在println中调用println方法

Question

I just wrote this code:

public class T 
{
  public String toString()
   {
     System.out.println("new line");
     return " "+4;
   }
}

and:

public static void main(String[] args)
{
     T t = new T();
     System.out.println("11"+t);
}

and while I expected it to print

11 new line
4

it actually printed:

new line
11 4

why is that? What is the order of this printing and what is the logic behind this?

Thanks!

Answer 1

The call for the toString happens before the "11" print.

The reason (like @Tom said), in order to concating the string, It have to create all the pieces first, and then it concating.

Only after the String is full ready - it will go to your screen.

Answer 2

When you call a function, Java evaluates all of its arguments before actually going into that function. So in you case, it first evaluates "11"+t . Now that calls t.toString(), which writes "new line". Then toString returns " 4", and that is concatenated to "11", then println is executed to write "11 4"