Android 从枚举中获取结果

Question

我正在尝试获取我在此处传入的枚举的值：

Intent intent = new Intent(MainActivity.this, SelectBrush.class);
                    Bundle bundle = new Bundle();
                    bundle.putString("Current brush", myFingerPainter.getBrush().name());
                    intent.putExtras(bundle);
                    startActivityForResult(intent, BRUSH_REQUEST_CODE );

我试着在第二个活动中把它弄出来：

  Paint.Cap brushStyle = Paint.Cap.valueOf(getIntent().getStringExtra("Current brush"));

这是我的日志：

    Process: com.example.myapplication, PID: 26592
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.myapplication/com.example.myapplication.SelectBrush}: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Intent.getStringExtra(java.lang.String)' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2567)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6119)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Intent.getStringExtra(java.lang.String)' on a null object reference
        at com.example.myapplication.SelectBrush.<init>(SelectBrush.java:17)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2557)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6119) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

我也试过

    Bundle bundle = getIntent().getExtras();
    String brushString = bundle.getString("Current brush");
    Paint.Cap brushStyle = Paint.Cap.valueOf(brushString);

但我遇到了类似的异常：

2020-10-31 11:55:38.921 28148-28148/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.myapplication, PID: 28148
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.myapplication/com.example.myapplication.SelectBrush}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.os.Bundle android.content.Intent.getExtras()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2567)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
        at android.app.ActivityThread.-wrap12(ActivityThread.java)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
        at android.os.Handler.dispatchMessage(Handler.java:102)
        at android.os.Looper.loop(Looper.java:154)
        at android.app.ActivityThread.main(ActivityThread.java:6119)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.os.Bundle android.content.Intent.getExtras()' on a null object reference
        at com.example.myapplication.SelectBrush.<init>(SelectBrush.java:16)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1078)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2557)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
        at android.app.ActivityThread.-wrap12(ActivityThread.java) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
        at android.os.Handler.dispatchMessage(Handler.java:102) 
        at android.os.Looper.loop(Looper.java:154) 
        at android.app.ActivityThread.main(ActivityThread.java:6119) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

我已经阅读过，我认为这是因为第二个活动认为 intent == null 但我不确定为什么，因为我显然是在第一堂课中创建它。

Answer 1

只需将这些行放在 onCreate 方法中：

    String brushString = bundle.getString("Current brush");
    brushStyle = Paint.Cap.valueOf(brushString);

这在班级层面：

Paint.Cap brushStyle;