Printing a primitive array

[Vinoth Kumar Kannan]

int arr[] = new int[]{ 1,2,3,41 };
System.out.println(months);

This prints some junk!! - [I@19821f or [I@3e25a5 - random of one of these every time this code is run
Why is this not working?
Is Arrays.toString(arr) the only way to print a primitive array?

[Henry Wong]

vinoth kumar k wrote:int arr[] = new int[]{ 1,2,3,41 };
System.out.println(months);

This prints some junk!! - [I@19821f or [I@3e25a5 - random of one of these every time this code is run
Why is this not working?
Is Arrays.toString(arr) the only way to print a primitive array?

The println() calls the toString() method to convert the object to a string before printing it. And arrays do not override the toString() method, so it is inherited from the object class.

The toString() method of the Object class prints the object type, followed by an @, followed by the identity hashcode in hex... so... [I@3e25a5 means array "[" of int "I" with hashcode 3e25a5.

Henry

[Janeice DelVecchio]

iterating through an array is another way to output.

[subhash kumar]

  

int arr[] = new int[]{ 1,2,3,41 }; 
	for(int k =0;k < arr.length;k++){
		System.out.println(arr[k]);
	}

[Jesper de Jong]

To print an array nicely formatted, you can also use Arrays.toString:

int arr[] = new int[]{ 1,2,3,41 };
System.out.println(Arrays.toString(arr));

[Abimaran Kugathasan]

Henry Wong wrote:
The println() calls the toString() method to convert the object to a string before printing it. And arrays do not override the toString() method, so it is inherited from the object class.
Henry

Henry, one question, which class you are mentioned here?

[Darryl Burke]

Abimaran Kugathasan wrote:
Henry, one question, which class you are mentioned here?

The array class
http://java.sun.com/docs/books/jls/second_edition/html/arrays.doc.html

[Abimaran Kugathasan]

Darryl Burke wrote:

Abimaran Kugathasan wrote:
Henry, one question, which class you are mentioned here?

The array class
http://java.sun.com/docs/books/jls/second_edition/html/arrays.doc.html

You mean to say Arrays class, there is no class in the API with the name array, that's why I asked!

[Campbell Ritchie]

The array class in the current question is the class called [I. Whether this is a notional class or an actual class is not important, but it is "whichever class arrays are made from."

[Abimaran Kugathasan]

Campbell Ritchie wrote:The array class in the current question is the class called [I. Whether this is a notional class or an actual class is not important, but it is "whichever class arrays are made from."

Campbell, I couldn't get it.

[fred rosenberger]

Anytime you use "System.out.println()" and pass in an object reference, java will call that underlying object's toString() method. It doesn't matter if it's an array, a String, an Integer, a Dog, a Fubar or what.

In your example, you are passing in an object that is an array (doesn't matter what the specific type is). Since the Arrays class does not override the Object class' toString() method, you get the default behavior of the Object class' toString() method.

You would see something VERY similar if you created your own Fubar class and did a System.out.println on it.

However, if in your class definition, you override the toString() method with something meaningful, when you sent it to S.o.p(), you will get your newly defined toString() behavior.

[Henry Wong]

Abimaran Kugathasan wrote:

Campbell Ritchie wrote:The array class in the current question is the class called [I. Whether this is a notional class or an actual class is not important, but it is "whichever class arrays are made from."

Campbell, I couldn't get it.

Basically, the java compiler create "array" classes, that mirror the regular classes, and for the primative types.

Meaning there is a class type called Object[], String[]. StringBuffer[]. Integer[], etc. And for the primatives, class types called int[], float[], char[], etc. These classes are created on-the-fly by the compiler (or the JVM, not sure). There is no java source for these classes, and hence, there is no JavaDoc for these classes.

Henry

[Campbell Ritchie]

I think Henry has explained it far better than I would have.

[Rob Spoor]

As for the [I in the output, this is how these classes are named: [ to indicate it's an array, followed by the native (JNI) name:
- Z for boolean (because B is used elsewhere)
- B for byte
- C for char
- S for short
- I for int
- J for long (because L is used elsewhere)
- F for float
- D for double
- L followed by the fully qualified name and a ; for objects; e.g. [Ljava.lang.String; for String[]

Each additional dimension simply gets a [ at the start; e.g. [[I for int[][].

You don't need to remember this (unless you're doing JNI programming), but it may be a fun fact for you.

[Abimaran Kugathasan]

Thanks Henry Wong and Campbell Ritchie, for this information, could you Please confirm the following....

Is this correct? I mean the hierarchy of the arrays?

                           Object
 +------+---------------------+----------------------+
 |      |                                        Object[]
int[] long[]             +----------+---------+---------+--------+------------+
(all 1D primitive        |          |         |         |        |            |
     arrays)          String[]  Integer[]  MyClass[]  int[][]  long[][]   Object[][]
                        (1D arrays of references)     (2D arrays of           |
                                                        primitives)           |
                                                                              |
                   +------------+-----------+----------+----------+-----------+
              String[][]  Integer[][]  MyClass[][]  int[][][] long[][][]      |
                                                                              |
                                                                         Object[][][]

Thanks in Advanced!

[Rob Spoor]

Yes and no.
Yes, Object[][] is a subtype of Object[] -- you can assign an Object[][] to an Object[], and instanceof also works this way.
No, Object[] is not a subclass of Object[], its super class is Object.

And there is no such thing as 2D or 3D array in Java. They are arrays of arrays (of arrays). And since an array is an object this explains why int[][][] is a subtype of Object[][].

[Abimaran Kugathasan]

Rob Prime wrote:Yes and no.
Yes, Object[][] is a subtype of Object[] -- you can assign an Object[][] to an Object[], and instanceof also works this way.
No, Object[] is not a subclass of Object[], its super class is Object.

And there is no such thing as 2D or 3D array in Java. They are arrays of arrays (of arrays). And since an array is an object this explains why int[][][] is a subtype of Object[][].

What about that bold line?

And this is for array assignments, that means, We can't assign different dimensional arrays....

[Henry Wong]

Abimaran Kugathasan wrote:

Rob Prime wrote:Yes and no.
Yes, Object[][] is a subtype of Object[] -- you can assign an Object[][] to an Object[], and instanceof also works this way.
No, Object[][] is not a subclass of Object[], its super class is Object.

And there is no such thing as 2D or 3D array in Java. They are arrays of arrays (of arrays). And since an array is an object this explains why int[][][] is a subtype of Object[][].

What about that bold line?

And this is for array assignments, that means, We can't assign different dimensional arrays....

First, I fixed the quote (to what I think Rob meant to say).

Basically, Rob is reporting that it is really weird. If you do assignments, or check it with the instanceof operator, then a Object[][] IS-A Object[] which IS-A Object. And it also enjoys being able to be assigned as such. ie. you can assign a Object[][] object to a reference for Object[].

However, if you actually try to follow the hierarchy, using the reflection libraries, you will see that the array hierarchy is flat. Every array class has java.lang.Object as its direct superclass.

Henry

[Rob Spoor]

Exactly.

[Vinoth Kumar Kannan]

Henry Wong wrote:

The toString() method of the Object class prints the object type, followed by an @, followed by the identity hashcode in hex... so... [I@3e25a5 means array "[" of int "I" with hashcode 3e25a5.

If 3e25a5 is the hashcode of the array, then on running again why do I get another value at times? Isn't the hashcode supposed to be same for a particular text/object/variable/whatever?

[Abimaran Kugathasan]

Vinoth Kumar Kannan wrote:
If 3e25a5 is the hashcode of the array, then on running again why do I get another value at times? Isn't the hashcode supposed to be same for a particular text/object/variable/whatever?

If you don't override the hashCode() method, which is inherited from Object class, it'll return the memory address of the object.(Condition applies! )

[Rob Spoor]

The identity hash code will be the same -- within the same JVM. If you restart it then all bets are off.

[Rob Spoor]

Abimaran Kugathasan wrote:it'll return the memory address of the object.

Whoever gave you that false idea? The memory address is shielded from you by the JVM. The identity hash code may be based on the (original) memory location, but there are no guarantees about this.

[Abimaran Kugathasan]

Rob Prime wrote:
Whoever gave you that false idea? The memory address is shielded from you by the JVM. The identity hash code may be based on the (original) memory location, but there are no guarantees about this.

That's why I add the Condition applies term, for a newbie, this is easy to understand the concept.