What does "()V" mean in a class signature?

Question

I created a constructor with Javassist which has no real method

CtConstructor c = CtNewConstructor.make ( argTypes, null, newClass );

When I'm trying to put out the signature of this class

c.getSignature();

I get

public Echo ()V

I'm confused what "V" means? I expected either public Echo (); or something similar...

Answer 1

The JVM uses a compact way of storing method signatures, of which constructors are considered a special case.

For your example:

• () indicates a method taking no arguments
• V indicates that it returns nothing

The other parts of the scheme are:

• B - byte
• C - char
• D - double
• F - float
• I - int
• J - long
• S - short
• V - void
• Z - boolean
• [ - array of the thing following the bracket
• L [class name] ; - instance of this class, with dots becoming slashes
• ( [args] ) [return type] - method signature

For example:

public int foo(String bar, long[][] baz)

would become

(Ljava/lang/String;[[J)I

See the spec at Sun^H^H^HOracle's web site

Answer 2

"V" determines the result type "void"

Answer 3

V in a type signature means void type. Bytecode does not differentiate constructors from other methods (other than using special method name).