java - 在没有成员变量的情况下通过实现创建抽象类有什么意义？

Question

这个问题实际上比主题更具体，但我认为主题涵盖了我想知道的内容。

我正在审查一个项目中的一些代码，这些代码看起来像是我想要使用的东西，我看到了一些让我感兴趣的东西，因为我不确定为什么要这样做。具体来说，在 Chris Banes 的 ActionBar-PullToRefresh 中，我在 .java 中看到了这个:

package uk.co.senab.actionbarpulltorefresh.library;

import android.app.Activity;
import android.content.res.Configuration;
import android.view.View;

/**
* HeaderTransformers are what controls and update the Header View to reflect the current state
* of the pull-to-refresh interaction. They are responsible for showing and hiding the header
* view, as well as update the state.
*/
public abstract class HeaderTransformer {

   /**
  * Called whether the header view has been inflated from the resources
 * defined in {@link Options#headerLayout}.
 *
 * @param activity The {@link android.app.Activity} that the header view is attached to.
 * @param headerView The inflated header view.
 */
public void onViewCreated(Activity activity, View headerView) {}

/**
 * Called when the header should be reset. You should update any child
 * views to reflect this.
 * <p/>
 * You should <strong>not</strong> change the visibility of the header
 * view.
 */
public void onReset() {}

/**
 * Called the user has pulled on the scrollable view.
 *
 * @param percentagePulled value between 0.0f and 1.0f depending on how far the
 *                         user has pulled.
 */
public void onPulled(float percentagePulled) {}

/**
 * Called when a refresh has begun. Theoretically this call is similar
 * to that provided from {@link uk.co.senab.actionbarpulltorefresh.library.listeners.OnRefreshListener} but is more suitable
 * for header view updates.
 */
public void onRefreshStarted() {}

/**
 * Called when a refresh can be initiated when the user ends the touch
 * event. This is only called when {@link Options#refreshOnUp} is set to
 * true.
 */
public void onReleaseToRefresh() {}

/**
 * Called when the current refresh has taken longer than the time
 * specified in {@link Options#refreshMinimizeDelay}.
 */
public void onRefreshMinimized() {}

/**
 * Called when the Header View should be made visible, usually with an animation.
 *
 * @return true if the visibility has changed.
 */
public abstract boolean showHeaderView();

/**
 * Called when the Header View should be made invisible, usually with an animation.
 *
 * @return true if the visibility has changed.
 */
public abstract boolean hideHeaderView();

/**
 * Called when the Activity's configuration has changed.
 *
 * @param activity The {@link android.app.Activity} that the header view is attached to.
 * @param newConfig New configuration.
 *
 * @see android.app.Activity#onConfigurationChanged(android.content.res.Configuration)
 */
public void onConfigurationChanged(Activity activity, Configuration newConfig) {}
}

我对这个文件的问题是，为什么要在这里做一个抽象类而不是接口(interface)，或者做一个抽象类的目的是什么？我看到该类有两个抽象方法。我的理解是抽象方法必须在子类中定义，否则该子类也是一个抽象类，对吗？那么这是作为一个抽象类完成的，以便只强制执行这两个抽象方法吗？这是在这里做抽象类而不是接口(interface)的唯一原因吗？

Answer 1

将其设为抽象类可能有很多原因，您自己指出了其中一些原因。

在这种情况下，我的猜测是因为它是一个图书馆。 假设您有一个庞大的应用程序，并使用此类定义许多其他类。

如果通过添加新方法更新类，如果它是一个接口(interface)，您的代码将无法编译，因为您的代码违反了接口(interface)的“契约”(它没有实现所有方法)。但在这种情况下，它是一个抽象类，可以定义默认行为:在这种情况下，什么也不做。这样您的应用程序就不会中断，并且可以毫无问题地进行编译。