Java 传递带有两个参数的 (lambda) 函数

Question

I'm trying to implement a function to read files, but I cannot change the signature of the method. There are a lot of cross references in the code but maybe someone can enlighten me, I have been stuck for 3 days right now.

The first method I'm trying to pass into the function is the following:

        Purchase newPurchase = null;
        String[] purchases = textLine.split(",");
        int foundBarcode = products.indexOf(getProductFromBarcode(products, Long.parseLong(purchases[0])));

        products.indexOf(purchases);
        newPurchase = new Purchase(
                products.get(foundBarcode),
                Integer.parseInt(purchases[1].trim())
        );

Somehow I want to pass this function into my import file function.

    public static <E> void importItemsFromFile(List<E> items, String filePath, Function<String,E> converter) {
        int originalNumItems = items.size();

        Scanner scanner = createFileScanner(filePath);

        // TODO read all source lines from the scanner,
        //  convert each line to an item of type E and
        //  and add each item to the list

        while (scanner.hasNext()) {
            // input another line with author information
            String line = scanner.nextLine();

            // TODO convert the line to an instance of E
            E newItem = converter.apply(line);

            // TODO add the item to the list of items
            items.add(newItem);
        }
        System.out.printf("Imported %d items from %s.\n", items.size() - originalNumItems, filePath);
    }

I hope someone can help me and explain how to pass this function into the other function's converter parameter. Tried to do a lot of research on this topic but I'm still not able to find the answer. Please help me stackoverflow community!:D

Answer 1

You need to declare this like this first inside a 'super - private function'

private Function<String, Purchase> doSomething() {
     return textLine -> {
        Purchase newPurchase = null;
        String[] purchases = textLine.split(",");
        int foundBarcode = products.indexOf(getProductFromBarcode(products, Long.parseLong(purchases[0])));

        products.indexOf(purchases);
        return new Purchase(products.get(foundBarcode), Integer.parseInt(purchases[1].trim())
     };
}

Then before calling this importItemsFromFile() method, you declare this function as a variable

Function<String, Purchase> abcFunction = doSomething();

and then call this importItemsFromFile() function like this

importItemsFromFile(items, filePath, abcFunction);

The reason why this works is, lambda operator returns a Functional Interface, here it is "Function", if you pass 3 arguments, you will need a BiFunction<T, U, R> for that.

Thats why these FunctionalInterfaces are sometimes called super private functions, as they are used inside another function.

And Hey give me extra points for this, as I even completed your half written function, and saved your 3 days time.

Answer 2

I am a little lost by the top snippet, but all you need to do is define an implementation of the Function interface. You can do this simply by defining a class that implements the interface or you can specify a lambda that would do the same thing.

Assuming the top code block is what you want in your lambada you would do something like this,

(textLine) ->{
        String[] purchases = textLine.split(",");
        int foundBarcode = products.indexOf(getProductFromBarcode(
            products, Long.parseLong(purchases[0])));
        products.indexOf(purchases);
        return new Purchase(
                products.get(foundBarcode),
                Integer.parseInt(purchases[1].trim())
        )