NullPointerException空指針異常——沒有事先載入佈局文件到acitivy——缺少：setContentView(R.layout.activity_setup_over);

空指針異常：

04-27 01:13:57.270: E/AndroidRuntime(4942): FATAL EXCEPTION: main
04-27 01:13:57.270: E/AndroidRuntime(4942): Process: com.itheima.mobilesafe74, PID: 4942
04-27 01:13:57.270: E/AndroidRuntime(4942): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.itheima.mobilesafe74/com.itheima.mobilesafe74.activity.SetupOverActivity}: java.lang.NullPointerException
04-27 01:13:57.270: E/AndroidRuntime(4942): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
04-27 01:13:57.270: E/AndroidRuntime(4942): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
04-27 01:13:57.270: E/AndroidRuntime(4942): at android.app.ActivityThread.access$800(ActivityThread.java:135)
04-27 01:13:57.270: E/AndroidRuntime(4942): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
04-27 01:13:57.270: E/AndroidRuntime(4942): at android.os.Handler.dispatchMessage(Handler.java:102)
04-27 01:13:57.270: E/AndroidRuntime(4942): at android.os.Looper.loop(Looper.java:136)
04-27 01:13:57.270: E/AndroidRuntime(4942): at android.app.ActivityThread.main(ActivityThread.java:5017)
04-27 01:13:57.270: E/AndroidRuntime(4942): at java.lang.reflect.Method.invokeNative(Native Method)
04-27 01:13:57.270: E/AndroidRuntime(4942): at java.lang.reflect.Method.invoke(Method.java:515)
04-27 01:13:57.270: E/AndroidRuntime(4942): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
04-27 01:13:57.270: E/AndroidRuntime(4942): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
04-27 01:13:57.270: E/AndroidRuntime(4942): at dalvik.system.NativeStart.main(Native Method)
04-27 01:13:57.270: E/AndroidRuntime(4942): Caused by: java.lang.NullPointerException
04-27 01:13:57.270: E/AndroidRuntime(4942): at com.itheima.mobilesafe74.activity.SetupOverActivity.initUI(SetupOverActivity.java:46)
04-27 01:13:57.270: E/AndroidRuntime(4942): at com.itheima.mobilesafe74.activity.SetupOverActivity.onCreate(SetupOverActivity.java:38)
04-27 01:13:57.270: E/AndroidRuntime(4942): at android.app.Activity.performCreate(Activity.java:5231)
04-27 01:13:57.270: E/AndroidRuntime(4942): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
04-27 01:13:57.270: E/AndroidRuntime(4942): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)

相應代碼：

 1 package com.itheima.mobilesafe74.activity;
 2 
 3 import com.itheima.mobilesafe74.R;
 4 import com.itheima.mobilesafe74.utils.ConstentValue;
 5 import com.itheima.mobilesafe74.utils.SpUtil;
 6 import android.app.Activity;
 7 import android.content.Intent;
 8 import android.os.Bundle;
 9 import android.view.View;
10 import android.view.View.OnClickListener;
11 import android.widget.TextView;
12 
13 /**
14  * 說明 :手機安全導航設置完成的界面
15  */
16 public class SetupOverActivity extends Activity
17 {
18     @Override
19     protected void onCreate(Bundle savedInstanceState)
20     {
21         super.onCreate(savedInstanceState);
22 //        setContentView(R.layout.activity_setup_over);
23         boolean setup_over = SpUtil.getBoolean(this, ConstentValue.SETUP_OVER,
24                 false);
25         if (setup_over)
26         {
27             // 手機安全導航設置完成跳轉到設置完成的界面
28             setContentView(R.layout.activity_setup_over);
29         } else
30         {
31             // 手機安全導航設置沒有完成跳轉到導航設置1的界面
32             Intent intent = new Intent(this, Setup1Activity.class);
33             startActivity(intent);
34             // 跳轉到一個新的界面時需要關閉當前界面
35             finish();
36         }
37         initUI();
38     }
39 
40     private void initUI()
41     {
42 
43         TextView tv_safe_number = (TextView) findViewById(R.id.tv_safe_number);
44         String phone = SpUtil.getString(this, ConstentValue.CONTACT_PHONE, "");
45         tv_safe_number.setText(phone);
46         // 重新進入設置嚮導textview點擊偵聽
47         TextView tv_reset_setting = (TextView) findViewById(R.id.tv_reset_setting);
48             tv_reset_setting.setOnClickListener(new OnClickListener()
49             {
50 
51                 @Override
52                 public void onClick(View v)
53                 {
54                     // 手機安全導航設置沒有完成跳轉到導航設置1的界面
55                     Intent intent = new Intent(getApplicationContext(),
56                             Setup1Activity.class);
57                     startActivity(intent);
58                     // 跳轉到一個新的界面時需要關閉當前界面
59                     finish();
60                 }
61             });
62     }
63 }

分析：由於調用initUI(),方法之前沒有載入佈局文件setContentView(R.layout.activity_setup_over);

導致後面45行tv_safe_number和48行tv_reset_setting通過findviewbyid方法調用時報出空指針

解決方案1：在protected void onCreate(Bundle savedInstanceState)方法體中添加setContentView(R.layout.activity_setup_over)，

解決方案2：在45行tv_safe_number和48行tv_reset_setting用到這兩個指針是做if（tv_safe_number！=null）判斷