ClassCastException when converting from String to Object.. why?

Question

I am just playing with MessageFormat but when I try to pass a String to MessageFormat format method it compiles fine but then I get a runtime classcast exception. Here is the code.

MessageFormat format = new MessageFormat(""); Object obj = Integer.toHexString(10); format.format(obj);

Now the runtime exception I get is as follows.

Exception in thread "main" java.lang.ClassCastException: java.lang.String cannot be cast to [Ljava.lang.Object; at java.text.MessageFormat.format(Unknown Source) at java.text.Format.format(Unknown Source) at JavaCore2.Codepoint.main(Codepoint.java:21)

andrewtc · Accepted Answer · 2011-09-20 15:41:53Z

6

MessageFormat.format() takes an argument of type Object[] (an Object array), whereas you are passing in a single Object.

You will have to create an array out of your Integer:

MessageFormat format = new MessageFormat("{0}");
Object[] args = { Integer.toHexString(10) };

String result = format.format(args);

answered Sep 20, 2011 at 15:41

andrewtc

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2 Comments

Boris Treukhov Over a year ago

This is correct answer javadoc clearly says that single argument format(obj) is equivalent to format(obj, new StringBuffer(), new FieldPosition(0)).toString(); which in turn(if you jump to another javadoc) is equivalent to format((Object[]) arguments, result, pos). What a nice people work in Oracle!

luis.espinal Over a year ago

Agreed. This gets me every time.

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