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“V不是功能接口”错误

java-8 elasticsearch java

Nariman Esmaiely Fard · 技术社区 · 7 年前

我正在基于本教程编写elasticsearch插件: Creating an elasticsearch plugin, the basics

这里我得到了“V不是功能接口”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);
}

1 回复 | 直到 7 年前

Holger 7 年前

的定义 AnalysisModule.AnalysisProvider 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到此接口,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);
    }
}

忽略该附加参数。如果不希望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 调用和方法引用。如果将显式类型用于 单音图

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

您将立即收到一条关于不匹配构造函数签名的有意义的错误消息。一般来说,如果用较新的Java语言构造得到奇怪的错误消息,请尝试插入显式类型,直到得到可以理解的错误消息。解决问题后,可以再次删除不必要的显式类型。