Question

We know that when we use objects in sysout(System.out.Println) statement internally it’s toString method is called. and with primitive it directly prints. but when we use any Wrapper class type of objects is used say suppose Integer like following

      Integer i = new Integer(10)
      System.out.Println(i);

is it toString() is responsible for printing it or Unboxing?

Answer 1

A quick test and debug run shows that print(Object) is invoked, not print(int).

A good way to check this is :

Integer val = null;
System.out.print(val);

If un-boxing were used, this would throw a NullPointerException. This doesn’t happen, though, it prints the string null, which is the output of String.valueOf(Object) when a null is passed in.

Another aspect to bear in mind is that PrintStream existed before Java 5. When autoboxing was introduced in Java 5, it had to be ensured that any existing code using PrintStream would not suddenly have its behaviour changed. So any existing code calling print(Object) must not suddenly have its behaviour changed to call print(int) instead, simply because of a new language feature. Backwards compatibility must always be maintained.