[JENKINS-43507] Code review comments

src/main/java/hudson/plugins/mercurial/MercurialSCMBuilder.java

@@ -29,11 +29,10 @@
    /**
     * The repository to track. This can be URL or a local file path.
     */
    private @Nonnull
    String source;
    private @Nonnull String source;

    public MercurialSCMBuilder(@Nonnull SCMHead head, @CheckForNull SCMRevision revision, String source,
                               String credentialsId) {
    public MercurialSCMBuilder(@Nonnull SCMHead head, @CheckForNull SCMRevision revision, @Nonnull String source,
                               @CheckForNull String credentialsId) {
        super(MercurialSCM.class, head, revision);
        this.source = source;
        this.credentialsId = credentialsId;

src/main/java/hudson/plugins/mercurial/MercurialSCMSource.java

@@ -54,6 +54,7 @@
import jenkins.scm.api.trait.SCMSourceTrait;
import jenkins.scm.api.trait.SCMSourceTraitDescriptor;
import jenkins.scm.impl.trait.RegexSCMHeadFilterTrait;
import jenkins.scm.impl.trait.RegexSCMSourceFilterTrait;
import org.apache.commons.lang.StringUtils;
import org.kohsuke.accmod.Restricted;
import org.kohsuke.accmod.restrictions.DoNotUse;
@@ -154,21 +155,13 @@ public void setCredentialsId(@CheckForNull String credentialsId) {
    }

    @Deprecated @Restricted(DoNotUse.class) @RestrictedSince("2.0") public String getInstallation() {
        for (SCMSourceTrait t: traits) {
            if (t instanceof MercurialInstallationSCMSourceTrait) {
                return ((MercurialInstallationSCMSourceTrait) t).getInstallation();
            }
        }
        return null;
        MercurialInstallationSCMSourceTrait t = getTrait(MercurialInstallationSCMSourceTrait.class);
        return t != null ? t.getInstallation() : null;
    }

    @Deprecated @Restricted(DoNotUse.class) @RestrictedSince("2.0") public String getBranchPattern() {
        for (SCMSourceTrait t: traits) {
            if (t instanceof RegexSCMHeadFilterTrait) {
                return ((RegexSCMHeadFilterTrait) t).getRegex();
            }
        }
        return "";
        RegexSCMHeadFilterTrait t = getTrait(RegexSCMHeadFilterTrait.class);
        return t != null ? t.getRegex() : "";
    }

    @Deprecated @Restricted(DoNotUse.class) @RestrictedSince("2.0") public String getModules() {
@@ -180,21 +173,21 @@ public void setCredentialsId(@CheckForNull String credentialsId) {
    }

    @Deprecated @Restricted(DoNotUse.class) @RestrictedSince("2.0") public HgBrowser getBrowser() {
        for (SCMSourceTrait t: traits) {
            if (t instanceof MercurialBrowserSCMSourceTrait) {
                return ((MercurialBrowserSCMSourceTrait) t).getBrowser();
            }
        }
        return null;
        MercurialBrowserSCMSourceTrait t = getTrait(MercurialBrowserSCMSourceTrait.class);
        return t != null ? t.getBrowser() : null;
    }

    @Deprecated @Restricted(DoNotUse.class) @RestrictedSince("2.0") public boolean isClean() {
        return getTrait(CleanMercurialSCMSourceTrait.class) != null;
    }

    private @CheckForNull <T extends SCMSourceTrait> T getTrait(@Nonnull Class<T> traitClass) {
        for (SCMSourceTrait t: traits) {
            if (t instanceof CleanMercurialSCMSourceTrait) {
                return true;
            if (traitClass.isInstance(t)) {
                return traitClass.cast(t);
            }
        }
        return false;
        return null;
    }

    @Override protected void retrieve(@CheckForNull SCMSourceCriteria criteria, @Nonnull SCMHeadObserver observer,