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Object.toString() does not return the hashcode value of the object in java but instead give some string

I have written the following code:-
Test ob = new Test();
System.out.println(ob.toString());
System.out.println(ob.hashCode());
and the output is
Test#15db9742
366712642
i understand that the second value is the hashcode of the object and it is an integer value but i am not able to understand what is the first value. If it is the hashcode of the object then how can it be string and not integer

If you read the docs for toString very carefully:
The toString method for class Object returns 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. In other words, this method returns a string equal to the value of:
getClass().getName() + '#' + Integer.toHexString(hashCode())
366712642 in hex is exactly 15DB9742!
If it is the hashcode of the object then how can it be string and not integer?
As you can see from the docs, it is the class name, plus #, plus the dashcode, not just the hash code. Also, the method's name is toString. It would be weird if it returned an int, wouldn't it?

It represents classname#HashCode_in_Hexadeciaml_form. So, the string which you are seeing is actually the hexadecimal form of the integer hashcode

You can look the source code of Object.java. toString method is meant to provide information about class at runtime, so can be overriden. What you're doing is calling the default toString method from Object.java. It simply returns following:
getClass().getName() + "#" + Integer.toHexString(hashCode()
Hence the output.
See code here

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

What is the connection between System.out.println() and toString() in Java? e.g:
public class A {
String x = "abc";
public String toString() {
return x;
}
}
public class ADemo {
public static void main(String[] args) {
A obj = new A();
System.out.println(obj);
}
}
If main class runs, it gives an output as "abc". When I remove the code which overrides toString(), it gives an output as "A#659e0bfd". So, can anyone explain what is the working principle of System.out.println() when I pass the obj object reference as an argument to it? Is it fully connected with 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".

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.println(obj) will print the returned string from obj.toString() if you dont override it it will call the base object.toString() method which by default the toString method for class Object returns 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. In other words, this method returns a string equal to the value of:
getClass().getName() + '#' + Integer.toHexString(hashCode())

hashCode method called for System.out.println() method in HashSet

import java.util.HashSet;
import java.util.Set;
class Employee {
#Override
public int hashCode() {
System.out.println("Hash");
return super.hashCode();
}
}
public class Test2 {
public static void main(String[] args) {
Set<Employee>set= new HashSet<>();
Employee employee = new Employee();
set.add(employee);
System.out.println(set);// if we comment this "Hash" will be printed once
}
}
Above code calls hashCode method 2 times if we print set. Why hashcode method is called on System.out.println()?

Find the following reason for printing Hash two times
For finding the hash value when you insert the Employee into the HashSet
When you print the set, it's calls the hashCode() method inside the default toString() method from Object class.
The default toString() method from Object class API docs says
The toString() method for class Object returns 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. In other words, this method returns a string equal
to the value of:
getClass().getName() + '#' + Integer.toHexString(hashCode())

See this. In short, the default toString() function calls hashCode() and uses a hexadecimal representation of the hash as part of the String.

The first call to hashCode() is executed when adding an Employee to your set variable, as it's needed in order to calculate which bucket to put it in.
The second call is a bit sneaker. Any Collection's default toString() is a coma-delimited concatination of all its elements toString()s enclosed by square brackets (e.g., [object1, object2]). Any object's default toString(), if you don't override it is getClass().getName() + "#" + Integer.toHexString(hashCode()). Here, since you don't override Employee's toString(), it's called again when you print set.

What does it mean when the jvm address of an object is 0?

I have an object, Mutation, that I have "new"ed. But when it prints out with toString(), the object says Mutation#0. That doesn't seem good to me. What might that mean?

That is Unsigned hexadecimal representation
As per Docs of toString() method in Object class
The toString method for class Object returns 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. In other words, this method returns a string equal to the value of:
That zero in the sense the Hashcode not yet calculated.
Source Code:
public String toString() {
237 return getClass().getName() + "#" + Integer.toHexString(hashCode());
238 }

If you look at the source code of Mutation, a good chance is you'll find this in there:
#Override public int hashCode() {
//TODO: implement this properly!
return 0;
}
The default implementation of toString() just reports this fact that hashCode returned 0.

printing an object

What does the following System.out print in the following code??
class ExampleTest {}
public class Test {
public static void main(String ... strings){
ExampleTest et=new ExampleTest();
System.out.println(et);
System.out.println(new ExampleTest());
}
}

When you give any object to a print method, such as in your code, it will call the toString() method.
In your example, your ExampleTest class does not override this toString() method, so it will call the Object.toString():
public String toString() {
return getClass().getName() + "#" + Integer.toHexString(hashCode());
}
So the output will be the full name of the class, and the result of the hashCode of this class.
Here is what the Javadoc of java.lang.Object.toString() says about that:
Returns a string representation of the
object. In general, the toString
method returns a string that
"textually represents" this object.
The result should be a concise but
informative representation that is
easy for a person to read. It is
recommended that all subclasses
override this method.
The toString method for class Object
returns 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. In other words, this method
returns a string equal to the value
of:
getClass().getName() + '#' + Integer.toHexString(hashCode())

It prints the class name followed by the # symbol, followed by the unsigned hexadecimal representation of the object's hashcode.