How many times can classes be nested within a class?

Question

I came across this questions on one of the online Java Tests. The options were 4,5,8 and any number of times.

I have only used one inner class, but have never tried multiple ones. I was wondering if anyone knows the answer.

Answer 1

It's a completely irrelevant question and I hope they weren't using the results for anything important. I guess the answer they were looking for was 'any number of times' but in practice there will be a limit in any given implementation of Java. If it's not defined directly it will be determined by something like the maximum file size, or some other (possibly undocumented) internal limit. The best way to find out is to try it.

Update: 30 works, but 300 gives this error:

A.java:3: error while writing B0.B1.B2.B3.B4.B5.B6.B7.B8.B9.B10.B11.B12.B13.B14.
B15.B16.B17.B18.B19.B20.B21.B22.B23.B24.B25.B26.B27.B28.B29.B30.B31.B32.B33.B34.
B35.B36.B37.B38.B39.B40.B41.B42.B43.B44.B45.B46.B47.B48.B49.B50.B51.B52.B53.B54.
B55.B56.B57.B58.B59.B60.B61.B62.B63.B64.B65.B66.B67.B68.B69.B70.B71.B72.B73.B74.
B75.B76.B77.B78.B79.B80.B81.B82.B83.B84.B85.B86.B87.B88.B89.B90.B91.B92.B93.B94.
B95.B96.B97.B98.B99.B100.B101.B102.B103.B104.B105.B106.B107.B108.B109.B110.B111.
B112.B113.B114.B115.B116.B117.B118.B119.B120.B121.B122.B123.B124.B125.B126.B127.
B128.B129.B130.B131.B132.B133.B134.B135.B136.B137.B138.B139.B140.B141.B142.B143.
B144.B145.B146.B147.B148.B149.B150.B151.B152.B153.B154.B155.B156.B157.B158.B159.
B160.B161.B162.B163.B164.B165.B166.B167.B168.B169.B170.B171.B172.B173.B174.B175.
B176.B177.B178.B179.B180.B181.B182.B183.B184.B185.B186.B187.B188.B189.B190.B191.
B192.B193.B194.B195.B196.B197.B198.B199.B200.B201.B202.B203.B204.B205.B206.B207.
B208.B209.B210.B211.B212.B213.B214.B215.B216.B217.B218.B219.B220.B221.B222.B223.
B224.B225.B226.B227.B228.B229.B230.B231.B232.B233.B234.B235.B236.B237.B238.B239.
B240.B241.B242.B243.B244.B245.B246.B247.B248.B249.B250.B251.B252.B253.B254.B255.
B256.B257.B258.B259.B260.B261.B262.B263.B264.B265.B266.B267.B268.B269.B270.B271.
B272.B273.B274.B275.B276.B277.B278.B279.B280.B281.B282.B283.B284.B285.B286.B287.
B288.B289.B290.B291.B292.B293.B294.B295.B296.B297.B298.B299: B0$B1$B2$B3$B4$B5$B
6$B7$B8$B9$B10$B11$B12$B13$B14$B15$B16$B17$B18$B19$B20$B21$B22$B23$B24$B25$B26$B
27$B28$B29$B30$B31$B32$B33$B34$B35$B36$B37$B38$B39$B40$B41$B42$B43$B44$B45$B46$B
47$B48$B49$B50$B51$B52$B53$B54$B55$B56$B57$B58$B59$B60$B61$B62$B63$B64$B65$B66$B
67$B68$B69$B70$B71$B72$B73$B74$B75$B76$B77$B78$B79$B80$B81$B82$B83$B84$B85$B86$B
87$B88$B89$B90$B91$B92$B93$B94$B95$B96$B97$B98$B99$B100$B101$B102$B103$B104$B105
$B106$B107$B108$B109$B110$B111$B112$B113$B114$B115$B116$B117$B118$B119$B120$B121
$B122$B123$B124$B125$B126$B127$B128$B129$B130$B131$B132$B133$B134$B135$B136$B137
$B138$B139$B140$B141$B142$B143$B144$B145$B146$B147$B148$B149$B150$B151$B152$B153
$B154$B155$B156$B157$B158$B159$B160$B161$B162$B163$B164$B165$B166$B167$B168$B169
$B170$B171$B172$B173$B174$B175$B176$B177$B178$B179$B180$B181$B182$B183$B184$B185
$B186$B187$B188$B189$B190$B191$B192$B193$B194$B195$B196$B197$B198$B199$B200$B201
$B202$B203$B204$B205$B206$B207$B208$B209$B210$B211$B212$B213$B214$B215$B216$B217
$B218$B219$B220$B221$B222$B223$B224$B225$B226$B227$B228$B229$B230$B231$B232$B233
$B234$B235$B236$B237$B238$B239$B240$B241$B242$B243$B244$B245$B246$B247$B248$B249
$B250$B251$B252$B253$B254$B255$B256$B257$B258$B259$B260$B261$B262$B263$B264$B265
$B266$B267$B268$B269$B270$B271$B272$B273$B274$B275$B276$B277$B278$B279$B280$B281
$B282$B283$B284$B285$B286$B287$B288$B289$B290$B291$B292$B293$B294$B295$B296$B297
$B298$B299.class (The filename, directory name, or volume label syntax is incorr
ect)

Code used to generate source (written in Python):

n = input()
print "class A{public static void main(String[] a){}}\n"
print ''.join("class B%d{" % x for x in range(n)) + '}' * n

Disappointing. I was actually hoping it would be a lot more. I wonder if it could get much further on another system which allows longer filenames.

Answer 2

Hmmm. I know that the online Java test you refer to is a lousy one. Does that count?

(This is the kind of limit that is irrelevant in practical experience. A similarly ridiculously question would be, "What is the maximum length of a function in bytes?")

Answer 3

I tried on my own: The answer is any number of times. The following is my test class, I had no compilation errors.

public class Test {


    public Test ()
    {

    }
    public static void main (String args[])
    {
    new Test ();
    }

    class Test2
    {
        class Test3
        {
            class Test4
            {
                class Test5{
                    class Test6{
                        class Test7{
                            class Test8{
                                class Test9
                                {

                                }
                            }
                        }
                    }
                }
            }

        }

    }


}

Answer 4

The true answer is none of the above. Javac will of course allow arbitrary nesting, but the underlying file system and/or OS have restrictions. We found recently that during startup the JVM running a Glassfish Java EE application server instance will hold open files to each .class during loading/JIT compilation. If you're running CentOS Linux say, the default limit on open files/process is 1024, so if you have a lot of classes being loaded at once and don't set the ulimit -n to a higher value, the JVM will hit "too many open files". And @Mark Byers' example shows that the .class file names get very long and can hit the file system name length limit, if there is one.

Answer 5

As a follow-up to Mark's trial on windows - a bit silly but fun nonetheless - I did a trial on AIX 5.3.

91 subclasses deep is ok, number 92 results in the following error :

-rw-r--r--   1 root     system        12813 Nov 24 15:15 A.class
Main class=class A

count=90 class=A$B$C$D$E$F$G$H$I$J$K$L$M$N$O$P$Q$R$S$T$U$V$W$X$Y$Z$AA$AB$AC$AD$AE$AF$AG$AH$AI$AJ$AK$AL$AM$AN$AO$AP$AQ$AR$AS$AT$AU$AV$AW$AX$AY$AZ$BA$BB$BC$BD$BE$BF$BG$BH$BI$BJ$BK$BL$BM$BN$BO$BP$BQ$BR$BS$BT$BU$BV$BW$BX$BY$BZ$CA$CB$CC$CD$CE$CF$CG$CH$CI$CJ$CK$CL$CM

-rw-r--r--   1 root     system        13081 Nov 24 15:16 A.class
Main class=class A

count=91 class=A$B$C$D$E$F$G$H$I$J$K$L$M$N$O$P$Q$R$S$T$U$V$W$X$Y$Z$AA$AB$AC$AD$AE$AF$AG$AH$AI$AJ$AK$AL$AM$AN$AO$AP$AQ$AR$AS$AT$AU$AV$AW$AX$AY$AZ$BA$BB$BC$BD$BE$BF$BG$BH$BI$BJ$BK$BL$BM$BN$BO$BP$BQ$BR$BS$BT$BU$BV$BW$BX$BY$BZ$CA$CB$CC$CD$CE$CF$CG$CH$CI$CJ$CK$CL$CM$CN

A.java:93: error while writing A.B.C.D.E.F.G.H.I.J.K.L.M.N.O.P.Q.R.S.T.U.V.W.X.Y.Z.AA.AB.AC.AD.AE.AF.AG.AH.AI.AJ.AK.AL.AM.AN.AO.AP.AQ.AR.AS.AT.AU.AV.AW.AX.AY.AZ.BA.BB.BC.BD.BE.BF.BG.BH.BI.BJ.BK.BL.BM.BN.BO.BP.BQ.BR.BS.BT.BU.BV.BW.BX.BY.BZ.CA.CB.CC.CD.CE.CF.CG.CH.CI.CJ.CK.CL.CM.CN.CO: A$B$C$D$E$F$G$H$I$J$K$L$M$N$O$P$Q$R$S$T$U$V$W$X$Y$Z$AA$AB$AC$AD$AE$AF$AG$AH$AI$AJ$AK$AL$AM$AN$AO$AP$AQ$AR$AS$AT$AU$AV$AW$AX$AY$AZ$BA$BB$BC$BD$BE$BF$BG$BH$BI$BJ$BK$BL$BM$BN$BO$BP$BQ$BR$BS$BT$BU$BV$BW$BX$BY$BZ$CA$CB$CC$CD$CE$CF$CG$CH$CI$CJ$CK$CL$CM$CN$CO.class (A file or path name is too long.)
static class CO {
       ^
1 error

The generated Java source looks like:

class A {
  static class B {
    void run() { 
      System.out.println("count=" + 2 + " class=" + B.class.getName() + "\n"); 
    }
  }

  public static void main(String[] a){
    System.out.println("Main class=" + A.class + "\n");
    (new A.B()).run();
  }
}

Answer 6

是的，你可以在Java中无限地嵌套一个类。