I get this when I call toString
on an object I received from a function call. I know the type of the object is encoded in this string, but I don't know how to read it.
What is this type of encoding called?
[Ljava.lang.Object;
is the name for Object[].class
, the java.lang.Class
representing the class of array of Object
.
The naming scheme is documented in 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 (§13.1).
If this class object represents a primitive type or
void
, then the name returned is the Java language keyword corresponding to the primitive type orvoid
.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 double D float F int I long J short S class or interface Lclassname;
Yours is the last on that list. Here are some examples:
// xxxxx varies
System.out.println(new int[0][0][7]); // [[[I@xxxxx
System.out.println(new String[4][2]); // [[Ljava.lang.String;@xxxxx
System.out.println(new boolean[256]); // [Z@xxxxx
The reason why the toString()
method on arrays returns String
in this format is because arrays do not @Override
the method inherited from Object
, which is specified as follows:
The
toString
method for classObject
returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. In other words, this method returns a string equal to the value of:getClass().getName() + '@' + Integer.toHexString(hashCode())
Note: you can not rely on the toString()
of any arbitrary object to follow the above specification, since they can (and usually do) @Override
it to return something else. The more reliable way of inspecting the type of an arbitrary object is to invoke getClass()
on it (a final
method inherited from Object
) and then reflecting on the returned Class
object. Ideally, though, the API should've been designed such that reflection is not necessary (see Effective Java 2nd Edition, Item 53: Prefer interfaces to reflection).
toString
for arraysjava.util.Arrays
provides toString
overloads for primitive arrays and Object[]
. There is also deepToString
that you may want to use for nested arrays.
Here are some examples:
int[] nums = { 1, 2, 3 };
System.out.println(nums);
// [I@xxxxx
System.out.println(Arrays.toString(nums));
// [1, 2, 3]
int[][] table = {
{ 1, },
{ 2, 3, },
{ 4, 5, 6, },
};
System.out.println(Arrays.toString(table));
// [[I@xxxxx, [I@yyyyy, [I@zzzzz]
System.out.println(Arrays.deepToString(table));
// [[1], [2, 3], [4, 5, 6]]
There are also Arrays.equals
and Arrays.deepEquals
that perform array equality comparison by their elements, among many other array-related utility methods.