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How to change the displayed name of an object in an ArrayList [duplicate]

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How do I print my Java object without getting "SomeType#2f92e0f4"?
(13 answers)
Closed 2 years ago.
I want to change an element name in an ArrayList, without changing its value.
Like :-
ArrayList<Integer> array1 = new ArrayList<>();
int num = 1;
array1.add(num);
Now I want to change num to number, but it's value will be 1.
(Is it possible?)
I want to display all the elements in a ArrayList in a swing combo box, but some of the variables show strange names, such as "Client#45673...". But I want the user to see a descriptive name. That is why I want to rename the variables.

OK, so you are wandering up the proverbial garden path with your "change the name of a variable".
Firstly, it is not a variable. When you put something into a collection, Java doesn't remember that at one point you had the value in a variable.
Secondly, values in general and objects in particular don't have names ... unless you specifically implemented some kind of "name" field as part the class definition.
But I think the clue is in this sentence:
... but some of the variables show strange names, such as "Client#45673..."
That's not a name! That looks like the output of the default implementation1 of toString that your Client class is inheriting from java.lang.Object.
If you want to print / display something something more meaningful, your Client class needs to override the toString() method with its own implementation.
For example:
public class Client {
private String myDescription;
public Client(...) {
...
myDescription = /* something */
}
#Override
public String toString() {
return myDescription;
}
}
Your toString method can return anything you want it to ... so long as it is represented as a string.
Incidentally, if you were to print the array1 object in your question, you would see the actual value 1 rather than a "name" (as you called it). This is because java.lang.Integer overrides the toString() method.
1 - The default toString() result consists of the internal class name and the object's identity hashcode value. The latter is a magic number that the JVM generates. It is not guaranteed unique, but it is guaranteed that it won't change for the lifetime of the object. At any rate, you cannot alter the string that is generated by that method ... except by overriding it.

What you're talking about is the output when printing an object. Those objects need to override toString() and return whatever string is deemed appropriate that describes the class or the contents thereof.
Try this.
class Foo {
//#Override
//public String toString() {
// return "This is a foo";
//}
}
Foo f = new Foo();
System.out.println(f);
Now uncomment the toString() method and print it again.
To print arrays, one way is to use Arrays.toString()
int[] arr = {1,2,3,4};
System.out.println(Arrays.toString(arr));

How come printing a string(an Object) in java prints it?

When we do:
String string = new String("Ralph");
//This creates a reference called string that points to a sequence of
//characters in memory
This is the same as:
String string = "Ralph";
When we print both, we get the actual value of the string.
If we print any other object in Java, we get an address for that object.
My question is, is there any dereferencing that is taking place behind the scenes?

When you pass an object reference to the System.out.println() method, for
example, the object's toString() method is called, and the returned value of toString() is shown in the following example:
public class HardToRead {
public static void main (String [] args) {
HardToRead h = new HardToRead();
System.out.println(h);
}
}
Running the HardToRead class gives us the lovely and meaningful,
% java HardToRead
HardToRead#a47e0
Now,
Trying to read this output might motivate you to override the toString()
method in your classes, for example,
public class BobTest {
public static void main (String[] args) {
Bob f = new Bob("GoBobGo", 19);
System.out.println(f);
}
}
class Bob {
int shoeSize;
String nickName;
Bob(String nickName, int shoeSize) {
this.shoeSize = shoeSize;
this.nickName = nickName;
}
public String toString() {
return ("I am a Bob, but you can call me " + nickName +". My shoe size is " + shoeSize);
}
}
This ought to be a bit more readable:
% java BobTest
I am a Bob, but you can call me GoBobGo. My shoe size is 19

The class String is a special class in Java.
But it gets out printed the same way every other class does.
If we call System.out.println("Ralph") the function println takes that String and then displays it.
The class Objects toString() method is implemented, so it displays the hash code of the Object, by calling the hashCode() function. If you overwrite the toString() method, it will display something else.
If you take any object other than a String and give it to a method that takes a String (or in fact cast it to a String) java will call the toString()method of that Object, to convert it to a String.
So 'printing' always does the same thing, it's just implemented in different ways, using the toString() method.

new String("Ralph") copies the character data array of the literal string and stores it in the new String instance.
However, you only get the address of an object when you print it because printing uses the toString() method of that object. If that method is not implemented, the default implementation defined in Object is used, which returns the class name plus the hash code (that seems like an address if hashCode() is not implemented).

I believe the primary types are printed with auto dereferencingString, int, float, etc., while the objects other than primary types, only with de-referencing function object.toString() that are implemented on the object level.

Why is this printing the memory address of array

So this first code returns the memory address of the integer array, but I want it to print out the actual array.
import java.util.Arrays;
public class FinalsReview{
int[] list = new int[]{12, 435,546, 7, 24, 4, 6, 45, 21, 1};
public static void main(String[]args){
FinalsReview hello = new FinalsReview();
System.out.print(hello.create());
}
public int[] create(){
Arrays.toString(list);
return list;
}
}
However, the following code prints the actual array.
import java.util.Arrays;
public class FinalsReview{
int[] list = new int[]{12, 435,546, 7, 24, 4, 6, 45, 21, 1};
public static void main(String[]args){
FinalsReview hello = new FinalsReview();
hello.create();
}
public void create(){
System.out.println(Arrays.toString(list));
}
}
Why does the first one return the memory address?

it is not memory address, it is the hashCode() and classname that is how toString() is defined in Object, when you don't specify toString() method for int[] class you inherit it from Object
and that is implemented like
public String toString() {
return getClass().getName() + "#" + Integer.toHexString(hashCode());
}
while this Arrays.toString(list) is explicitly iterates over the Collection and printing value of each element
and why that from http://bugs.java.com/view_bug.do?bug_id=4168079
One caveat: the toString() method is often used in printing diagnostics.
One would have to be careful if very large arrays were involved. The
Lisp language deals with this using the print-level/print-length mechanism.
Something similar would be needed in Java as well. In practice, the
'toString' method provided in every class should be preferred as the
brief option suitable for use in concise diagnostics, and a more verbose
representation provided by additional application-specific conversion methods
if needed by the application logic.
Regardless of its technical merit, however, it is doubtful that we can make
such a change at this late date due to compatibility/stability concerns.
william.maddox#Eng 1998-08-31
I concur. It would definitely have been the right thing in 1.0, or
maybe even 1.1, but it's almost certainly too late for all of these changes
except perhaps the toString change. One consolation is that it's amazingly easy

It's because you're returning the array from the method create, and when you attempt to print it, the display is what Jigar Joshi described. Simply calling Arrays.toString(list) will not reformat the array so that it will give the output that you expect when you try to print it.
If you want it to print the elements of the array, have your method return String instead, and use return Arrays.toString(list).

Arrays.toString(list) merely returns a String that is being printed at System.out.println(Arrays.toString(list)), however in your first method you are just returning the Array without saving the result of Arrays.toString(list). You're only printing yourList.toString(), so to say:
int[] myList = ... //initialize
String s = Arrays.toString(list); //Save the returned String in a variable
System.out.println(myList); //Prints myList.toString()
System.out.println(s); //Prints out the contents of the array

The line
System.out.print(hello.create());
is evaluated by first invoking the create() method, the passing its return value (a reference to an object of type int[]) to the print() method of System.out. That method's Javadoc reads:
Prints an Object and then terminate the line. This method calls at first String.valueOf(x) to get the printed object's string value, then behaves as though it invokes print(String) and then println().
and the Javadoc of String.valueOf() reads:
if the argument is null, then a string equal to "null"; otherwise, the value of obj.toString() is returned.
Ok, our array reference isn't null, so what does its toString method do? The answer is in section 10.7 of the Java Language Specification, which reads:
The members of an array type are all of the following:
The public final field length, which contains the number of components of the array. length may be positive or zero.
The public method clone, which overrides the method of the same name in class Object and throws no checked exceptions. The return type of the clone method of an array type T[] is T[].
A clone of a multidimensional array is shallow, which is to say that it creates only a single new array. Subarrays are shared.
All the members inherited from class Object; the only method of Object that is not inherited is its clone method.
Therefore, the toString method must be inherited from Object. The Javadoc of Object.toString reads:
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())
... and that's why you see the type and hashCode of the array object when you print your array. (Incidentally, a hash code is not quite the same as a memory address. For one, several objects may have the same hash code; otherwise java would be limited to about 4 billion objects per application).
That's why Arrays.toString is a useful method to have, as it returns a String with the contents of the array. (You do invoke this method in create(), and it builds that String, but then you don't do anything with that String object, which is why it isn't printed).

In your first example, you are running Arrays.toString(list); but youare throwing away the result (a single String), and returning the original value of list. System.out.println is then trying to convert the whole int[] array into a string, which it doesn't know how to do, so it is giving you the unexpected output.
In the second example, you are running the same utility function, but this time passing its output to System.out.println. The input to System.out.println is now the string you wanted.
What you need to do is make the create function return the String given by Arrays.toString(list), rather than the original int[].

What happens when printing an object in java

class Data {
int a = 5;
}
class Main {
public static void main(String[] args) {
int b=5;
Data dObj = new Data();
System.out.println(dObj);
System.out.println(b);
}
}
I want to know what's happening when printing a object or number or string.
I ran the above code, I'm getting the result as "data#1ae73783" for System.out.println(dObj); and "5" for System.out.println(b);
Then I did debug to check whats really happening when printing a object, there was lot of parameter called in a debug mode(like classloader,theards)
I know for the first print the value represent class name followed by address. But don't know what's really happening in debug mode, for the 2nd print only variable assignment happened in the debug mode i.e b=5.
Please explain whats really happening?

You don't need a debugger to know what's happening. System.out is of type PrintStream. The javadoc of PrintStream.println(Object) says:
Prints an Object and then terminate the line. This method calls at first String.valueOf(x) to get the printed object's string value, then behaves as though it invokes print(String) and then println().
The javadoc of String.valueOf(Object) says:
if the argument is null, then a string equal to "null"; otherwise, the value of obj.toString() is returned.
And the javadoc of Object.toString() says:
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())

Please explain whats really happening?
As other have told you, using System.out.println with an object will call to toString method on that object. If the class doesn't have it's own toString method, then it's a call to the super class's toString. If the super class call goes all the way back to java.lang.Object, the default toString method prints the name of the object's type (what class it is), followed by an # sign, and the memory location of the object--the hexidecimal address of where that object is stored in memory.
ClassName#MemoryLocation

when we print object of any class System.out.print() gives string of class name along with memory address of object (ClassName#MemoryAdress)

All objects inherit from java.lang.Object which has a default implementation of toString. If an object overrides this method then out.print (obj) will put something useful on the screen.
Primitive data types are handled by a different, much simpler implementation of println. The println method is overridden for every data type in addition to Object.

First, int isn't an Object. It's primitive type.
Second, when Class haven't overrived toString() method, toString() from Object class is invoked.

data dObj = new data() does not exist in the source code;
you want to print the string value of the object (Data), you have to override the toString method;
try this
public class Program {
public static void main(String[] args) {
Data data = new Data();
System.out.println(data);
}
}
class Data {
int a = 5;
#Override
public String toString() {
return String.valueOf(a);
}
}

In Addition to #JB Nizet answer,
To Provide our own string representation, we have to override toString() in our class which is highly recommended because
There are some classes in which toString() is overridden already to get the proper string representation.
Examle: String, StringBuffer, StringBuilder and all wrapper classes
Integer ob1 = new Integer("10");
String ob2 = new String("Doltan Roy");
StringBuffer ob3 = new StringBuffer("The Rock");
StringBuilder ob4 = new StringBuilder("The Joshua");
System.out.println(ob1);
System.out.println(ob2);
System.out.println(ob3);
System.out.println(ob4);
Output:
10
Doltan Roy
The Rock
The Joshua
Hope this would help!

How can println print an object

{
List list= new ArrayList();
list.add("one");
list.add("second");
System.out.println(list);
}
How can the object "list" be used like it has been in the print statement? Don't we need to use the object to access a method to print the list?

prinln(someObject) will print out whatever is implemented in someObject's toString() method.

You can use toString() which is (supposed) to be implemented for all objects:
System.out.println(list.toString())
Note that you ought not to use the returned string as anything you can actually parse; it's really for a visual representation. It also doesn't need to uniquely represent the object.
When you write
System.out.println(list)
you are, in fact, using the toString() method implicitly.

Docs Says about toString() in Collections:
Returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String.valueOf(Object).

When we pass any object to println() method, it will implicitly call that object's toString() method. So, what is actually executed is
System.out.println( list.toString() );
ArrayList is inherited from the class java.util.AbstractCollection and that class has toString() method. So, in your case, that toString() should be executed.
That toString() method returns a string representation of this collection. The string representation consists of a list of the collection's elements in the order they are returned by its iterator, enclosed in square brackets ("[]"). Adjacent elements are separated by the characters ", " (comma and space). Elements are converted to strings as by String.valueOf(Object).

Looking at your question and comments, I think your confusion ultimately stems from you being unsure how printing works. In general in such cases, I recommend to get the JDK sources and simply take look inside.
In this case, we would first go to the System class and check out the out member (because println is called on System.out):
public final class System {
...
public final static PrintStream out = null;
Since we know now that out is a PrintStream, let's check out that class:
public class PrintStream extends FilterOutputStream
implements Appendable, Closeable
{
...
public void println(Object x) {
String s = String.valueOf(x);
synchronized (this) {
print(s);
newLine();
}
}
(we know it's calling this method, since the other println signatures don't match the type List)
OK, so we see that println converts the given Object (your List in this case) to a String using String.valueOf(Object). Let's check out that method:
public final class String {
...
public static String valueOf(Object obj) {
return (obj == null) ? "null" : obj.toString();
}
So now we know that your List's toString() method is used to generate the output. As others have pointed out, it is overriden in ArrayList to provide the output you see.

It can be used like this because class List inherits the Object class, and the Object class has the toString() method, which means every object can be turned into a String.

printing a list directly will give the the string representation of the list.
If you want to access the objects in the list you need to use iterator or loops or advanced for loops for collections.
e.g.
for(String s : list){
System.out.println(s);
}
Well you can study and try out all the list methods to modify it. e.g. add, remove.
Also if you are printing an object, then toString() method is implicitly called.

You can loop through all the items in the list in order to print them out. you can do this using a while loop of a for loop.
while (list.hasNext) {
System.out.println((String) list.next());
}
for(String i in list){
System.out.println(i) ;
}
Not sure if this is what your looking for but its an option.

Yo need toString() method. when you print object Reference , by default toString() method is called. If you look at toString() method by default it prints
getClass().getName() + '#' + Integer.toHexString(hashCode())
Now as per your requirements you can override this to print the values in list(String Class overrides it too)

Here you are creating List interface.So it will allow to add duplicate values in list also.Also we can directly print object like System.out.println(obj);
And inside the List class the, toString method is being overridden so that it will print all its contents rather than the address of the object.

System.out.println(Arrays.toString(myList.toArray()));