我應該在哪里初始化我的開關小部件 Android

Question

我的應用程序中有一個選項菜單，其中有一個選項可以打開一個包含一些小部件的對話框。 我無法弄清楚我應該在哪里初始化我的小部件，以便它不會給出空指針異常。

我應該把Switch sw = findViewById(R.id.switch1)放在哪里，以便我可以在對話框打開之前使用sw.setChecked(<condition gets decided in the code>) 。

主活動.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;
        }
    }

        

當我在 OnCreate() 中調用 sw.setChecked 時出現錯誤日志

-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

當我注釋掉 sw.setChecked(true) 語句時，此錯誤消失

Answer 1

你應該做這個。

主要活動

 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;
        }
    }

Answer 2

我認為這會起作用，因為 Switch 視圖在對話框內，你應該使用 dialog.findViewById(R.id.switch1); 獲取它的實例。

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;
        }
    }