText)findViewById(R.id.edit1);
buttonSet=(Button) findViewById(R.id.btn_Set);
buttonRead=(Button) findViewById(R.id.btn_Read);
//设置监听事件
buttonSet.setOnClickListener(this);
buttonRead.setOnClickListener(this);
}
@Override
public void onClick(View view) {
switch (view.getId()){
case R.id.btn_Set:
editText.setText("圆周率为：");
pai=3.141592654;
break;
case R.id.btn_Read:
String str=editText.getText().toString()+pai;
Toast.makeText(MainActivity.this,str,Toast.LENGTH_SHORT).show();
break;
}
}
}

<?xml version="1.0" encoding="utf-8"?>
<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"
tools:context="com.king.activitytest2.MainActivity">
<EditText
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:id="@+id/edit1"/>
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="设置"
android:id="@+id/btn_Set"
android:layout_marginTop="46dp"
android:layout_below="@+id/edit1"
android:layout_alignParentLeft="true"
android:layout_alignParentStart="true"
android:layout_marginLeft="35dp"
android:layout_marginStart="35dp" />
<Button
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="读取"
android:id="@+id/btn_Read"
android:layout_alignTop="@+id/btn_Set"
android:layout_alignParentRight="true"
android:layout_alignParentEnd="true"
android:layout_marginRight="46dp"
android:layout_marginEnd="46dp"
/>
</RelativeLayout>

@Override
protected void onSaveInstanceState(Bundle outState) {
super.onSaveInstanceState(outState);
//将需要保存的数据放入Bundle中
outState.putDouble("pai",pai);
}

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//将pai保存
if(savedInstanceState!=null)
pai=savedInstanceState.getDouble("pai");
}