java为什么实现了两个方法的TextWatcher inheritor和作为一个匿名类方法实现的第三个方法不能转换为lambda？

Question

我有一个abstract class实现TextWatcher，在只需要afterTextChanged(Editable s)方法的情况下缩短代码

这门课看起来像这样

public abstract class AfterTextChangedTextWatcher implements TextWatcher {
    @Override
    public void beforeTextChanged(CharSequence s, int start, int count, int after) {

    }

    @Override
    public void onTextChanged(CharSequence s, int start, int before, int count) {

    }
}

问题是，为什么当我创建这个类的匿名实现时，afterTextChanged(Editable s)的调用不能转换为lambda（我使用retrolambda库）？有什么变通方法或模式可以将其转换为lambda吗

现在这个类的用法如下所示

currentPasswordInput.addTextChangedListener(new AfterTextChangedTextWatcher() {
            @Override
            public void afterTextChanged(Editable s) {
                presenter.queryCurrentPw(s.toString());
            }
        });

UPD:感谢tamtom的伟大answer

我将把我的新代码放在这里，它可以很好地总结一切

所以现在我的后文本更改侦听器变成了一个功能接口

@FunctionalInterface//this annotation ensures you, that the class has only 
//one unimplemented method. Otherwise, the program won't compile.
public interface AfterTextChangedTextWatcher extends TextWatcher {
@Override
default public void beforeTextChanged(CharSequence s, int start, int count, int after) {
    // default keyword in method specifier allows you to add basic implementation right in the interface. In my case, this code should do nothing.
}

@Override
default public void onTextChanged(CharSequence s, int start, int before, int count) {

    //now we have two methods of TextWatcher interface implemented by 
    //default. The only afterTextChange remains abstract. This allows us to 
    //consider our AfterTextChangeListener interface as abstract
}

}

现在，我们的EditText字段已经准备好作为lambda获取AfterTextChangeTextWatcher。我们只需要将TextWatcher类型指定为AfterTextChangedTextWatcher

currentPasswordInput.addTextChangedListener((AfterTextChangedTextWatcher) s -> presenter.queryCurrentPw(s.toString()));

Answer 1

A functional interface is an interface that has just one abstract method, and thus represents a single function contract.

lambda需要这些功能接口，因此仅限于它们的单一方法。匿名接口仍然需要用于实现多方法接口

addMouseListener(new MouseAdapter() {

    @Override
    public void mouseReleased(MouseEvent e) {
       ...
    }

    @Override
    public void mousePressed(MouseEvent e) {
      ...
    }

如果您使用的是Java 8 通过使用助手接口，可以将多方法接口用于lambdas。这适用于这样的侦听器接口，其中不需要的方法的实现非常简单（即，我们也可以做MouseAdapter提供的）：

interface AfterTextChangedTextWatcher extends TextWatcher
{
    @Override
    public default void beforeTextChanged(CharSequence s, int start, int count, int after) {}


    @Override
    public default void onTextChanged(CharSequence s, int start, int before, int count) {}

}