如何在Java 8中使用Lambda表达式覆盖基类方法？

使用 委托 。对于此任务，您需要一个只能执行一次的帮助程序类：

interface IoBiFunction<T, U, R> {  R apply(T t, U u) throws IOException;}class LambdaFileVisitor<T> extends SimpleFileVisitor<T> {    IoBiFunction<T, BasicFileAttributes, FileVisitResult> preVisitDir=super::preVisitDirectory;    IoBiFunction<T, BasicFileAttributes, FileVisitResult> visitFile=super::visitFile;    IoBiFunction<T, IOException, FileVisitResult> visitFailed=super::visitFileFailed;    IoBiFunction<T, IOException, FileVisitResult> postVisitDir=super::postVisitDirectory;    public LambdaFileVisitor<T> onVisitFile(IoBiFunction<T, BasicFileAttributes, FileVisitResult> f) {        this.visitFile = Objects.requireNonNull(f);        return this;    }    public LambdaFileVisitor<T> onVisitFailed(IoBiFunction<T, IOException, FileVisitResult> f) {        this.visitFailed = Objects.requireNonNull(f);        return this;    }    public LambdaFileVisitor<T> onPreVisitDir(IoBiFunction<T, BasicFileAttributes, FileVisitResult> f) {        this.preVisitDir = Objects.requireNonNull(f);        return this;    }    public LambdaFileVisitor<T> onPostVisitDir(IoBiFunction<T, IOException, FileVisitResult> f) {        this.postVisitDir = Objects.requireNonNull(f);        return this;    }    @Override    public FileVisitResult visitFile(T file, BasicFileAttributes attrs) throws IOException {        return visitFile.apply(file, attrs);    }    @Override    public FileVisitResult visitFileFailed(T file, IOException exc) throws IOException {        return visitFailed.apply(file, exc);    }    @Override    public FileVisitResult preVisitDirectory(T dir, BasicFileAttributes attrs) throws IOException {        return preVisitDir.apply(dir, attrs);    }    @Override    public FileVisitResult postVisitDirectory(T dir, IOException exc) throws IOException {        return postVisitDir.apply(dir, exc);    }}

有了辅助类后，就可以将其与lambda表达式一起使用，例如

FileVisitor<Path> fv=new LambdaFileVisitor<Path>()  .onVisitFile((f,a)->{System.out.println(f); return CONTINUE; })  .onVisitFailed((f,e)->{ throw e; });

要么

FileVisitor<Path> fv2=new LambdaFileVisitor<Path>()  .onPreVisitDir((f,a)->{System.out.println("ENTER "+f); return CONTINUE; })  .onPostVisitDir((f,e)->{      System.out.println("LEAVE "+f);      if(e!=null) throw e; else return CONTINUE;  });