java - Android Studio 中的空指针

Question

您好，由于某种原因，我的项目在启动模拟器时有一个空指针，我不确定原因是什么。这是我收到的错误:

W/System.err: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setBackgroundDrawable(android.graphics.drawable.Drawable)' on a null object reference
W/System.err:     at com.panda.cleaner_batterysaver.ui.MainActivity.applyKitKatTranslucency(MainActivity.java:201)
W/System.err:     at com.panda.cleaner_batterysaver.ui.MainActivity.onCreate(MainActivity.java:83)

代码部分:

private void applyKitKatTranslucency() {

        // KitKat translucent navigation/status bar.
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
            setTranslucentStatus(true);
            SystemBarTintManager mTintManager = new SystemBarTintManager(this);
            mTintManager.setStatusBarTintEnabled(true);
            mTintManager.setNavigationBarTintEnabled(true);
            // mTintManager.setTintColor(0xF00099CC);

            mTintManager.setTintDrawable(UIElementsHelper
                    .getGeneralActionBarBackground(this));

            getActionBar().setBackgroundDrawable(
                    UIElementsHelper.getGeneralActionBarBackground(this));

        }

    }

调用该方法的可绘制文件:

  public class UIElementsHelper {

    private static final String NOW_PLAYING_COLOR = "NOW_PLAYING_COLOR";
    private static final String BLUE = "BLUE";
    private static final String RED = "RED";
    private static final String GREEN = "GREEN";
    private static final String ORANGE = "ORANGE";
    private static final String PURPLE = "PURPLE";
    private static final String MAGENTA = "MAGENTA";
    private static final String GRAY = "GRAY";
    private static final String WHITE = "WHITE";
    private static final String BLACK = "BLACK";

    /**
     * Returns the ActionBar color based on the selected color theme (not used
     * for the player).
     */
    public static Drawable getGeneralActionBarBackground(Context context) {
        final SharedPreferences settings = PreferenceManager
                .getDefaultSharedPreferences(context);

        Drawable drawable = new ColorDrawable(0xFF9acd32);


        return drawable;

    }

}

Answer 1

在默认的 styles.xml 文件中，检查您正在使用的主题。如果您创建空 Activity ，则默认主题是:

<style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">

但是，您可能使用过如下内容:

<style name="AppTheme" parent="Theme.AppCompat.Light.NoActionBar">

在这种情况下，您需要在 onCreate 中手动设置操作栏:

// Assuming you have a toolbar in your xml layout with id toolbar
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);

setSupportActionBar(toolbar);
ActionBar actionBar = getSupportActionBar();
if (actionBar != null) {
    actionBar.setBackgroundDrawable(
                UIElementsHelper.getGeneralActionBarBackground(this));
}

如果您不使用 AppCompat，那么您将需要使用 getActionBar 和适当的主题。