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Message:

surprise with static classes having private constructors

Posted by Neville on April 01, 2001 at 12:19 AM

(delete "antispam" from the domain name above to obtain my true email address)

Can anyone explain to me the strange treatment by the JDK 1.3 javac compiler of a static nested class with a private constructor? An unexpected, apparently local, class file is compiled from the sample code shown below:

class Outer
{
Outer(){
Nested i = new Nested();
}
static class Nested
{
// Outer$1.class is created if constructor is private
private Nested(){
}
}
}

A file Outer$1.class is created in addition to the expected Outer.class and Outer$Nested.class. The compiler does not generate Outer$1.class if the private access modifier is deleted from the Nested constructor declaration as follows:

class Outer
{
Outer(){
Nested i = new Nested();
}
static class Nested
{
// Outer$1.class is created if constructor is private
Nested(){
}
}
}

When the Nested constructor is declared private, a disassembly
of the Outer$Nested.class shows an unexpected extra constructor
that takes an argument of type Outer$1:

---- disassembly of Outer$Nested (private constructor) ----

Compiled from Outer.java
public static class Outer. Nested extends java.lang.Object
/* ACC_SUPER bit NOT set */
{
private Outer.Nested();
/* ()V */
/* Stack=1, Locals=1, Args_size=1 */
Outer.Nested(Outer$1);
/* (LOuter$1;)V */
/* Stack=1, Locals=2, Args_size=2 */
}

Method Outer. Nested()
0 aload_0
1 invokespecial #2
4 return

Line numbers for method Outer. Nested()
line 12: 0
line 13: 4

Method Outer. Nested(Outer$1)
0 aload_0
1 invokespecial #1
4 return

Line numbers for method Outer. Nested(Outer$1)
line 8: 0

--- end disassembly of Outer$Nested (private constructor) ---

A disassembly of Outer.class contains the following entry
that shows the undeclared Nested constructor is performed
on the Nested object created in the Outer constructor:

9 invokespecial #3

A disassembly of Outer$1 shows it to be an empty class:

Compiled from Outer.java
class Outer$1 extends java.lang.Object {
}

When the Nested constructor has package access,
a disassembly shows only one constructor as expected:

---- disassembly of Outer$Nested (package constructor) ----

Compiled from Outer.java
public static class Outer. Nested extends java.lang.Object
/* ACC_SUPER bit NOT set */
{
Outer.Nested();
/* ()V */
/* Stack=1, Locals=1, Args_size=1 */
}

Method Outer. Nested()
0 aload_0
1 invokespecial #1
4 return

Line numbers for method Outer. Nested()
line 12: 0
line 13: 4

--- end disassembly of Outer$Nested (package constructor) ---

When the Nested constructor has package access,
the Outer constructor calls the expected Nested constructor:

8 invokespecial #3

Why this strange distinction between public and private
static classes? Does this have anything to do with
security?

• Just show the supremacy of Outer class Kishori Sharan April 01, 2001 at 4:52 PM (0)