activity执行onDestroy后最大化回收内存

public class ActivityReleaseMemoryUtil {
    private static final ActivityReleaseMemoryUtil ourInstance = new ActivityReleaseMemoryUtil();

    public static ActivityReleaseMemoryUtil getInstance() {
        return ourInstance;
    }

    private ActivityReleaseMemoryUtil() {
    }
    public void setContentViewNull(Activity activity){
         //一个空的FrameLayout布局
        View layoutNull = LayoutInflater.from(activity).inflate(R.layout.view_null, null);
        activity.setContentView(layoutNull);
        if(null!=layoutNull)layoutNull = null;
    }
    public void clearViewGroups(ViewGroup...viewGroups){
        for (ViewGroup vg :
                viewGroups) {
            if (null != vg){
                vg.removeAllViews();
                vg = null;
            }
        }
    }
    public void clearList(List...lists){
        for (List list :
                lists) {
            if (null != list) {
                list.clear();
                list = null;
            }
        }
    }
    //viewpager recyclerview listview,要设置setAdapter(null);后再置为空等等等......

}