Android学习笔记（四）

案例一：图片获得焦点事件

MainActivity

package com.example.onfocuschagelistenerdemo;

import java.util.Iterator;

import android.os.Bundle;

import android.app.Activity;

import android.view.View;

import android.view.View.OnFocusChangeListener;

import android.widget.ImageButton;

import android.widget.TextView;

public class MainActivity extends Activity implements OnFocusChangeListener{

//焦点事件

private TextView show ;

ImageButton [] imageButtons = new ImageButton[2];

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

show =(TextView)findViewById(R.id.show);

imageButtons[0] =(ImageButton)this.findViewById(R.id.Imgshow1);

imageButtons[1]= (ImageButton)this.findViewById(R.id.imgshow2);

imageButtons[0].setOnFocusChangeListener(this);

imageButtons[1].setOnFocusChangeListener(this);

* for (ImageButton imagebutton : imageButtons) {

* imagebutton.setOnFocusChangeListener(this);

* }

*for (int i = 0; i < imageViews.length; i++) {

* //普通循环

* }

* for (Iterator iterator = collection.iterator(); iterator.hasNext();) {

* type type = (type) iterator.next();

* //遍历针对于Collection 集合

* }

* **/

}

@Override

public void onFocusChange(View v, boolean hasFocus) {

// TODO Auto-generated method stub

if(v.getId()==R.id.Imgshow1){

this.show.setText("你选中了羊");

}else if(v.getId()==R.id.imgshow2){

this.show.setText("你选中的是够");

}

Activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="fill_parent"

android:layout_height="fill_parent"

android:orientation="vertical" >

<LinearLayout

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:orientation="horizontal"

<ImageButton

android:id="@+id/Imgshow1"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:focusableInTouchMode="true"

android:src="@drawable/png1" />

<ImageButton

android:id="@+id/imgshow2"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:focusableInTouchMode="true"

android:src="@drawable/png2"

</LinearLayout>

<TextView

android:id="@+id/show"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

</LinearLayout>

效果图

案例二：焦点事件的验证

MainActivity

package com.example.onfocuschangelinstenerdemo2;

import android.view.View.OnFocusChangeListener;

public class MainActivity extends Activity {

private EditText ed1;

private TextView show ;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

show=(TextView)findViewById(R.id.show);

ed1=(EditText)findViewById(R.id.et1);

ed1.setOnFocusChangeListener(new OnFocusChangeListenerText());

}

class OnFocusChangeListenerText implements OnFocusChangeListener{

@Override

public void onFocusChange(View v, boolean hasFocus) {

// TODO Auto-generated method stub

if(v.getId()==MainActivity.this.ed1.getId()){

if(hasFocus){

MainActivity.this.show.setText("文本输入获得焦点的组件是ed1");

}else {

MainActivity.this.show.setText("你ed1中输入的是合法");

}

}else{

if(hasFocus){

MainActivity.this.show.setText("文本输入获得焦点的组件是ed2");

}else {

MainActivity.this.show.setText("你ed2中输入的是合法");

}

Activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="fill_parent"

android:layout_height="fill_parent"

android:orientation="vertical" >

<EditText

android:id="@+id/et1"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="请输入内容"

<EditText

android:id="@+id/ed2"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="请输入内容"

<TextView

android:id="@+id/show"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

</LinearLayout>

SharedPreferencs

Android 操作系统提供五种存储方式， SharedPreference ，文件存储，SQLite数据存储，ConnectionProvider存储，网络存储。

SharedPreferencs保存数据

package com.example.gdsharepreferfacedemo;

public class MainActivity extends Activity {

//SharedPreferences 文件保存

private static final String FILENAME ="gangdan";//文件名称

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

SharedPreferences share =super.getSharedPreferences(FILENAME, Activity.MODE_PRIVATE);

//指定文件的名字和编辑的模式

SharedPreferences.Editor edit =share.edit();//获得edit()接口

edit.putString("gqy", "guoqingyu");//保存字符串类型；

edit.putInt("年龄", 18);//保存整型

edit.commit();//提交

}

SharedPerferences 获取数据

package com.example.hxsharedpreference;

public class OtherActivity extends Activity {

private TextView msgName, msgAge;

private static final String FILENAME="xuesheng";

@Override

protected void onCreate(Bundle savedInstanceState) {

// TODO Auto-generated method stub

super.onCreate(savedInstanceState);

setContentView(R.layout.other);

msgName=(TextView)findViewById(R.id.msgName);

msgAge=(TextView)findViewById(R.id.msgAge);

SharedPreferences sharedPreferences =

getSharedPreferences(FILENAME, Activity.MODE_PRIVATE);

msgName.setText(sharedPreferences.getString("Name", "没有此用户"));

int age= sharedPreferences.getInt("Age", 0);

String msg= Integer.toString(age);

msgAge.setText(msg );

}

注：TextView.SetText()这能设置String类型

把整形转化成String类型两种常用方式

Int i=10；

String msg= Integer.toString(i);

String xx = String.valueOf(1);

File存储

FileOutStream(向文件写数据)

package com.example.hxfile;

import java.io.FileNotFoundException;

import java.io.FileOutputStream;

import java.io.PrintStream;

public class MainActivity extends Activity {

private static final String FILENAME="file.txt";

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

FileOutputStream out=null;

try {

out =openFileOutput(FILENAME, Activity.MODE_PRIVATE);

} catch (FileNotFoundException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

PrintStream output= new PrintStream(out);

output.print("北京中软国际");

output.print("哈尔滨学院");

}

FileInputStream(从文件中读取数据)

package com.example.hxfile;

public class OtherActivity extends Activity {

private TextView showmsg =null;

private static final String FILENAME="file.txt";

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_other);

showmsg=(TextView)findViewById(R.id.msg);

FileInputStream input=null;

try {

input=openFileInput(FILENAME);

} catch (FileNotFoundException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

Scanner scanner = new Scanner(input);

while(scanner.hasNext()){

showmsg.append(scanner.next()+"\n");

}

JSON

JSON 常用的几个对象 JSONObject ,JSONArray(), JSONStringer和JSONException

Json数据格式

Json对象是一个无序的“名称/值”对的集合

如1{“name”：“郭庆雨”，

“age”：18}

如2 {“syudent“ [

{ “name”:”郭庆雨” ，“age“：100}

{ “ name”:”张三”,”age”:100}

] }

JSONObject allData = new JSONObject();

allData.put(“name“，”郭庆雨“)

//用put 方法（Key，value）形式添加和Map集合是一样的

allData.toString();

//转化成JSONObject对象转化成String 类型

//JSONObject 存储是一个对象

实例一：举例说明什么是JSONObject

JSONObject alljson = new JSONObject();

try {

alljson.put("person", "郭庆雨");

System.out.println(alljson.toString());

} catch (JSONException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

结果 {“person”,”郭庆雨”}

实例二：举例说明什么是JSONObject和JSONArray

String data []={"郭庆雨","18","北京"};

JSONObject alljson = new JSONObject();

JSONArray sing = new JSONArray();

for(int x=0;x<data.length;x++){

JSONObject msg = new JSONObject();

try {

msg.put("name",data[x] );

} catch (JSONException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

sing.put(msg);

}

try {

alljson.put("url", sing);

} catch (JSONException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

System.out.println(alljson);

结果：{"person":[{"name":"郭庆雨"},{"name":"文章"},{"name":"同大卫"}]}

实例三：开发常用实例

一个Json工具包 JsonTooles 一个实体类工具包Person 一个JSON服务包JsonServices 一个Activity包jsonActivity

JsonTooles 包

public class JsonTooles {

public static String createJsonString (String key ,Object object){

//解析其他的Object 类型的数据除了List内含有对象的数据

JSONObject allDataString = new JSONObject();

try {

allDataString.put(key, object);

} catch (JSONException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

return allDataString.toString();

}

public String createJsonList(String key ,List<Person> list) {

//获取List内部是一个对象

JSONObject allData = new JSONObject();

JSONArray sing= new JSONArray();

Person person=null;

for(int x=0;x<list.size();x++){

person =list.get(x);

JSONObject msg = new JSONObject();

try {

msg.put("name", person.getName());

msg.put("age", person.getAge());

msg.put("address", person.getAddress());

sing.put(msg);

allData.put(key, sing) ;

} catch (JSONException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

return allData.toString();

}

JSON 后台解析

MainActivity

public class MainActivity extends Activity {

private Button preson,presons,listString ,listMap;

private static final String TAG ="MainActivity";

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

preson= (Button)findViewById(R.id.person);

//获取文本服务器的personJson

preson.setOnClickListener(new OnPersonListenner());

}

class OnPersonListenner implements OnClickListener{

@Override

public void onClick(View v) {

// TODO Auto-generated method stub

new Thread(){

public void run() {

HttpTooles httpTooles = new HttpTooles();

String jsonString=httpTooles.getJsonContent("http://192.168.1.108:8080/jsonProject2/Action?type=person");

JsonTooles jsonTooles = new JsonTooles();

Person person= jsonTooles.getPerson(jsonString);

//System.out.println(str);

System.out.println(person.getAddress());

System.out.println(person.getAge());

System.out.println(person.getName());

Log.i(TAG, person.toString());

};

}.start();

}

HttpTooles

public class HttpTooles {

public HttpTooles() {

super();

// TODO Auto-generated constructor stub

}

public static String getJsonContent(String urlpath){

String str=null;

try {

URL url = new URL(urlpath);

HttpURLConnection httpURLConnection = (HttpURLConnection)url.openConnection();

httpURLConnection.setConnectTimeout(3000);

httpURLConnection.setRequestMethod("GET");

//httpURLConnection.getContentEncoding();

httpURLConnection.setDoInput(true);//允许读服务器内容

int code =httpURLConnection.getResponseCode();

if(code==200){

//当这个状态码是200的时候说明服务器状态已经准备好了可以获得一个流

InputStream in= httpURLConnection.getInputStream();

BufferedReader buf =

new BufferedReader(new InputStreamReader(new BufferedInputStream(in)));

str =buf.readLine();

}

} catch (MalformedURLException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}catch(IOException e){

e.printStackTrace();

}

return str;

}

JsonTooles

public class JsonTooles {

public JsonTooles() {

super();

// TODO Auto-generated constructor stub

}

public static Person getPerson(String jsonString){

Person person = new Person();

try {

JSONObject jsonperson = new JSONObject(jsonString);

JSONObject personObject = jsonperson.getJSONObject("person");

person.setAddress(personObject.getString("address"));

person.setAge(personObject.getInt("age"));

person.setName(personObject.getString("name"));

} catch (JSONException e) {

// TODO Auto-generated catch block

e.printStackTrace();

}

return person;

}

实体类工具包Person

public class Person {

private String name ;

private int age ;

private String address;

public Person() {

super();

// TODO Auto-generated constructor stub

}

public Person(String name, int age, String address) {

this.name = name;

this.age = age;

this.address = address;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

public String getAddress() {

return address;

}

public void setAddress(String address) {

this.address = address;

}

@Override

public String toString() {

return "Person [name=" + name + ", age=" + age + ", address=" + address

+ "]";

}

JSON服务包JsonServices

public class JsonService {

public JsonService() {

}

public Person getPerson(){

//含有一个对象

Person person = new Person("郭庆雨", 18,"北京");

return person;

}

public List <Person> getListperson(){

//含有多个对象的List集合

List<Person> list = new ArrayList<Person>();

Person person = new Person("郭庆雨", 18,"北京");

Person person1 = new Person("赵薇", 18,"南京");

Person person2 = new Person("文章", 18,"天津");

list.add(person);

list.add(person1);

list.add(person2);

return list;

}

public List <String >getListString(){

//普通的List集合

List<String> list = new ArrayList<String>();

list.add("北京");

list.add("上海");

list.add("南京");

return list;

}

public List<Map<String ,Object>> getListMaps(){

//list集合的Map集合

List <Map<String ,Object>> list = new ArrayList<Map<String,Object>>();

Map<String,Object> map= new HashMap<String,Object>();

map.put("地址", "北京");

Map<String,Object> map2= new HashMap<String,Object>();

map2.put("地址", "北京");

list.add(map);

list.add(map2);

return list;

}

Activity包jsonActivity

public class MainActivity extends Activity {

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

JsonService service = new JsonService();

List <Person> list=service.getListperson();

JsonTooles jsontooles = new JsonTooles();

String str= jsontooles.createJsonString("persons",list);

System.out.println(str);

}

@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;

}

Intent

认识Intent

Intent传值

Intent值的回调

Intent深入

打开网页

Activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="fill_parent"

android:layout_height="fill_parent"

android:orientation="vertical" >

<Button

android:id="@+id/mybut"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="打开网页" />

</LinearLayout>

MainActivity

package com.example.intentdemo2;

import android.net.Uri;

import android.os.Bundle;

import android.app.Activity;

import android.content.Intent;

import android.view.Menu;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

public class MainActivity extends Activity {

private Button but ;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

but=(Button)findViewById(R.id.mybut);

but.setOnClickListener(new OnClickListenerImp());

}

class OnClickListenerImp implements OnClickListener{

@Override

public void onClick(View v) {

// TODO Auto-generated method stub

Uri uri = Uri.parse("http://www.baidu.com");

Intent intent = new Intent();

intent.setAction(Intent.ACTION_VIEW);

intent.setData(uri);

MainActivity.this.startActivity(intent);

}

Intent打电话

案例一：调用拨号键

MainActivity

package com.example.intentdemo4;

import android.net.Uri;

import android.os.Bundle;

import android.app.Activity;

import android.content.Intent;

import android.view.Menu;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

import android.widget.EditText;

public class MainActivity extends Activity {

private Button but;

private EditText tel;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

but=(Button)findViewById(R.id.myBut);

tel=(EditText)findViewById(R.id.tel);

but.setOnClickListener(new OnClickListener() {

@Override

public void onClick(View v) {

// TODO Auto-generated method stub

String telStr= MainActivity.this.tel.getText().toString();

Uri uri=Uri.parse("tel:"+telStr);

Intent intent= new Intent();

intent.setAction(Intent.ACTION_DIAL);

intent.setData(uri);

MainActivity.this.startActivity(intent);

}

});

}

Activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="fill_parent"

android:layout_height="fill_parent"

android:orientation="vertical" >

<EditText

android:id="@+id/tel"

android:layout_width="match_parent"

android:layout_height="wrap_content"

</EditText>

<Button

android:id="@+id/myBut"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="打电话" />

</LinearLayout>

AndroidManifest.xml

<uses-permission android:name="android.permission.CALL_PHONE"/>

案例二：直接打电话

MainActivity

package com.example.intentdemo4;

import android.net.Uri;

import android.os.Bundle;

import android.app.Activity;

import android.content.Intent;

import android.view.Menu;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

import android.widget.EditText;

public class MainActivity extends Activity {

private Button but;

private EditText tel;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

but=(Button)findViewById(R.id.myBut);

tel=(EditText)findViewById(R.id.tel);

but.setOnClickListener(new OnClickListener() {

@Override

public void onClick(View v) {

// TODO Auto-generated method stub

String telStr= MainActivity.this.tel.getText().toString();

Uri uri=Uri.parse("tel:"+telStr);

Intent intent= new Intent();

intent.setAction(Intent.ACTION_CALL);

intent.setData(uri);

MainActivity.this.startActivity(intent);

}

});

}

Activity_main.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="fill_parent"

android:layout_height="fill_parent"

android:orientation="vertical" >

<EditText

android:id="@+id/tel"

android:layout_width="match_parent"

android:layout_height="wrap_content"

</EditText>

<Button

android:id="@+id/myBut"

android:layout_width="wrap_content"

android:layout_height="wrap_content"

android:text="直接打电话" />

</LinearLayout>

AndroidManifest.xml

<uses-permission android:name="android.permission.CALL_PHONE"/>

Activity生命周期

什么是Activity

Activity 实际上是一个人机的交互程序，用于存放各个显示控件，也是Android的基本组成部分

每一个继承了android.app.Activity的java类都将成为一个Activity程序。

创建Activity的要点

创建Activity的要点

1，一个Activity就是一个类，并且这个类要继承Activity

2，需要复写Oncreate 方法

3，每一个Activity都需要在AndroidManifest.xml 文件当中进行配置

4，为Activity添加必要的控件

Activit七个生命周期函数

Public class Activity extends ApplicationContext{

Protected void onCreate ()

一个不可以关闭的方法；当Activity程序启动之后会首次调用此方法。

Protected void onStart ()

一个不可以关闭的方法；当用户第一次显示界面时调用此方法。

Protected void onRestart ()

一个不可以关闭的方法；当Activity 程序停止后再次显示给用户是调用。

Protected void onResume ()

一个不可以关闭的方法；当获得用户焦点时候调用此方法

Protected void onPause ()

一个可以关闭的方法；当启动其他Activity程序时调用此方法用于进行数据的提交，动画处理等操作

Protected void onStop ()

一个可以关闭的方法；当一个Activity程序完全不可见时调用此方法，此时并不会销毁Activity程序

Protected void onDestroy ()

一个可以关闭的方法；程序被销毁时调用，当调用finish（）方法或系统资源不够用时将调用此方法。

}

Activity流程图

案例一：

Mainactivity

package com.example.gdactivity2;

import android.os.Bundle;

import android.app.Activity;

import android.content.Intent;

import android.view.Menu;

import android.view.View;

import android.view.View.OnClickListener;

import android.widget.Button;

public class MainActivity extends Activity {

private Button start;

@Override

protected void onCreate(Bundle savedInstanceState) {

super.onCreate(savedInstanceState);

setContentView(R.layout.activity_main);

System.out.println("FriestActivity------->onCreate()");

* 第一次创建时候加载整个组件*/

start=(Button)findViewById(R.id.starOtherActivity);

start.setOnClickListener(new ButttonClick());

}

class ButttonClick implements OnClickListener{

@Override

public void onClick(View v) {

// TODO Auto-generated method stub

Intent intent = new Intent();

intent.setClass(MainActivity.this,OtherActivity.class);

MainActivity.this.startActivity(intent);

}

@Override

protected void onDestroy() {

// TODO Auto-generated method stub

System.out.println("FriestActivity------->onDestroy()");

super.onDestroy();

* 当调用finish（）方法或系统资源不够用时将调用此方法**/

}

@Override

protected void onPause() {

// TODO Auto-generated method stub

System.out.println("FriestActivity------->onPause()");

* 暂停当前方法

* **/

super.onPause();

}

@Override

protected void onRestart() {

// TODO Auto-generated method stub

System.out.println("FriestActivity------->onRestart()");

super.onRestart();

* 当Activity重新被调用时候用此方法**/

}

@Override

protected void onResume() {

// TODO Auto-generated method stub

System.out.println("FriestActivity------->onResume() ");

* 当用户在点击或是使用组件时候获取焦点

* **/

super.onResume();

}

@Override

protected void onStart() {

// TODO Auto-generated method stub

System.out.println("FriestActivity------->onStart() ");

* 当用户第一次看见模拟器出现组件时候被调用

* **/

super.onStart();

}

@Override

protected void onStop() {

// TODO Auto-generated method stub

System.out.println("FriestActivity------->onStop()");

/*当Activity全部不可见状态是调用此方法

* **/

super.onStop();

}

OtherActivity

package com.example.gdactivity2;

import android.app.Activity;

import android.os.Bundle;

public class OtherActivity extends Activity {

@Override

protected void onCreate(Bundle savedInstanceState) {

// TODO Auto-generated method stub

super.onCreate(savedInstanceState);

setContentView(R.layout.other);

System.out.println("OtherActivity------->onCreate()");

}

@Override

protected void onDestroy() {

// TODO Auto-generated method stub

super.onDestroy();

System.out.println("OtherActivity------->onDestroy()");

}

@Override

protected void onPause() {

// TODO Auto-generated method stub

super.onPause();

System.out.println("OtherActivity------->onPause()");

}

@Override

protected void onRestart() {

// TODO Auto-generated method stub

super.onRestart();

System.out.println("OtherActivity------->onRestart()");

}

@Override

protected void onResume() {

// TODO Auto-generated method stub

super.onResume();

System.out.println("OtherActivity------->onResume()");

}

@Override

protected void onStart() {

// TODO Auto-generated method stub

super.onStart();

System.out.println("OtherActivity------->onStart()");

}

@Override

protected void onStop() {

// TODO Auto-generated method stub

super.onStop();

System.out.println("OtherActivity------->onStop()");

}

AndroidManifest

<application

android:allowBackup="true"

android:icon="@drawable/ic_launcher"

android:label="@string/app_name"

android:theme="@style/AppTheme" >

<activity

android:name="com.example.gdactivity4.MainActivity"

android:label="@string/app_name" >

<intent-filter>

</intent-filter>

</activity>

<activity

android:name="com.example.gdactivity4.SecodActivity"

android:label="@string/title_activity_secod"

android:theme="@android:style/Theme.Dialog"

</activity>

</application>

运行本节案例一

操作一：首先启动程序

结果一：启动了FirstActivity的onCreate onStart onResume

操作二：点击启动第二个Activity

结果二：1： FirstActivity的onPause ，2：SecondActivity onCreate onStart onResume，3：FirstActivity onStop

操作三：点击back按钮的时候

结果三：SecondActivity onPause FirstActivity onRestart onStart onResume SecondActivity onStop onDestory

注意：什么时候调用onDestroy方法

1 在一个Activity调用了finish（）方法

（此效果在下一节案例1给大家展示）

2 资源不够或是系统认为此Activity没有用的时候系统调用onDestroy方法