Java 什么是RecyclerView.Adapter<；MyAdapter.MyViewHolder>；它与Android中的RecyclerView.Adapter有何不同？

我正在学习RecyclerView，在的站点中，Adapter类扩展了

RecyclerView.Adapter

。实施情况表明：

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.MyViewHolder> {
private String[] mDataset;


public static class MyViewHolder extends RecyclerView.ViewHolder {

    public TextView textView;
    public MyViewHolder(TextView v) {
        super(v);
        textView = v;
    }
}

public MyAdapter(String[] myDataset) {
    mDataset = myDataset;
}

@Override
public MyAdapter.MyViewHolder onCreateViewHolder(ViewGroup parent,
                                               int viewType) {

    TextView v = (TextView) LayoutInflater.from(parent.getContext())
            .inflate(R.layout.my_text_view, parent, false);

    MyViewHolder vh = new MyViewHolder(v);
    return vh;
}

@Override
public void onBindViewHolder(MyViewHolder holder, int position) {

    holder.textView.setText(mDataset[position]);

}

@Override
public int getItemCount() {
    return mDataset.length;
}
}

那么，

RecyclerView.Adapter

和

RecyclerView.Adapter

之间的区别是什么？在这种情况下，

RecyclerView.Adapter

中的语法是什么意思？我知道它代表泛型。我什么时候用这个

那么，RecyclerView.Adapter和RecyclerView.Adapter之间有什么区别呢？在本例中，RecyclerView.Adapter的语法是什么意思？我知道它代表泛型。我什么时候用这个

首先，让我们回顾一下

适配器
类的定义。它在
RecyclerView
中定义如下：

public abstract static class Adapter<VH extends ViewHolder>


或者，您发布的教程不指定类型

public class SimpleAdapter extends RecyclerView.Adapter

当您扩展一个需要泛型类型而不指定泛型类型的类时，编译器默认使用最小公分母，即继承层次结构中保证该类可以按设计使用的最小类类型。由于适配器的泛型类型定义为
VH extends ViewHolder
，编译器知道类类型必须至少是
ViewHolder
，并且默认为该类型。因此，本例覆盖的相同方法返回
RecyclerView.ViewHolder
，而不是
MyViewHolder
：

@Override
public RecyclerView.ViewHolder onCreateViewHolder(final ViewGroup parent, final int viewType) {
    final View view = LayoutInflater.from(parent.getContext()).inflate(viewType, parent, false);
    return new SimpleViewHolder(view);
}

如果将
RecyclerView.Adapter
定义为
Adapter
（无边界类型），并且您在未指定类型的情况下对其进行了扩展，则该类型将默认为
Object
（所有Java类的根类）：


那么，最后，为什么要用一个而不是另一个呢？一般来说：应该始终为泛型类指定类型。这保证了键入的安全性。在本教程示例中，您可能返回错误类型的视图持有者，该持有者将编译，但在运行时崩溃：

@Override
public RecyclerView.ViewHolder onCreateViewHolder(final ViewGroup parent, final int viewType) {
    final View view = LayoutInflater.from(parent.getContext()).inflate(viewType, parent, false);

    // Returning wrong view holder, but because it still extends ViewHolder and
    // that is all this method requires, this compiles
    return new SomeOtherViewHolder(view);
}


@Override
public void onBindViewHolder(final RecyclerView.ViewHolder holder, final int position) {
    // Above we created a SomeOtherViewHolder but here we're expecting a SimpleViewHolder
    // This will crash trying to cast to the wrong type
    ((SimpleViewHolder) holder).bindData(models.get(position));
}

但在第一个示例中不可能犯此错误，因为
ViewHolder
类型是显式声明的：

@Override
public MyAdapter.MyViewHolder onCreateViewHolder(ViewGroup parent,
                                               int viewType) {

    TextView v = (TextView) LayoutInflater.from(parent.getContext())
            .inflate(R.layout.my_text_view, parent, false);

    // Trying to return SomeOtherViewHolder when method expects MyViewHolder
    // Compiler's like (*waves finger*) "nuh-uh, not on my watch" and fails
    // to compile
    SomeOtherViewHolder vh = new SomeOtherViewHolder(v);
    return vh;
}

@Override
public void onBindViewHolder(MyViewHolder holder, int position) {
    // Notice here that holder is explicitly of MyViewHolder type and casting
    // is not necessary! That's TYPE SAFETY y'all.
    holder.textView.setText(mDataset[position]);
}

由于指定了所需的显式具体类型，因此不能意外返回错误的类型，不需要强制转换，并且可以利用自定义
ViewHolder
类中定义的所有公共方法

希望有帮助
 如果类是泛型类型，则应始终指定类型参数。所以我应该使用“公共类SimpleAdapter扩展RecyclerView.Adapter”而不是“公共类SimpleAdapter扩展RecyclerView.Adapter”？这两种方法都可以，您应该使用第一种方法，尽管您显然会用应用程序正在使用的任何类型替换类型参数。虽然这两种方法都有效，但它们并不等价，第二种是原始类型。有关原始类型的更多信息，以及为什么应该避免原始类型，请参见我在前面的评论中链接的Q&A。
@Override
public Object onCreateViewHolder(final ViewGroup parent, final int viewType) {
    final View view = LayoutInflater.from(parent.getContext()).inflate(viewType, parent, false);
    return new SimpleViewHolder(view);
}

@Override
public RecyclerView.ViewHolder onCreateViewHolder(final ViewGroup parent, final int viewType) {
    final View view = LayoutInflater.from(parent.getContext()).inflate(viewType, parent, false);

    // Returning wrong view holder, but because it still extends ViewHolder and
    // that is all this method requires, this compiles
    return new SomeOtherViewHolder(view);
}


@Override
public void onBindViewHolder(final RecyclerView.ViewHolder holder, final int position) {
    // Above we created a SomeOtherViewHolder but here we're expecting a SimpleViewHolder
    // This will crash trying to cast to the wrong type
    ((SimpleViewHolder) holder).bindData(models.get(position));
}

@Override
public MyAdapter.MyViewHolder onCreateViewHolder(ViewGroup parent,
                                               int viewType) {

    TextView v = (TextView) LayoutInflater.from(parent.getContext())
            .inflate(R.layout.my_text_view, parent, false);

    // Trying to return SomeOtherViewHolder when method expects MyViewHolder
    // Compiler's like (*waves finger*) "nuh-uh, not on my watch" and fails
    // to compile
    SomeOtherViewHolder vh = new SomeOtherViewHolder(v);
    return vh;
}

@Override
public void onBindViewHolder(MyViewHolder holder, int position) {
    // Notice here that holder is explicitly of MyViewHolder type and casting
    // is not necessary! That's TYPE SAFETY y'all.
    holder.textView.setText(mDataset[position]);
}