External parameters referenced by methods in Java 8

I want to pass external parameters to method references:

String prefix = "The number is :";
numbers.forEach(Main::printWithPrefix);

private static void printWithPrefix(Integer number) {
    System.out.println(number);
}

I don't know what to do I can do this with lambda:

String prefix = "The number is :";
numbers.forEach(number -> {
    System.out.println(prefix + number);
});

Can external parameters be passed to method references?

Solution

No, you cannot pass parameters to a method reference What you can do is create a method to return to the consumer:

private static Consumer<Integer> printWithPrefix(String prefix) {
    return number -> System.out.println(prefix + number);
}

This can then be used as a factory for creating consumers, which you can pass to numbers forEach：

String prefix = "The number is :";
numbers.forEach(printWithPrefix(prefix));

You can even make it more generic by creating a printwithprefix method with consumer as a parameter so that you can pass in another one. If you want to:

private static Consumer<Integer> printWithPrefix(String prefix,Consumer<Integer> printer) {
    return number -> {
        System.out.print(prefix);
        printer.accept(number);
    };
}

For example, you can use the printnumber method:

private static void printNumber(Integer number) {
    System.out.println(number);
}

String prefix = "The number is :";
numbers.forEach(printWithPrefix(prefix,Main::printNumber));