CODEWITHSUNDEEP
javastringcastinginteger
user710818
user710818

Reputation: 24248

Why cannot cast Integer to String in java?

I found some strange exception:

java.lang.ClassCastException: java.lang.Integer 
 cannot be cast to java.lang.String

How it can be possible? Each object can be casted to String, doesn't it?

The code is:

String myString = (String) myIntegerObject;

Thanks.

Upvotes: 115

Views: 235070

Answers (11)

A.Aleem11
A.Aleem11

Reputation: 1964

Use .toString instead like below:

String myString = myIntegerObject.toString();

Upvotes: 0

Dmytro Sokolyuk
Dmytro Sokolyuk

Reputation: 1076

In your case don't need casting, you need call toString().

Integer i = 33;
String s = i.toString();
//or
s = String.valueOf(i);
//or
s = "" + i;

Casting. How does it work?

Given:

class A {}
class B extends A {}

(A)
  |
(B)

B b = new B(); //no cast
A a = b;  //upcast with no explicit cast
a = (A)b; //upcast with an explicit cast
b = (B)a; //downcast

A and B in the same inheritance tree and we can this:

a = new A();
b = (B)a;  // again downcast. Compiles but fails later, at runtime: java.lang.ClassCastException

The compiler must allow things that might possibly work at runtime. However, if the compiler knows with 100% that the cast couldn't possibly work, compilation will fail.
Given:

class A {}
class B1 extends A {}
class B2 extends A {}

        (A)
      /       \
(B1)       (B2)

B1 b1 = new B1();
B2 b2 = (B2)b1; // B1 can't ever be a B2

Error: Inconvertible types B1 and B2. The compiler knows with 100% that the cast couldn't possibly work. But you can cheat the compiler:

B2 b2 = (B2)(A)b1;

but anyway at runtime:

Exception in thread "main" java.lang.ClassCastException: B1 cannot be cast to B2

in your case:

          (Object)
            /       \
(Integer)       (String)

Integer i = 33;
//String s = (String)i; - compiler error
String s = (String)(Object)i;

at runtime: Exception in thread "main" java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String

Upvotes: 1

Sachin
Sachin

Reputation: 18747

Why this is not possible:

Because String and Integer are not in the same Object hierarchy.

      Object
     /      \
    /        \
String     Integer

The casting which you are trying, works only if they are in the same hierarchy, e.g.

      Object
     /
    /
   A
  /
 /
B

In this case, (A) objB or (Object) objB or (Object) objA will work.

Hence as others have mentioned already, to convert an integer to string use:

String.valueOf(integer), or Integer.toString(integer) for primitive,

or

Integer.toString() for the object.

Upvotes: 177

DRiFTy
DRiFTy

Reputation: 11369

For int types use:

int myInteger = 1;
String myString = Integer.toString(myInteger);

For Integer types use:

Integer myIntegerObject = new Integer(1);
String myString = myIntegerObject.toString();

Upvotes: 22

Jonathan
Jonathan

Reputation: 7604

You can't cast explicitly anything to a String that isn't a String. You should use either:

"" + myInt;

or:

Integer.toString(myInt);

or:

String.valueOf(myInt);

I prefer the second form, but I think it's personal choice.

Edit OK, here's why I prefer the second form. The first form, when compiled, could instantiate a StringBuffer (in Java 1.4) or a StringBuilder in 1.5; one more thing to be garbage collected. The compiler doesn't optimise this as far as I could tell. The second form also has an analogue, Integer.toString(myInt, radix) that lets you specify whether you want hex, octal, etc. If you want to be consistent in your code (purely aesthetically, I guess) the second form can be used in more places.

Edit 2 I assumed you meant that your integer was an int and not an Integer. If it's already an Integer, just use toString() on it and be done.

Upvotes: 6

yshavit
yshavit

Reputation: 43391

Casting is different than converting in Java, to use informal terminology.

Casting an object means that object already is what you're casting it to, and you're just telling the compiler about it. For instance, if I have a Foo reference that I know is a FooSubclass instance, then (FooSubclass)Foo tells the compiler, "don't change the instance, just know that it's actually a FooSubclass.

On the other hand, an Integer is not a String, although (as you point out) there are methods for getting a String that represents an Integer. Since no no instance of Integer can ever be a String, you can't cast Integer to String.

Upvotes: 3

RanRag
RanRag

Reputation: 49557

Use String.valueOf(integer).

It returns a string representation of integer.

Upvotes: 0

millimoose
millimoose

Reputation: 39950

Objects can be converted to a string using the toString() method:

String myString = myIntegerObject.toString();

There is no such rule about casting. For casting to work, the object must actually be of the type you're casting to.

Upvotes: 5

andri
andri

Reputation: 11292

No. Every object can be casted to an java.lang.Object, not a String. If you want a string representation of whatever object, you have to invoke the toString() method; this is not the same as casting the object to a String.

Upvotes: 6

Savino Sguera
Savino Sguera

Reputation: 3572

You should call myIntegerObject.toString() if you want the string representation.

Upvotes: 4

Petar Minchev
Petar Minchev

Reputation: 47373

No, Integer and String are different types. To convert an integer to string use: String.valueOf(integer), or Integer.toString(integer) for primitive, or Integer.toString() for the object.

Upvotes: 51

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