What do those strange class names in a java heap dump mean?

Question

I'm trying to track down a memory leak in a java process, using jmap and jhat. Every time I do this I see those weird notation for specific object types, like [S for string arrays and [C for Character arrays. I never remember what means what, and it's very hard to google this stuff.

(EDIT: to prove my point, it turns out that [S is array of short and [C is array of char.)

Anyone care to make a table listing all the different class names and what they mean? Or point me to such table?

Specifically I'd like to know what [Ljava.lang.Object; means.

Answer 1

You'll find the complete list documented under Class.getName():

If this class object represents a reference type that is not an array type then the binary name of the class is returned, as specified by the Java™ Language Specification, Second Edition.

If this class object represents a primitive type or void, then the name returned is a String equal to the Java language keyword corresponding to the primitive type or void.

If this class object represents a class of arrays, then the internal form of the name consists of the name of the element type preceded by one or more '[' characters representing the depth of the array nesting. The encoding of element type names is as follows:

 Element Type        Encoding boolean             Z byte                B char                C class or interface  Lclassname; double              D float               F int                 I long                J short               S  

Answer 2

it is an array of objects as specified by JVM Specifications for internal representation of class names:

• a single [ means an array of
• L followed by a fully qualified class name (e.g. java/lang/Object) is the class name terminated by semicolon ;

so [Ljava.lang.object; means Object[]