[L array notation - where does it come from?

Question

I've often seen messages that use [L then a type to denote an array, for instance:

[Ljava.lang.Object; cannot be cast to [Ljava.lang.String;

(The above being an arbitrary example I just pulled out.) I know this signifies an array, but where does the syntax come from? Why the beginning [ but no closing square bracket? And why the L? Is it purely arbitrary or is there some other historical/technical reason behind it?

Answer 1

[ stands for Array, the Lsome.type.Here means the type. That's similar to the type descriptors used internally in the bytecode seen in §4.3 of the Java Virtual Machine Specification -- picked to be as brief as possible. The only difference is in that the real descriptors use / rather than . for denoting packages.

For instance, for primitives the value is: [I for array of int, a two-dimensional array would be: [[I.

Since classes may have any name, it would be harder to identify what class it is, hence the L, the class name ends with a ;

Descriptors are also used to represent the types of fields and methods.

For instance:

(IDLjava/lang/Thread;)Ljava/lang/Object; 

... corresponds to a method whose parameters are int, double, and Thread and the return type is Object

edit

You can also see this in .class files using the java dissambler

C:>more > S.java class S {   Object  hello(int i, double d, long j, Thread t ) {    return new Object();   } } ^C C:>javac S.java  C:>javap -verbose S class S extends java.lang.Object   SourceFile: "S.java"   minor version: 0   major version: 50   Constant pool: const #1 = Method       #2.#12; //  java/lang/Object."<init>":()V const #2 = class        #13;    //  java/lang/Object const #3 = class        #14;    //  S const #4 = Asciz        <init>; const #5 = Asciz        ()V; const #6 = Asciz        Code; const #7 = Asciz        LineNumberTable; const #8 = Asciz        hello; const #9 = Asciz        (IDJLjava/lang/Thread;)Ljava/lang/Object;; const #10 = Asciz       SourceFile; const #11 = Asciz       S.java; const #12 = NameAndType #4:#5;//  "<init>":()V const #13 = Asciz       java/lang/Object; const #14 = Asciz       S;  { S();   Code:    Stack=1, Locals=1, Args_size=1    0:   aload_0    1:   invokespecial   #1; //Method java/lang/Object."<init>":()V    4:   return   LineNumberTable:    line 1: 0   java.lang.Object hello(int, double, long, java.lang.Thread);   Code:    Stack=2, Locals=7, Args_size=5    0:   new     #2; //class java/lang/Object    3:   dup    4:   invokespecial   #1; //Method java/lang/Object."<init>":()V    7:   areturn   LineNumberTable:    line 3: 0   } 

And in raw class file ( look at line 5 ):

Reference: Field description on the JVM specification