Java中奇怪的printf（）行为

Question

I have the following code fragment

System.out.printf("%b\n", 123);

which prints "true".

Can somebody explain this behavior? shouldn't this throw a IllegalFormatException?

Answer 1

Well since the specification says:

"If the argument arg is null, then the result is "false". If arg is a boolean or Boolean, then the result is the string returned by String.valueOf(). Otherwise, the result is "true". " ( src )

The behavior is quite expected isn't it? Why they decided to implement it that way - no idea, I'd agree that it's not intuitive (but well it follows C which also prints just anything if you give it the wrong arguments ;) )

Answer 2

From the JavaDocs:

If the argument arg is null, then the result is "false". If arg is a boolean or Boolean, then the result is the string returned by String.valueOf(). Otherwise, the result is "true".

The argument you're giving it isn't null , boolean , or Boolean , so it falls under "Otherwise" and therefore is true