How to print out an object if it's created with lambda?

Question

Here is my interface example

public interface Card {
    String name();
}

I use it in ArrayList and create objects like this.

public static void main(String[] args) {
    List<Card> cards = new ArrayList<>();
    cards.add(() -> "2");
  cards.add(() -> "4");
    System.out.println(cards);
}

How can I print a number to console if I create an object like this? Here is the example of the printed text.

[d$$Lambda$14/0x0000000800bb1438@2d98a335]

But I want 2 and 4 to be printed out.

Or `cards.stream().map(Card::name).forEach(System.out::println);` — Turing85, Jan 07 '22 at 21:45
You might want to check https://stackoverflow.com/questions/23628631/how-to-make-a-lambda-expression-define-tostring-in-java-8 — Progman, Jan 07 '22 at 21:46

score 0 · Accepted Answer · 2022-01-07T23:01:44.320

Try this.

public interface Card {
    String name();

    static Card of(Supplier<String> supplier) {
        return new Card() {
            @Override public String name() { return supplier.get(); }
            @Override public String toString() { return name(); }
        };
    }
}

public static void main(String[] args) {
    List<Card> cards = new ArrayList<>();
    cards.add(Card.of(() -> "2"));
    cards.add(Card.of(() -> "4"));
    System.out.println(cards);
}

output:

[2, 4]

Note: You can't override toString() method.

public interface Card {
    String name();

    public default String toString() {  // compile error
                                        // "A default method cannot override a method from java.lang.Object "
        return name();
    }
}

How to print out an object if it's created with lambda?

1 Answers1