Why the system invokes card.toString() by itself

Question

Hi All!
When Card3 is instantiated and then printed out.The result is, as if the card.toString()
was invoked ( Ten of Clubs). Could somebody tell,why the system invokes card.toString()
by itself.
Thanks.
public class card3Demo {
public static void main(String[] args) {
Card3 card=new Card3(10,2);
System.out.println(card);
public class Card3 {
private int suit = UNASSIGNED;
private int number = UNASSIGNED;
public final static int UNASSIGNED = -1;
public final static int DIAMONDS = 1;
public final static int CLUBS = 2;
public final static int HEARTS = 3;
public final static int SPADES = 4;
public final static int ACE = 1;
public final static int KING = 13;
public Card3(int number, int suit) {
this.suit=suit;
this.number=number;
public String toString123() {
return numberToString(this.number) + " of "
+ suitToString(this.suit);
public static String numberToString(int number) {
String result = "";
switch (number) {
case ACE: result = "Ace"; break;
case 2: result = "Two"; break;
case 3: result = "Three"; break;
case 4: result = "Four"; break;
case 5: result = "Five"; break;
case 6: result = "Six"; break;
case 7: result = "Seven"; break;
case 8: result = "Eight"; break;
case 9: result = "Nine"; break;
case 10: result = "Ten"; break;
case 11: result = "Jack"; break;
case 12: result = "Queen"; break;
case KING: result = "King"; break;
case UNASSIGNED: result = "Invalid Number"; break;
return result;
public static String suitToString(int suit) {
String result = "";
switch (suit) {
case DIAMONDS: result = "Diamonds"; break;
case CLUBS: result = "Clubs"; break;
case HEARTS: result = "Hearts"; break;
case SPADES: result = "Spades"; break;
case UNASSIGNED: result = "Invalid Suit"; break;
return result;

Answer 1

Actually it will call String.valueOf(o) which works like this:
public static String valueOf(Object obj) {
  return (obj == null) ? "null" : obj.toString();