Question

mCallback = (OnHeadlineSelectedListener) activity;

这是一个Android应用教程的一些java代码。我不明白这是做什么的。我知道它正在为mCallback赋值但是什么？为什么OnHeadlineSelectedListener在括号中然后活动对象就在它后面，那该怎么办？

/*
 * Copyright (C) 2012 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.vizoplex.my.first.app;

import android.app.Activity;
import android.os.Build;
import android.os.Bundle;
import android.support.v4.app.ListFragment;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.ListView;

public class HeadlinesFragment extends ListFragment {
    OnHeadlineSelectedListener mCallback;

    // The container Activity must implement this interface so the frag can deliver messages
    public interface OnHeadlineSelectedListener {
        /** Called by HeadlinesFragment when a list item is selected */
        public void onArticleSelected(int position);
    }

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // We need to use a different list item layout for devices older than Honeycomb
        int layout = Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB ?
                android.R.layout.simple_list_item_activated_1 : android.R.layout.simple_list_item_1;

        // Create an array adapter for the list view, using the Ipsum headlines array
        setListAdapter(new ArrayAdapter<String>(getActivity(), layout, Ipsum.Headlines));
    }

    @Override
    public void onStart() {
        super.onStart();

        // When in two-pane layout, set the listview to highlight the selected list item
        // (We do this during onStart because at the point the listview is available.)
        if (getFragmentManager().findFragmentById(R.id.article_fragment) != null) {
            getListView().setChoiceMode(ListView.CHOICE_MODE_SINGLE);
        }
    }

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);

        // This makes sure that the container activity has implemented
        // the callback interface. If not, it throws an exception.
        try {
            mCallback = (OnHeadlineSelectedListener) activity;
        } catch (ClassCastException e) {
            throw new ClassCastException(activity.toString()
                    + " must implement OnHeadlineSelectedListener");
        }
    }

    @Override
    public void onListItemClick(ListView l, View v, int position, long id) {
        // Notify the parent activity of selected item
        mCallback.onArticleSelected(position);

        // Set the item as checked to be highlighted when in two-pane layout
        getListView().setItemChecked(position, true);
    }
}

Answer 1

它正在尝试将该活动投射到OnHeadlineSelectedListener，可能是因为此实际活动扩展了Listener。这样，mCallback充当正确定义回调的侦听器。

修改：

你有：

try { mCallback = (OnHeadlineSelectedListener) activity; } catch (ClassCastException e) { throw new ClassCastException(activity.toString() + " must implement OnHeadlineSelectedListener"); }

它正试图将activity投射到OnHeadlineSelectedListener。如果activity未实现此侦听器，则抛出异常。这条消息很简单：

activity.toString()+ " must implement OnHeadlineSelectedListener"

Answer 2

那是类型转换。例如：你有Animal接口有两种方法，运行和吃。

public interface Animal {
    public String run();
    public String eat(String food);
}

你有课程，它实现了它，比如，Cat课程。 Cat类有一些额外的方法，划痕和喵喵。

public class Cat implements Animal{
   public String run(){
     return "Run as a cat";
   }
   public String eat(){
     return "Eat fish";
   }
   public String mew(){
     return "MEW!!!!";
   }
   public String scratch(){
     return "scratch-scratch-scratch";
   }
}

另外，你有Dog类，它有树皮方法。

public class Dog implements Animal{
   public String run(){
     return "Run as a dog";
   }
   public String eat(){
     return "Eat meat";
   }
   public String bark(){
     return "BARK!!!!";
   }
}

所以，你经常需要和一些动物（狗或猫）一起使用动物。但编译器cat没有你的帮助就不能进行这种投射。所以，你必须告诉它你到底想要什么：

public void Show{
  Cat cat = new Cat(); //you have a cat
  Dog dog = new Dog(); //and a dog
  Animal animal = null; //and animal
  dog = (Dog)animal; //and here is where compiler can't convert animal to dog without your help.
  Animal animal2 = cat; //But this casting it can make without your help, as it exactly knows what to do.
}

This one book对学习Java非常有用。如果你真的对理解这些东西感兴趣，你应该阅读它。关于铸造如何工作的完整解释也可以在本书中找到。

Answer 3

activity与OnHeadlineSelectedListener对象casted mCallback，{{1}}正在分配给该对象。

Answer 4

我认为mCallback对象是activity对象类的子类。因此，此语句将活动对象强制转换为更具体的mCallback对象。

有关投射Click here

的更多信息

Answer 5

它正在将活动转换为OnHeadlineSelectedListener。假设该活动实现了OnHeadlineSelectedListener。这样做是为了您可以直接调用OnHeadlineSelectedListener接口中的方法。

Answer 6

我最近已经转出Java了，但它看起来好像该行是投射 activity到类型OnHeadlineSelectedListener。如果类型不兼容，则转换将一种类型转换为另一种类型，并抛出异常（一种或另一种形式）。

语法是：

variable = (Type) variableOfType2;

试图理解这行java代码

6 个答案: