The connection between 'System.out.println()' and 'toString()' in Java

toString() is a method that exist in the Object class (Root of the inheritence tree) for all classes.

System.out.print() (SOP) will call the toString method when fed an object.

If you don't overwrite the method toString(), SOP will call the parent toString() which, if parent is the Object class, it will print the hashCode of the object

If you overwrite the method, SOP will call your toString() method

System.out is a PrintStream. Printstream defines several versions of the println() function to handle numbers, strings, and so on. When you call PrintStream.println() with an arbitrary object as a parameter, you get the version of the function that acts on an Object. This version of the function

...calls at first String.valueOf(x) to get the printed object's string value...

Looking at String.valueOf(Object), we see that it returns

if the argument is null, then a string equal to "null"; otherwise, the value of obj.toString() is returned.

So, long story short, System.out.println(someObject) calls that object's toString() function to convert the object to a string representation.

If your object defines its own toString() function, then that is what will be called. If you don't provide such a function, then your object will inherit toString() from one of its parent classes. In the worst case, it will inherit Object.toString(). That version of toString() is defined to return

a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object.

Or, in other words:

getClass().getName() + '@' + Integer.toHexString(hashCode())

So, when you call System.out.println() on an object that doesn't define its own version of toString(), you might get the Object version which looks like "classname@someHexNumber".