英文:
Where should I initialize my switch widget Android
问题
我在我的应用程序中有一个选项菜单,在其中我有一个选项,它打开一个包含一些小部件的对话框。我无法弄清楚在哪里初始化我的小部件,以便它不会产生空指针异常。
我应该在哪里放置 Switch sw = findViewById(R.id.switch1)
,以便在对话框打开之前可以使用 sw.setChecked(<在代码中确定的条件>)
。
MainActivity.java:
package com.example
//导入语句
public class MainActivity extends Activity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
switch (item.getItemId())
{
case R.id.option:
// sw.setChecked(true); // 这里的代码会导致问题
Dialog dialog = new Dialog(this);
dialog.setContentView(R.layout.dialog);
dialog.show();
default:
return false;
}
}
}
在OnCreate()中调用sw.setChecked()时的错误日志:
09-04 16:38:48.358 2395 2395 E AndroidRuntime FATAL EXCEPTION: main
09-04 16:38:48.358 2395 2395 E AndroidRuntime Process: com.PjMathematician.ImgMath, PID: 2395
09-04 16:38:48.358 2395 2395 E AndroidRuntime java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.PjMathematician.ImgMath/com.PjMathematician.ImgMath.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
...
当我注释掉 sw.setChecked(true)
语句时,此错误消失。
英文:
I have an options menu in my app, and in that I have an option which opens a dialog box which has some widgets. I am not being able to figure out where should I initialize my widgets so that it doesnt give a null pointer exception.
Where should I put Switch sw = findViewById(R.id.switch1)
so that i can use sw.setChecked(<condition gets decided in the code>)
before the dialog box opens.
MainActivity.java:
package com.example
//imports
public class MainActivity extends Activity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
switch (item.getItemId())
{
case R.id.option:
sw.setChecked(true);
Dialog dialog = new Dialog(this);
dialog.setContentView(R.layout.dialog);
dialog.show();
default:
return false;
}
}
Error log when i call sw.setChecked inside OnCreate()
-04 16:38:48.358 2395 2395 E AndroidRuntime FATAL EXCEPTION: main
09-04 16:38:48.358 2395 2395 E AndroidRuntime Process: com.PjMathematician.ImgMath, PID: 2395
09-04 16:38:48.358 2395 2395 E AndroidRuntime java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.PjMathematician.ImgMath/com.PjMathematician.ImgMath.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2855)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3093)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1816)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.os.Handler.dispatchMessage(Handler.java:106)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.os.Looper.loop(Looper.java:193)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.ActivityThread.main(ActivityThread.java:6865)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at java.lang.reflect.Method.invoke(Native Method)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:504)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
09-04 16:38:48.358 2395 2395 E AndroidRuntime Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.view.Window.findViewById(int)' on a null object reference
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.Activity.findViewById(Activity.java:4125)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at com.PjMathematician.ImgMath.MainActivity.<init>(MainActivity.java:307)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at java.lang.Class.newInstance(Native Method)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:69)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.Instrumentation.newActivity(Instrumentation.java:1231)
09-04 16:38:48.358 2395 2395 E AndroidRuntime at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2842)
09-04 16:38:48.358 2395 2395 E AndroidRuntime ... 11 more
This error disappears when I comment out the sw.setChecked(true) statement
答案1
得分: 0
主活动
package com.example;
//导入语句
public class MainActivity extends Activity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
//在这里初始化开关
Switch sw = findViewById(R.id.switch1);
if(sharedPreferenceObj == 你的逻辑){
//在这里放置你的业务逻辑
}else{
//如果条件为假,则在这里执行操作
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
switch (item.getItemId())
{
case R.id.option:
sw.setChecked(true);
Dialog dialog = new Dialog(this);
dialog.setContentView(R.layout.dialog);
dialog.show();
default:
return false;
}
}
}
英文:
You should do this.
Main Activity
package com.example
//imports
public class MainActivity extends Activity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
//initialize the switch here
Switch sw = findViewById(R.id.switch1);
if(sharedPreferenceObj == your logic){
//your business logic here
}else{
//if condition is false then do stuff here
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
switch (item.getItemId())
{
case R.id.option:
sw.setChecked(true);
Dialog dialog = new Dialog(this);
dialog.setContentView(R.layout.dialog);
dialog.show();
default:
return false;
}
}
答案2
得分: 0
我认为这会起作用,因为Switch视图位于对话框内,您应该使用dialog.findViewById(R.id.switch1)来获取其实例;
```java
package com.example
//导入
public class MainActivity extends Activity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
switch (item.getItemId())
{
case R.id.option:
sw.setChecked(true);
Dialog dialog = new Dialog(this);
dialog.setContentView(R.layout.dialog);
Switch sw = dialog.findViewById(R.id.switch1);
sw.setChecked(condition);
dialog.show();
default:
return false;
}
}
英文:
I think this will work, since the Switch view is inside the dialog box, you should get its instance using dialog.findViewById(R.id.switch1);
package com.example
//imports
public class MainActivity extends Activity
{
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
}
@Override
public boolean onCreateOptionsMenu(Menu menu)
{
MenuInflater inflater = getMenuInflater();
inflater.inflate(R.menu.main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item)
{
switch (item.getItemId())
{
case R.id.option:
sw.setChecked(true);
Dialog dialog = new Dialog(this);
dialog.setContentView(R.layout.dialog);
Switch sw = dialog.findViewById(R.id.switch1);
sw.setChecked(condition);
dialog.show();
default:
return false;
}
}
通过集体智慧和协作来改善编程学习和解决问题的方式。致力于成为全球开发者共同参与的知识库,让每个人都能够通过互相帮助和分享经验来进步。
评论