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

Answer 1

Means Object[]

Answer 2

The rules are listed in the API doc of Class.getName().

[Ljava.lang.Object; would be an instance of Object[]. Note that multidimensional arrays are displayed with multiple opening braces.

Answer 3

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[]

Answer 4

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