通过Lambda采取参数方法

Question

I'm looking for a way to pass a code block to a method, which will then perform actions on other arguments passed to method, take the results of those actions, and pass those results to the code block passed to the method. For clarity:

private static void method1(String filename, int sheetNum) {
    runOnSheet(filename, () -> {
        doStuffWithStream(FileInputStream fileStream);         // Does something with a file stream
        doOtherStuffWithStream(FileInputStream fileStream);    // Does something else with a file stream
    });
}

// Elsewhere
private static void runOnFile(String fileName, Runnable block1) {
    try {
        fileStream = new FileInputStream(fileName);
    } catch (IOException e) {
        e.printStackTrace();
    }

    block1.run();    // I'd like to pass fileStream to this code block. Ideally i could do block1.run(fileStream );

    fileStream.close();
}

I want to be able to reuse runOnFile anywhere I need to open a file, run some code on the stream, and close the stream.
What I actually want to do is more complicated, and uses other libraries in addition to FileInputStream, but the structure of what I wish to accomplish is the same.

Thanks for any help!

Answer 1

Java 8+ has a Class called Consumer that can be used for your usecase: https://docs.oracle.com/javase/8/docs/api/java/util/function/Consumer.html

private static void method1(String filename, int sheetNum) {
    runOnFile(filename, (fileStream) -> {
        doStuffWithStream(fileStream);
        doOtherStuffWithStream(fileStream);
    });
}

// Elsewhere
private static void runOnFile(String fileName, Consumer<FileInputStream> block1) {
    try {
        fileStream = new FileInputStream(fileName);
    } catch (IOException e) {
        e.printStackTrace();
    }

    block1.accept(fileStrean);

    fileStream.close();
}

EDIT: As suggested by Dimitri using the try-with-resource syntax:

// Elsewhere
private static void runOnFile(String fileName, Consumer<FileInputStream> block1) {
    try (FileInputStream fis = new FileInputStream(fileName)) {
        block1.accept(fis);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

Answer 2

Try something like this:

private static void method1(String filename, int sheetNum)
{
  try ( final FileInputStream fileStream = new FileInputStream(filename))
  {
    runOnSheet(filename, () -> 
    {
      doStuffWithStream(fileStream);         // Does something with a file stream
      doOtherStuffWithStream(fileStream);    // Does something else with a file stream
    });
  }
}