Botsford, J.
The question we here consider concerns the application of this court’s decision in Johnstone, petitioner, 453 Mass. 544 (2009) (Johnstone), to a discharge petition brought by a sexually dangerous person (SDP) that was pending on appeal on the date of the Johnstone decision. In Johnstone, this court held that “in order for the Commonwealth to proceed to trial in a [SDP] discharge proceeding under G. L. c. 123A, § 9, at least one of the two qualified examiners [appointed to examine the petitioner] must opine that the petitioner remains sexually dangerous.” Id. at 553. If neither qualified examiner so opines, the petitioner is entitled to a directed verdict and judgment in his favor, and an order of discharge. See id. at 546, 551-553.
The petitioner in this case, John Mclntire, brings a direct appeal from an adverse judgment in a discharge proceeding that was tried in the Superior Court in 2002, long before the Johnstone decision (2002 discharge proceeding). The Commonwealth proceeded to trial although neither of the qualified examiners appointed to examine the petitioner under G. L. c. 123A, § 9, had opined that he remained sexually dangerous. We agree with the petitioner that Johnstone applies to his direct appeal from the judgment in the 2002 discharge proceeding, and requires that the judgment be vacated. However, we disagree that as a result of a Johnstone error, the judgment in that case was void from inception; the judgment was voidable on account of error, but not void. Accordingly, we conclude that the judgments adverse to the petitioner in his subsequent discharge proceedings — tried in 2005, 2008, and 2010,1 respectively — also are not void, and that the petitioner presently is not entitled to an order of discharge.
1. Background. We recite the facts relevant to this appeal, which are undisputed.2 In March, 1984, when he was twenty years old, the petitioner pleaded guilty to charges of indecent assault and battery on a child under the age of fourteen, G. L. [259]*259c. 265, § 13B; rape and abuse of a child under the age of sixteen (two indictments), G. L. c. 265, § 23; assault and battery by means of a dangerous weapon, G. L. c. 265, § 15A; kidnapping, G. L. c. 265, § 26; and larceny, G. L. c. 266, § 30. Following his guilty pleas, he was committed to the Massachusetts Treatment Center in Bridgewater (treatment center) for sixty days for examination, and in November, 1984, he was adjudicated an SDP by a judge in the Superior Court. As a result of that adjudication, and in lieu of sentencing on the indecent assault and battery and rape and abuse charges, the petitioner was committed to the treatment center in November, 1984; he was twenty-one years old.3
On January 31, 2000, the petitioner filed a petition pursuant to G. L. c. 123A, § 9, seeking discharge from the treatment center on the ground that he was no longer an SDP. During a jury trial in the Superior Court on the petition in June, 2002, the two appointed qualified examiners,4 called as witnesses by the petitioner, both testified that, in their professional opinion, the petitioner was no longer an SDP. The petitioner also called two other psychologists who testified to the same effect. The Commonwealth relied exclusively on a report by the Community Access Board (CAB) and the testimony of the CAB’s chair, a licensed psychologist, who opined that the petitioner remained sexually dangerous. The jury found that the petitioner remained an SDP, and judgment entered on the jury’s verdict. In July, 2002, the petitioner filed a notice of appeal in the trial court; the appeal was entered in the Appeals Court in August, 2006. In [260]*260December, 2006, the petitioner sought a stay of appeal in order to file a motion for a new trial in the Superior Court, which the Appeals Court granted. The petitioner filed his new trial motion in the Superior Court in January, 2007. In July, 2007, his counsel filed a motion to amend the petitioner’s memorandum in support of the motion for a new trial. The amended memorandum raised, for the first time, the petitioner’s argument that the 2002 judgment was improper because both qualified examiners had opined that he was no longer an SDP, and only the chair of the CAB offered an opinion that the petitioner was still an SDP. In November, 2007, the judge who had presided at the 2002 discharge proceeding denied the petitioner’s motion for a new trial. The petitioner appealed from the denial, and the Appeals Court consolidated that appeal with his direct appeal from the 2002 judgment.
In a memorandum and order issued pursuant to its rule 1:28 in 2010, the Appeals Court concluded that the interpretation of G. L. c. 123A, § 9, set forth in Johnstone governed the petitioner’s appeal, and that, accordingly, the judgment must be reversed because neither qualified examiner had determined that the petitioner remained an SDP. Mclntire, petitioner, 76 Mass. App. Ct. 1117 (2010). That court also concluded that contrary to the petitioner’s argument, the error affecting the judgment in the 2002 discharge proceeding neither rendered the 2002 judgment void ab initio nor invalidated the judgments in later discharge proceedings in which the petitioner had been found to remain an SDP. We granted the petitioner’s application for further appellate review.
2. Discussion, a. Application of Johnstone. The petitioner argues that our holding in Johnstone, 453 Mass, at 553, must be applied retroactively to invalidate the jury verdict and resulting judgment against him in 2002; for its part, the Commonwealth contends that the court can, and should, apply Johnstone only on a prospective basis to discharge petitions filed after the date of the Johnstone decision in 2009. We agree with the Appeals Court that a retroactive-prospective analysis is unnecessary.
“In general, changes in the common law brought about by judicial decisions are given retroactive effect.” Halley v. Birbiglia, 390 Mass. 540, 544 (1983). See Papadopoulos v. Target Corp., 457 Mass. 368, 384-385 (2010) (applying retroactively [261]*261new common-law rule of premises liability based on abolition of distinction between natural and unnatural accumulation of snow and ice); Schrottman v. Barnicle, 386 Mass. 627, 631 (1982) (“Decisional law is generally applied ‘retroactively’ to past events”). Where a decision does not announce new common-law rules or rights but rather construes a statute, no analysis of retroactive or prospective effect is required because at issue is the meaning of the statute since its enactment. See Shawmut Worcester County Bank v. Miller, 398 Mass. 273, 281 (1986) (rejecting argument that court’s construction of G. L. c. 106, § 9-504 [1] and [3], adopted in that case, should be applied on prospective basis only: “although this court has not previously dealt with the issues raised here, we are not announcing common law rules but rather are construing certain statutory provisions. Those provisions have had the same meaning since the effective date of the statutes”). See also Mouradian v. General Elec. Co., 23 Mass. App. Ct. 538, 542 n.3 (1987).
The question in Johnstone was purely one of statutory interpretation: whether, under G. L. c.
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Botsford, J.
The question we here consider concerns the application of this court’s decision in Johnstone, petitioner, 453 Mass. 544 (2009) (Johnstone), to a discharge petition brought by a sexually dangerous person (SDP) that was pending on appeal on the date of the Johnstone decision. In Johnstone, this court held that “in order for the Commonwealth to proceed to trial in a [SDP] discharge proceeding under G. L. c. 123A, § 9, at least one of the two qualified examiners [appointed to examine the petitioner] must opine that the petitioner remains sexually dangerous.” Id. at 553. If neither qualified examiner so opines, the petitioner is entitled to a directed verdict and judgment in his favor, and an order of discharge. See id. at 546, 551-553.
The petitioner in this case, John Mclntire, brings a direct appeal from an adverse judgment in a discharge proceeding that was tried in the Superior Court in 2002, long before the Johnstone decision (2002 discharge proceeding). The Commonwealth proceeded to trial although neither of the qualified examiners appointed to examine the petitioner under G. L. c. 123A, § 9, had opined that he remained sexually dangerous. We agree with the petitioner that Johnstone applies to his direct appeal from the judgment in the 2002 discharge proceeding, and requires that the judgment be vacated. However, we disagree that as a result of a Johnstone error, the judgment in that case was void from inception; the judgment was voidable on account of error, but not void. Accordingly, we conclude that the judgments adverse to the petitioner in his subsequent discharge proceedings — tried in 2005, 2008, and 2010,1 respectively — also are not void, and that the petitioner presently is not entitled to an order of discharge.
1. Background. We recite the facts relevant to this appeal, which are undisputed.2 In March, 1984, when he was twenty years old, the petitioner pleaded guilty to charges of indecent assault and battery on a child under the age of fourteen, G. L. [259]*259c. 265, § 13B; rape and abuse of a child under the age of sixteen (two indictments), G. L. c. 265, § 23; assault and battery by means of a dangerous weapon, G. L. c. 265, § 15A; kidnapping, G. L. c. 265, § 26; and larceny, G. L. c. 266, § 30. Following his guilty pleas, he was committed to the Massachusetts Treatment Center in Bridgewater (treatment center) for sixty days for examination, and in November, 1984, he was adjudicated an SDP by a judge in the Superior Court. As a result of that adjudication, and in lieu of sentencing on the indecent assault and battery and rape and abuse charges, the petitioner was committed to the treatment center in November, 1984; he was twenty-one years old.3
On January 31, 2000, the petitioner filed a petition pursuant to G. L. c. 123A, § 9, seeking discharge from the treatment center on the ground that he was no longer an SDP. During a jury trial in the Superior Court on the petition in June, 2002, the two appointed qualified examiners,4 called as witnesses by the petitioner, both testified that, in their professional opinion, the petitioner was no longer an SDP. The petitioner also called two other psychologists who testified to the same effect. The Commonwealth relied exclusively on a report by the Community Access Board (CAB) and the testimony of the CAB’s chair, a licensed psychologist, who opined that the petitioner remained sexually dangerous. The jury found that the petitioner remained an SDP, and judgment entered on the jury’s verdict. In July, 2002, the petitioner filed a notice of appeal in the trial court; the appeal was entered in the Appeals Court in August, 2006. In [260]*260December, 2006, the petitioner sought a stay of appeal in order to file a motion for a new trial in the Superior Court, which the Appeals Court granted. The petitioner filed his new trial motion in the Superior Court in January, 2007. In July, 2007, his counsel filed a motion to amend the petitioner’s memorandum in support of the motion for a new trial. The amended memorandum raised, for the first time, the petitioner’s argument that the 2002 judgment was improper because both qualified examiners had opined that he was no longer an SDP, and only the chair of the CAB offered an opinion that the petitioner was still an SDP. In November, 2007, the judge who had presided at the 2002 discharge proceeding denied the petitioner’s motion for a new trial. The petitioner appealed from the denial, and the Appeals Court consolidated that appeal with his direct appeal from the 2002 judgment.
In a memorandum and order issued pursuant to its rule 1:28 in 2010, the Appeals Court concluded that the interpretation of G. L. c. 123A, § 9, set forth in Johnstone governed the petitioner’s appeal, and that, accordingly, the judgment must be reversed because neither qualified examiner had determined that the petitioner remained an SDP. Mclntire, petitioner, 76 Mass. App. Ct. 1117 (2010). That court also concluded that contrary to the petitioner’s argument, the error affecting the judgment in the 2002 discharge proceeding neither rendered the 2002 judgment void ab initio nor invalidated the judgments in later discharge proceedings in which the petitioner had been found to remain an SDP. We granted the petitioner’s application for further appellate review.
2. Discussion, a. Application of Johnstone. The petitioner argues that our holding in Johnstone, 453 Mass, at 553, must be applied retroactively to invalidate the jury verdict and resulting judgment against him in 2002; for its part, the Commonwealth contends that the court can, and should, apply Johnstone only on a prospective basis to discharge petitions filed after the date of the Johnstone decision in 2009. We agree with the Appeals Court that a retroactive-prospective analysis is unnecessary.
“In general, changes in the common law brought about by judicial decisions are given retroactive effect.” Halley v. Birbiglia, 390 Mass. 540, 544 (1983). See Papadopoulos v. Target Corp., 457 Mass. 368, 384-385 (2010) (applying retroactively [261]*261new common-law rule of premises liability based on abolition of distinction between natural and unnatural accumulation of snow and ice); Schrottman v. Barnicle, 386 Mass. 627, 631 (1982) (“Decisional law is generally applied ‘retroactively’ to past events”). Where a decision does not announce new common-law rules or rights but rather construes a statute, no analysis of retroactive or prospective effect is required because at issue is the meaning of the statute since its enactment. See Shawmut Worcester County Bank v. Miller, 398 Mass. 273, 281 (1986) (rejecting argument that court’s construction of G. L. c. 106, § 9-504 [1] and [3], adopted in that case, should be applied on prospective basis only: “although this court has not previously dealt with the issues raised here, we are not announcing common law rules but rather are construing certain statutory provisions. Those provisions have had the same meaning since the effective date of the statutes”). See also Mouradian v. General Elec. Co., 23 Mass. App. Ct. 538, 542 n.3 (1987).
The question in Johnstone was purely one of statutory interpretation: whether, under G. L. c. 123A, § 9, at a discharge proceeding “in which both qualified examiners form the opinion that the petitioner is no longer sexually dangerous, the testimony of a CAB member and the accompanying CAB report constitute evidence that is sufficient for the Commonwealth to avoid a directed verdict.” Johnstone, 453 Mass, at 545. We construed § 9 to mean that the qualified examiners perform an essential gatekeeping function with respect to whether a person may be classified as an SDP.5 Id. at 549-553. Although we had noted this issue of statutory interpretation before in a similar context, [262]*262see Commonwealth v. Poissant, 443 Mass. 558, 560 n.6 (2005),6 it is true that Johnstone represents the first time we considered it directly and resolved it. The fact that the question had not been answered before, however, does not mean that it represented a “new” interpretation.7 Rather, as in Shawmut Worcester County Bank v. Miller, 398 Mass, at 281, G. L. c. 123A, § 9, has “had the same meaning since the effective date of the statute[].”
The interpretation of c. 123A, § 9, set forth in Johnstone, therefore, is the one we apply in considering here the petitioner’s direct appeal from the judgment in the 2002 discharge proceeding. There is no dispute that both qualified examiners opined there that the petitioner no longer remained sexually dangerous. Given this fact, under § 9, the Commonwealth in this case was not entitled to rely on the CAB report and the testimony of the CAB’s chair to meet its burden of proof with respect to the petitioner’s then current sexual dangerousness, see Andrews, petitioner, 368 Mass. 468, 489-490 (1975), and, as in Johnstone, the petitioner was entitled to a directed verdict in his favor. Johnstone, 453 Mass, at 546, 553. The judgment entered in the Superior Court on July 3, 2002, must be reversed.
[263]*263b. Validity and effect of subsequent SDP discharge proceedings.
We must decide whether the reversal of the judgment in the 2002 discharge proceeding currently entitles the petitioner to an order of discharge from the treatment center. While his appeal from the 2002 judgment was pending, the petitioner filed additional petitions for discharge pursuant to G. L. c. 123A, § 9, as the statute entitled him to do. See G. L. c. 123A, § 9, first par. (“Any person committed to the treatment center shall be entitled to file a petition for examination and discharge once in every twelve months”). Three such petitions went to trial in 2005, 2008, and 2010, respectively; at the conclusion of each, the petitioner was found to remain an SDP.8
The petitioner contends that these post-2002 proceedings are irrelevant to the disposition of this appeal, because when the two qualified examiners “found him not sexually dangerous” in connection with his 2002 discharge petition, the Superior Court lost jurisdiction over him. As a result of the qualified examiners’ determinations, he claims, his immediate discharge is required on both statutory and substantive due process grounds. We disagree.
(i) The petitioner’s statutory claim is that under the reading of G. L. c. 123A, § 9, adopted by this court in Johnstone, his petition for discharge should never have gone to trial in 2002 in light of the qualified examiners’ negative opinions on his sexual [264]*264dangerousness, and that the resulting Superior Court judgment was without authority and void ah initio. Given the voidness of the original judgment, the argument continues, the subsequent judgments are also void and “nullities,” a conclusion flowing from the fact that after 2002 the petitioner was a person whom the Commonwealth did not hold legally in its custody, and therefore the Superior Court lacked jurisdiction over him. The petitioner’s argument fails.
“A void judgment is to be distinguished from an erroneous one, in that the latter is subject only to direct attack. A void judgment is one which, from its inception, was a complete nullity and without legal effect. In the interest of finality, the concept of void judgments is narrowly construed.” Harris v. Sannella, 400 Mass. 392, 395 (1987), quoting Lubben v. Selective Serv. Sys., 453 F.2d 645, 649 (1st Cir. 1972). To be void, a judgment must issue from a court that “lacked jurisdiction over the parties, lacked jurisdiction over the subject matter, or failed to provide due process of law.” Harris v. Sannella, supra. A mere error in a court’s determination of its own jurisdiction, however, will not render a judgment void; “[ojnly in the rare instance of a clear usurpation of power will a judgment be rendered void.” Id., quoting Lubben v. Selective Serv. Sys., supra.
The 2002 judgment was not void. Clearly, the Superior Court had jurisdiction over the petitioner and the subject matter of the case: G. L. c. 123A, § 9, expressly authorizes an SDP to file a petition for discharge and provides that it must be filed in the Superior Court; and the petitioner makes no argument that the Superior Court conducted the proceeding in a manner that failed to afford the petitioner the process that he was due. As previously discussed, what Johnstone establishes is that the failure of at least one qualified examiner to opine that a petitioner remains an SDP renders the Commonwealth unable to meet its burden of proof. Johnstone, 453 Mass, at 552. A judgment entered after a trial where the Commonwealth has not introduced sufficient evidence is not jurisdictionally flawed, but legally erroneous.9 Moreover, even if one were to classify the error in the 2002 discharge proceeding judgment as raising a jurisdictional issue, [265]*265the erroneous interpretation of a statutory grant of jurisdiction is not “equivalent to acting with total want of jurisdiction and does not render the judgment a complete nullity.” Harris v. Sannella, 400 Mass, at 395, quoting Hooks v. Hooks, 771 F.2d 935, 950 (6th Cir. 1985). Cf. Chicot County Drainage Dist. v. Baxter State Bank, 308 U.S. 371, 374, 376-378 (1940) (assumption of jurisdiction based on statute later declared unconstitutional may be appealed but is not nullity).
Where the underlying judgment is erroneous, it is voidable for error and not void, and is therefore “valid until reversed through the legal process.” Lynch, petitioner, 379 Mass. 757, 760 (1980). See Lewis v. Commonwealth, 329 Mass. 445, 447-448 (1952); DuPont v. Commissioner of Correction, 59 Mass. App. Ct. 908, 910 (2003), cert, denied, 542 U.S. 943 (2004).10 See also Streeter v. Worcester, 336 Mass. 469, 472 (1957), quoting Moll v. Wakefield, 274 Mass 505, 507 (1931) (“A court with jurisdiction may make a wrong decision . . . and thus a decision may not be a true adjudication. But such decisions are not void. They stand unless reversed under some recognized and available procedure”). Accordingly, because the 2002 judgment had not been reversed at the time of the petitioner’s 2005, 2008, and 2010 discharge proceedings, that earlier judgment remained validly in effect, and the petitioner was therefore legally held at the treatment center as an SDP when he filed the three later discharge petitions.11 As a result, the Superior Court had jurisdiction to entertain those petitions and to enter judg[266]*266merits on them.12 Because those judgments effectively adjudicated the petitioner as remaining an SDP, he is not entitled to an order of discharge from the treatment center at this time.
(ii) The petitioner also argues that his continued confinement following the qualified examiners’ opinions in 2002 that he was no longer an SDP violates principles of substantive due process; he claims that consistent with this court’s decision in Commonwealth v. Travis, 372 Mass. 238, 246-251 (1977), he should be released immediately. There is a significant distinction between the petitioner’s situation and that of the defendant in Travis. In that case, a Superior Court judge had found, after a trial, that the defendant was no longer an SDP, and had ordered him to be conditionally released. Id. at 239. We held that when the defendant later violated terms of his conditional release, the trial judge was without authority to recommit him as an SDP — that it would be “intolerable” and contrary to due process “that a person found not sexually dangerous should be indefinitely burdened with the spectre of vacation of that finding.” Id. at 250. In the present case, there has been no finding or judgment that the petitioner is no longer an SDP; the opinions of the qualified examiners to that effect in 2002 are opinions, not [267]*267adjudications. The petitioner has the continuing ability under G. L. c. 123A, § 9, to bring annual petitions for discharge, and if the Commonwealth is not able to prove beyond a reasonable doubt that he remains an SDP, he will be entitled to an order of discharge.13 There has been no denial of the petitioner’s substantive due process rights.
3. Conclusion. The judgment of the Superior Court dated July 3, 2002, is reversed, and the jury verdict set aside. In light of the judgments entered in 2005, 2008, and 2010, on subsequent petitions for discharge filed by the petitioner, he is not entitled to an order of discharge at this time.
So ordered.