@Override
    @SuppressWarnings({"unchecked", "checkstyle:NPathComplexity"})
    public void merge(AuthenticationInfo info) {
        if (info == null || info.getPrincipals() == null || info.getPrincipals().isEmpty()) {
            return;
        }

        if (this.principals == null) {
            this.principals = info.getPrincipals();
        } else {
            if (!(this.principals instanceof MutablePrincipalCollection)) {
                this.principals = new SimplePrincipalCollection(this.principals);
            }
            ((MutablePrincipalCollection) this.principals).addAll(info.getPrincipals());
        }

        //only mess with a salt value if we don't have one yet.  It doesn't make sense
        //to merge salt values from different realms because a salt is used only within
        //the realm's credential matching process.  But if the current instance's salt
        //is null, then it can't hurt to pull in a non-null value if one exists.
        //
        //since 1.1:
        if (this.credentialsSalt == null && info instanceof SaltedAuthenticationInfo) {
            this.credentialsSalt = ((SaltedAuthenticationInfo) info).getCredentialsSalt();
        }

        Object thisCredentials = getCredentials();
        Object otherCredentials = info.getCredentials();

        if (otherCredentials == null) {
            return;
        }

        if (thisCredentials == null) {
            this.credentials = otherCredentials;
            return;
        }

        if (!(thisCredentials instanceof Collection)) {
            Set newSet = new HashSet();
            newSet.add(thisCredentials);
            setCredentials(newSet);
        }

        // At this point, the credentials should be a collection
        Collection credentialCollection = (Collection) getCredentials();
        if (otherCredentials instanceof Collection) {
            credentialCollection.addAll((Collection) otherCredentials);
        } else {
            credentialCollection.add(otherCredentials);
        }
    }
Takes the specified <code>info</code> argument and adds its principals and credentials into this instance.

@param info the <code>AuthenticationInfo</code> to add into this instance.
core/src/main/java/org/apache/shiro/authc/SimpleAuthenticationInfo.java