java - "V is not a functional interface"错误

Question

我正在根据本教程编写一个 elasticsearch 插件: Creating an elasticsearch plugin, the basics

这里出现“V is not a functional interface”错误:

@Override
    public Map<String, AnalysisModule.AnalysisProvider<TokenFilterFactory>> getTokenFilters() {
        return Collections.singletonMap("jettro", MyTokenFilterFactory::new);
    }

MyTokenFilterFactory 的代码:

public class MyTokenFilterFactory extends AbstractTokenFilterFactory {

public MyTokenFilterFactory(IndexSettings indexSettings, String name, Settings settings) {
    super(indexSettings, name, settings);
}

@Override
public TokenStream create(TokenStream tokenStream) {
    return new TokenFilter(tokenStream);
}

Answer 1

AnalysisModule.AnalysisProvider的定义是

public interface AnalysisProvider<T> {
    T get(IndexSettings indexSettings, Environment environment,
          String name, Settings settings) throws IOException;
    // irrelevant default methods omitted
}

要通过方法引用将构造函数绑定(bind)到此接口(interface)，它必须具有完全相同的签名，即您必须将声明更改为

public class MyTokenFilterFactory extends AbstractTokenFilterFactory {

    public MyTokenFilterFactory(IndexSettings indexSettings, Environment environment,
                                String name, Settings settings) {
        super(indexSettings, name, settings);
    }

    @Override
    public TokenStream create(TokenStream tokenStream) {
        return new TokenFilter(tokenStream);
    }
}

忽略那个额外的参数。如果不想让普通调用者提供不需要的参数，可以重载构造函数

public class MyTokenFilterFactory extends AbstractTokenFilterFactory {

    public MyTokenFilterFactory(IndexSettings indexSettings, Environment environment,
                                String name, Settings settings) {
        super(indexSettings, name, settings);
    }
    public MyTokenFilterFactory(IndexSettings indexSettings,
                                String name, Settings settings) {
        super(indexSettings, name, settings);
    }

    @Override
    public TokenStream create(TokenStream tokenStream) {
        return new TokenFilter(tokenStream);
    }
}

或者您只保留原始构造函数只声明所需的参数并将方法引用更改为 lambda 表达式:

public Map<String, AnalysisModule.AnalysisProvider<TokenFilterFactory>> getTokenFilters() {
    return Collections.singletonMap("jettro",
                                    (is, env, n, s) -> new MyTokenFilterFactory(is, n, s));
}

---

请注意，误导性编译器错误消息很可能是由于 singletonMap 调用的类型推断与方法引用之间的复杂交互造成的。如果您对 singletonMap 调用使用显式类型，即

public Map<String, AnalysisModule.AnalysisProvider<TokenFilterFactory>> getTokenFilters() {
    return Collections.
        <String, AnalysisModule.AnalysisProvider<TokenFilterFactory>>
        singletonMap("jettro", MyTokenFilterFactory::new);
}

您会立即收到有关构造函数签名不匹配的有意义的错误消息。作为一般经验法则，如果您在使用较新的 Java 语言结构时收到奇怪的错误消息，请尝试插入显式类型，直到您收到可以理解的错误消息为止。解决问题后，您可以再次删除当时不需要的显式类型。