在非 activity class 比较器中获取上下文

Question

我必须从我的一个实体 class 的比较器 class 访问我的数据库。为了实例化数据库连接，我需要一个应用程序上下文。

在这种情况下如何获取应用程序上下文：

实体 - class 带有比较器

static public class CowLastInspectionComparator implements Comparator<Cow> {
    public int compare(Cow cow1, Cow cow2) {
        DbAdapter adapter = DbAdapter.getInstance(new MainView().getApplicationContext()); //this doesn't work
        List<Inspection> inspectionsCow1 = adapter.getInspectionByCow(cow1);
        List<Inspection> inspectionsCow2 = adapter.getInspectionByCow(cow2);

        return inspectionsCow1.get(0).getJob().getJobDate().compareTo(inspectionsCow2.get(0).getJob().getJobDate());
    }
}

在 Activity 中排序：

 if (sortOption == "Name") Collections.sort(cows, new Cow.CowLastInspectionComparator());

Answer 1

I have to access my database from the comparator class from one of my entity-classes

如果您的意思是要在 compare() 中执行数据库 I/O，那将使您的 compare() 调用变慢，而您的 sort() 调用非常慢。如果你想对你的，嗯，奶牛进行排序，但它们并不都在内存中，请使用 ORDER BY 子句执行 single 数据库查询以正确排序它们。

For instancing the database connection I need an application context.

您需要 Context。它不一定必须是 Application.

How do I get the application context in this case

向 CowLastInspectionComparator 提供一个，例如通过构造函数参数。

Answer 2

static public class CowLastInspectionComparator implements Comparator<Cow> {
  private Context context;
  CowLastInspectionComparator(Context context){
    this.context=context;
  }
  public int compare(Cow cow1, Cow cow2) {
    DbAdapter adapter = DbAdapter.getInstance(context); //this doesn't work
    List<Inspection> inspectionsCow1 = adapter.getInspectionByCow(cow1);
    List<Inspection> inspectionsCow2 = adapter.getInspectionByCow(cow2);

    return inspectionsCow1.get(0).getJob().getJobDate().compareTo(inspectionsCow2.get(0).getJob().getJobDate());
  }
}

在Activity中：

 if (sortOption == "Name"){
    Cow.CowLastInspectionComparator comparator=new Cow.CowLastInspectionComparator(this);
    Collections.sort(cows,comparator);
 }

在非 activity class 比较器中获取上下文

Getting context in non-activity class comparator

java

android

comparator

android-context