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分享关于android activity界面切换
小白一枚，最近在自学android，在activity界面切换时遇到了些问题，自行百度了1天没找到相应的内容。
话不多说，上代码。
1.activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
tools:context="com.demo.MainActivity">
<Button
android:id="@+id/btn1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="跳转1"/>
<com.demo.UnderMeun
android:id="@+id/underMeun"
android:layout_width="match_parent"
android:layout_height="wrap_content">
</com.demo.UnderMeun>
</LinearLayout>
2.MainActivity.java
public class MainActivity extends AppCompatActivity {
private Button btn1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btn1 = (Button) findViewById(R.id.btn1);
btn1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Intent intent = new Intent();
intent.setClass( MainActivity.this, ClockActivity.class);
MainActivity.this.startActivity(intent);
}
});
}
}
3.UnderMeun.java
public class UnderMeun extends LinearLayout {
private MainActivity mainActivity;
public UnderMeun(Context context) {
super(context);
initUnderMeun();
}
public UnderMeun(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
initUnderMeun();
}
public UnderMeun(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
initUnderMeun();
}
/**
* 初始化底部菜单栏
*/
private void initUnderMeun() {
// 横向布局，纵向排列 从左往右
setOrientation(LinearLayout.HORIZONTAL);
// 设置背景色
setBackgroundColor(Color.parseColor("#00CACA"));
// 添加菜单选项 -- 跳转按钮
Button btn2 = new Button(getContext());
btn2.setText("跳转2");
btn2.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View view) {
mainActivity = new MainActivity();
Intent intent = new Intent();
intent.setClass( mainActivity, ClockActivity.class);
mainActivity.startActivity(intent);
}
});
addView(btnClock);
}
}
4.AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.demo">
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<!-- icon:app图标
label:app名称 -->
<activity android:name=".MainActivity"
android:screenOrientation="portrait">
<!-- screenOrientation=portrait 禁止横屏 -->
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".clock.ClockActivity"
android:label="This is clockView" >
</activity>
</application>
</manifest>
代码贴完了，下面说下问题：
共有两个activity界面，main_activity.xml和clock.xml，main_activity的代码贴出来了，clock的就不贴了，里面一片空白。
然后问题就是：MainActivity.java类中的btn1可以跳转到clock界面，而UnderMeun.java类中的btn2无法跳转，报以下的错误。
error：
08-22 10:43:33.793 18136-18136/com.game2048 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.demo, PID: 18136
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ContextWrapper.getPackageName(ContextWrapper.java:142)
at android.content.ComponentName.<init>(ComponentName.java:128)
at android.content.Intent.setClass(Intent.java:7984)
at com.demo.UnderMeun$2.onClick(UnderMeun.java:42)
at android.view.View.performClick(View.java:5611)
at android.view.View$PerformClick.run(View.java:22276)
at android.os.Handler.handleCallback(Handler.java:754)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:160)
at android.app.ActivityThread.main(ActivityThread.java:6275)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:877)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
错误位置我已用红色字体标注，求大佬来看看什么情况，指导知道。
话不多说，上代码。
1.activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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:orientation="vertical"
tools:context="com.demo.MainActivity">
<Button
android:id="@+id/btn1"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="跳转1"/>
<com.demo.UnderMeun
android:id="@+id/underMeun"
android:layout_width="match_parent"
android:layout_height="wrap_content">
</com.demo.UnderMeun>
</LinearLayout>
2.MainActivity.java
public class MainActivity extends AppCompatActivity {
private Button btn1;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
btn1 = (Button) findViewById(R.id.btn1);
btn1.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Intent intent = new Intent();
intent.setClass( MainActivity.this, ClockActivity.class);
MainActivity.this.startActivity(intent);
}
});
}
}
3.UnderMeun.java
public class UnderMeun extends LinearLayout {
private MainActivity mainActivity;
public UnderMeun(Context context) {
super(context);
initUnderMeun();
}
public UnderMeun(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
initUnderMeun();
}
public UnderMeun(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
initUnderMeun();
}
/**
* 初始化底部菜单栏
*/
private void initUnderMeun() {
// 横向布局，纵向排列 从左往右
setOrientation(LinearLayout.HORIZONTAL);
// 设置背景色
setBackgroundColor(Color.parseColor("#00CACA"));
// 添加菜单选项 -- 跳转按钮
Button btn2 = new Button(getContext());
btn2.setText("跳转2");
btn2.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View view) {
mainActivity = new MainActivity();
Intent intent = new Intent();
intent.setClass( mainActivity, ClockActivity.class);
mainActivity.startActivity(intent);
}
});
addView(btnClock);
}
}
4.AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.demo">
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<!-- icon:app图标
label:app名称 -->
<activity android:name=".MainActivity"
android:screenOrientation="portrait">
<!-- screenOrientation=portrait 禁止横屏 -->
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<activity
android:name=".clock.ClockActivity"
android:label="This is clockView" >
</activity>
</application>
</manifest>
代码贴完了，下面说下问题：
共有两个activity界面，main_activity.xml和clock.xml，main_activity的代码贴出来了，clock的就不贴了，里面一片空白。
然后问题就是：MainActivity.java类中的btn1可以跳转到clock界面，而UnderMeun.java类中的btn2无法跳转，报以下的错误。
error：
08-22 10:43:33.793 18136-18136/com.game2048 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.demo, PID: 18136
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ContextWrapper.getPackageName(ContextWrapper.java:142)
at android.content.ComponentName.<init>(ComponentName.java:128)
at android.content.Intent.setClass(Intent.java:7984)
at com.demo.UnderMeun$2.onClick(UnderMeun.java:42)
at android.view.View.performClick(View.java:5611)
at android.view.View$PerformClick.run(View.java:22276)
at android.os.Handler.handleCallback(Handler.java:754)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:160)
at android.app.ActivityThread.main(ActivityThread.java:6275)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:877)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
错误位置我已用红色字体标注，求大佬来看看什么情况，指导知道。
...全文
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江三木洛 2017-08-23
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private MainActivity mainActivity;
mainActivity = (MainActivity) context;
改了
private Activity mctivity;
mActivity = (Activity) context;
不要转成特定的Activity
名不副实的书山 2017-08-23
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@江三沐洛
这个问题解决了，但是又迎来了第二个后续问题。
就是如果在UnderMeun中使用getContext这个方法，那么这个自定义扩展控件就不能在被activity_main.xml引用的同时再被clock.xml界面所引用，会包如下错误。
error：
08-23 09:53:26.734 4605-4605/com.game2048 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.game2048, PID: 4605
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.game2048/com.game2048.clock.ClockActivity}: android.view.InflateException: Binary XML file line #14: Binary XML file line #14: Error inflating class <unknown>
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2715)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2776)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1525)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:160)
at android.app.ActivityThread.main(ActivityThread.java:6275)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:877)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Caused by: android.view.InflateException: Binary XML file line #14: Binary XML file line #14: Error inflating class <unknown>
Caused by: android.view.InflateException: Binary XML file line #14: Error inflating class <unknown>
Caused by: java.lang.reflect.InvocationTargetException
at java.lang.reflect.Constructor.newInstance0(Native Method)
at java.lang.reflect.Constructor.newInstance(Constructor.java:430)
at android.view.LayoutInflater.createView(LayoutInflater.java:652)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:812)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:752)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:883)
at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:846)
at android.view.LayoutInflater.inflate(LayoutInflater.java:522)
at android.view.LayoutInflater.inflate(LayoutInflater.java:430)
at android.view.LayoutInflater.inflate(LayoutInflater.java:377)
at com.android.internal.policy.PhoneWindow.setContentView(PhoneWindow.java:414)
at android.app.Activity.setContentView(Activity.java:2433)
at com.game2048.clock.ClockActivity.onCreate(ClockActivity.java:18)
at android.app.Activity.performCreate(Activity.java:6736)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1119)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2668)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2776)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1525)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:160)
at android.app.ActivityThread.main(ActivityThread.java:6275)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:877)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
Caused by: java.lang.ClassCastException: com.game2048.clock.ClockActivity cannot be cast to com.game2048.MainActivity
at com.game2048.UnderMeun.initUnderMeun(UnderMeun.java:46)
at com.game2048.UnderMeun.<init>(UnderMeun.java:29)
at java.lang.reflect.Constructor.newInstance0(Native Method)
at java.lang.reflect.Constructor.newInstance(Constructor.java:430)
at android.view.LayoutInflater.createView(LayoutInflater.java:652)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:812)
at android.view.LayoutInflater.createViewFromTag(LayoutInflater.java:752)
at android.view.LayoutInflater.rInflate(LayoutInflater.java:883)
at android.view.LayoutInflater.rInflateChildren(LayoutInflater.java:846)
at android.view.LayoutInflater.inflate(LayoutInflater.java:522)
at android.view.LayoutInflater.inflate(LayoutInflater.java:430)
at android.view.LayoutInflater.inflate(LayoutInflater.java:377)
at com.android.internal.policy.PhoneWindow.setContentView(PhoneWindow.java:414)
at android.app.Activity.setContentView(Activity.java:2433)
at com.game2048.clock.ClockActivity.onCreate(ClockActivity.java:18)
at android.app.Activity.performCreate(Activity.java:6736)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:877)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
PS：回复内容有限，一些底层的log被我删了，如果需要补全说一下，我再发
其实我的想法就是弄一个公共菜单栏供多个activity界面使用，这样既减少代码量又方便管理
方便的话加下我微信吧：smallerLife
江三木洛 2017-08-22
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能告诉我怎么解决吗？小白一只看不懂太深的东西，想先实现功能再研究原理。[/quote]
我贴的就是改好的
名不副实的书山 2017-08-22
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- 举报
能告诉我怎么解决吗？小白一只看不懂太深的东西，想先实现功能再研究原理。
江三木洛 2017-08-22
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- 举报
public class UnderMeun extends LinearLayout {
private MainActivity mainActivity;
public UnderMeun(Context context) {
super(context);
initUnderMeun(context);
}
public UnderMeun(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
initUnderMeun(context);
}
public UnderMeun(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
initUnderMeun(context);
}
/**
* 初始化底部菜单栏
*/
private void initUnderMeun(Context context) {
mainActivity = (MainActivity) context;
// 横向布局，纵向排列 从左往右
setOrientation(LinearLayout.HORIZONTAL);
// 设置背景色
setBackgroundColor(Color.parseColor("#00CACA"));
// 添加菜单选项 -- 跳转按钮
Button btn2 = new Button(getContext());
btn2.setText("跳转2");
btn2.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View view) {
Intent intent = new Intent();
intent.setClass(mainActivity, ClockActivity.class);
mainActivity.startActivity(intent);
}
});
addView(btn2);
}
}
直接new Activity是不行的，系统除了new之外，还做了不少额外的工作，你去看看Activity的attach方法
这里用它做参数是为了取得他的包名，但是由于你只new了，Activity里面有的参数没有被赋值，就是说那个应该调用getPackageName方法的对象没有值，为空，所以抛出了异常
