diff --git a/icu4c/source/common/locid.cpp b/icu4c/source/common/locid.cpp
index f7899cc4dc..9bf3a1eb98 100644
--- a/icu4c/source/common/locid.cpp
+++ b/icu4c/source/common/locid.cpp
@@ -1226,7 +1226,7 @@ public:
         return new KeywordEnumeration(keywords, length, (int32_t)(current - keywords), status);
     }
 
-    int32_t count(UErrorCode &/*status*/) const {
+    virtual int32_t count(UErrorCode &/*status*/) const {
         char *kw = keywords;
         int32_t result = 0;
         while(*kw) {
@@ -1236,7 +1236,7 @@ public:
         return result;
     }
 
-    const char* next(int32_t* resultLength, UErrorCode& status) {
+    virtual const char* next(int32_t* resultLength, UErrorCode& status) {
         const char* result;
         int32_t len;
         if(U_SUCCESS(status) && *current != 0) {
@@ -1255,13 +1255,13 @@ public:
         return result;
     }
 
-    const UnicodeString* snext(UErrorCode& status) {
+    virtual const UnicodeString* snext(UErrorCode& status) {
         int32_t resultLength = 0;
         const char *s = next(&resultLength, status);
         return setChars(s, resultLength, status);
     }
 
-    void reset(UErrorCode& /*status*/) {
+    virtual void reset(UErrorCode& /*status*/) {
         current = keywords;
     }
 };
diff --git a/icu4c/source/common/ucnv_u16.c b/icu4c/source/common/ucnv_u16.c
index 9341cc1050..86fd19991c 100644
--- a/icu4c/source/common/ucnv_u16.c
+++ b/icu4c/source/common/ucnv_u16.c
@@ -19,7 +19,6 @@
 #if !UCONFIG_NO_CONVERSION
 
 #include "unicode/ucnv.h"
-#include "unicode/ucnv_err.h"
 #include "ucnv_bld.h"
 #include "ucnv_cnv.h"
 #include "cmemory.h"
diff --git a/icu4c/source/common/ucnv_u32.c b/icu4c/source/common/ucnv_u32.c
index 96fe68ee2f..1a37e96f59 100644
--- a/icu4c/source/common/ucnv_u32.c
+++ b/icu4c/source/common/ucnv_u32.c
@@ -19,7 +19,6 @@
 #if !UCONFIG_NO_CONVERSION
 
 #include "unicode/ucnv.h"
-#include "unicode/ucnv_err.h"
 #include "ucnv_bld.h"
 #include "ucnv_cnv.h"
 #include "cmemory.h"
diff --git a/icu4c/source/common/ucnv_u7.c b/icu4c/source/common/ucnv_u7.c
index ac46536760..f15da50331 100644
--- a/icu4c/source/common/ucnv_u7.c
+++ b/icu4c/source/common/ucnv_u7.c
@@ -19,7 +19,6 @@
 #if !UCONFIG_NO_CONVERSION
 
 #include "unicode/ucnv.h"
-#include "unicode/ucnv_err.h"
 #include "ucnv_bld.h"
 #include "ucnv_cnv.h"
 
diff --git a/icu4c/source/common/ucnv_u8.c b/icu4c/source/common/ucnv_u8.c
index 7123d574d8..d9114dfdc2 100644
--- a/icu4c/source/common/ucnv_u8.c
+++ b/icu4c/source/common/ucnv_u8.c
@@ -23,7 +23,6 @@
 #if !UCONFIG_NO_CONVERSION
 
 #include "unicode/ucnv.h"
-#include "unicode/ucnv_err.h"
 #include "ucnv_bld.h"
 #include "ucnv_cnv.h"
 #include "cmemory.h"
diff --git a/icu4c/source/common/ucnvhz.c b/icu4c/source/common/ucnvhz.c
index 9bec70a403..c030f1825c 100644
--- a/icu4c/source/common/ucnvhz.c
+++ b/icu4c/source/common/ucnvhz.c
@@ -19,7 +19,6 @@
 #if !UCONFIG_NO_CONVERSION && !UCONFIG_NO_LEGACY_CONVERSION
 
 #include "cmemory.h"
-#include "unicode/ucnv_err.h"
 #include "unicode/ucnv.h"
 #include "unicode/ucnv_cb.h"
 #include "unicode/uset.h"
diff --git a/icu4c/source/common/ucnvisci.c b/icu4c/source/common/ucnvisci.c
index 57e9f015f7..7eb3994ea0 100644
--- a/icu4c/source/common/ucnvisci.c
+++ b/icu4c/source/common/ucnvisci.c
@@ -20,7 +20,6 @@
 #if !UCONFIG_NO_CONVERSION && !UCONFIG_NO_LEGACY_CONVERSION
 
 #include "cmemory.h"
-#include "unicode/ucnv_err.h"
 #include "ucnv_bld.h"
 #include "unicode/ucnv.h"
 #include "ucnv_cnv.h"
diff --git a/icu4c/source/common/ucnvlat1.c b/icu4c/source/common/ucnvlat1.c
index 109a1dc0a0..7ddf376aa4 100644
--- a/icu4c/source/common/ucnvlat1.c
+++ b/icu4c/source/common/ucnvlat1.c
@@ -17,7 +17,6 @@
 #if !UCONFIG_NO_CONVERSION
 
 #include "unicode/ucnv.h"
-#include "unicode/ucnv_err.h"
 #include "unicode/uset.h"
 #include "ucnv_bld.h"
 #include "ucnv_cnv.h"
diff --git a/icu4c/source/common/unicode/uniset.h b/icu4c/source/common/unicode/uniset.h
index b15235815c..7cb4410120 100644
--- a/icu4c/source/common/unicode/uniset.h
+++ b/icu4c/source/common/unicode/uniset.h
@@ -715,12 +715,12 @@ public:
      * Implement UnicodeMatcher::matches()
      * @stable ICU 2.4
      */
-    UMatchDegree matches(const Replaceable& text,
+    virtual UMatchDegree matches(const Replaceable& text,
                          int32_t& offset,
                          int32_t limit,
                          UBool incremental);
 
- private:
+private:
     /**
      * Returns the longest match for s in text at the given position.
      * If limit > start then match forward from start+1 to limit
@@ -757,7 +757,7 @@ public:
      */
     int32_t findCodePoint(UChar32 c) const;
 
- public:
+public:
 
     /**
      * Implementation of UnicodeMatcher API.  Union the set of all
@@ -766,7 +766,7 @@ public:
      * @param toUnionTo the set into which to union the source characters
      * @stable ICU 2.4
      */
-    void addMatchSetTo(UnicodeSet& toUnionTo) const;
+    virtual void addMatchSetTo(UnicodeSet& toUnionTo) const;
 
     /**
      * Returns the index of the given character within this set, where
diff --git a/icu4c/source/common/unicode/utf16.h b/icu4c/source/common/unicode/utf16.h
index aad6c81e5e..d5e4ca4643 100644
--- a/icu4c/source/common/unicode/utf16.h
+++ b/icu4c/source/common/unicode/utf16.h
@@ -100,7 +100,7 @@
  * @stable ICU 2.4
  */
 #define U16_GET_SUPPLEMENTARY(lead, trail) \
-    (((lead)<<10UL)+(trail)-U16_SURROGATE_OFFSET)
+    (((UChar32)(lead)<<10UL)+(UChar32)(trail)-U16_SURROGATE_OFFSET)
 
 
 /**
diff --git a/icu4c/source/i18n/nfsubs.h b/icu4c/source/i18n/nfsubs.h
index fce41403f5..ec03fee9c4 100644
--- a/icu4c/source/i18n/nfsubs.h
+++ b/icu4c/source/i18n/nfsubs.h
@@ -267,11 +267,11 @@ public:
         const UnicodeString& description,
         UErrorCode& status);
     
-    int64_t transformNumber(int64_t number) const { return number; }
-    double transformNumber(double number) const { return number; }
-    double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const { return newRuleValue; }
-    double calcUpperBound(double oldUpperBound) const { return oldUpperBound; }
-    UChar tokenChar() const { return (UChar)0x003d; } // '='
+    virtual int64_t transformNumber(int64_t number) const { return number; }
+    virtual double transformNumber(double number) const { return number; }
+    virtual double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const { return newRuleValue; }
+    virtual double calcUpperBound(double oldUpperBound) const { return oldUpperBound; }
+    virtual UChar tokenChar() const { return (UChar)0x003d; } // '='
 private:
     static const char fgClassID;
     
@@ -299,7 +299,7 @@ public:
         }
     }
     
-    void setDivisor(int32_t radix, int32_t exponent, UErrorCode& status) { 
+    virtual void setDivisor(int32_t radix, int32_t exponent, UErrorCode& status) { 
         divisor = uprv_pow(radix, exponent);
         ldivisor = util64_fromDouble(divisor);
 
@@ -308,23 +308,23 @@ public:
         }
     }
     
-    UBool operator==(const NFSubstitution& rhs) const;
+    virtual UBool operator==(const NFSubstitution& rhs) const;
     
-    int64_t transformNumber(int64_t number) const {
+    virtual int64_t transformNumber(int64_t number) const {
         return number / ldivisor;
     }
     
-    double transformNumber(double number) const {
+    virtual double transformNumber(double number) const {
         return uprv_floor(number / divisor);
     }
     
-    double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const {
+    virtual double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const {
         return newRuleValue * divisor;
     }
     
-    double calcUpperBound(double /*oldUpperBound*/) const { return divisor; }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const { return divisor; }
     
-    UChar tokenChar() const { return (UChar)0x003c; } // '<'
+    virtual UChar tokenChar() const { return (UChar)0x003c; } // '<'
 private:
     static const char fgClassID;
     
@@ -346,7 +346,7 @@ public:
         const UnicodeString& description,
         UErrorCode& status);
     
-    void setDivisor(int32_t radix, int32_t exponent, UErrorCode& status) { 
+    virtual void setDivisor(int32_t radix, int32_t exponent, UErrorCode& status) { 
         divisor = uprv_pow(radix, exponent);
         ldivisor = util64_fromDouble(divisor);
 
@@ -355,30 +355,30 @@ public:
         }
     }
     
-    UBool operator==(const NFSubstitution& rhs) const;
+    virtual UBool operator==(const NFSubstitution& rhs) const;
     
-    void doSubstitution(int64_t number, UnicodeString& toInsertInto, int32_t pos) const;
-    void doSubstitution(double number, UnicodeString& toInsertInto, int32_t pos) const;
+    virtual void doSubstitution(int64_t number, UnicodeString& toInsertInto, int32_t pos) const;
+    virtual void doSubstitution(double number, UnicodeString& toInsertInto, int32_t pos) const;
     
-    int64_t transformNumber(int64_t number) const { return number % ldivisor; }
-    double transformNumber(double number) const { return uprv_fmod(number, divisor); }
+    virtual int64_t transformNumber(int64_t number) const { return number % ldivisor; }
+    virtual double transformNumber(double number) const { return uprv_fmod(number, divisor); }
     
-    UBool doParse(const UnicodeString& text, 
+    virtual UBool doParse(const UnicodeString& text, 
         ParsePosition& parsePosition,
         double baseValue,
         double upperBound,
         UBool lenientParse,
         Formattable& result) const;
     
-    double composeRuleValue(double newRuleValue, double oldRuleValue) const {
+    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const {
         return oldRuleValue - uprv_fmod(oldRuleValue, divisor) + newRuleValue;
     }
     
-    double calcUpperBound(double /*oldUpperBound*/) const { return divisor; }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const { return divisor; }
     
-    UBool isModulusSubstitution() const { return TRUE; }
+    virtual UBool isModulusSubstitution() const { return TRUE; }
     
-    UChar tokenChar() const { return (UChar)0x003e; } // '>'
+    virtual UChar tokenChar() const { return (UChar)0x003e; } // '>'
 private:
     static const char fgClassID;
     
@@ -396,11 +396,11 @@ public:
         UErrorCode& status)
         : NFSubstitution(_pos, _ruleSet, formatter, description, status) {}
     
-    int64_t transformNumber(int64_t number) const { return number; }
-    double transformNumber(double number) const { return uprv_floor(number); }
-    double composeRuleValue(double newRuleValue, double oldRuleValue) const { return newRuleValue + oldRuleValue; }
-    double calcUpperBound(double /*oldUpperBound*/) const { return DBL_MAX; }
-    UChar tokenChar() const { return (UChar)0x003c; } // '<'
+    virtual int64_t transformNumber(int64_t number) const { return number; }
+    virtual double transformNumber(double number) const { return uprv_floor(number); }
+    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const { return newRuleValue + oldRuleValue; }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const { return DBL_MAX; }
+    virtual UChar tokenChar() const { return (UChar)0x003c; } // '<'
 private:
     static const char fgClassID;
     
@@ -420,23 +420,23 @@ public:
         const UnicodeString& description,
         UErrorCode& status);
     
-    UBool operator==(const NFSubstitution& rhs) const;
+    virtual UBool operator==(const NFSubstitution& rhs) const;
     
-    void doSubstitution(double number, UnicodeString& toInsertInto, int32_t pos) const;
-    void doSubstitution(int64_t /*number*/, UnicodeString& /*toInsertInto*/, int32_t /*_pos*/) const {}
-    int64_t transformNumber(int64_t /*number*/) const { return 0; }
-    double transformNumber(double number) const { return number - uprv_floor(number); }
+    virtual void doSubstitution(double number, UnicodeString& toInsertInto, int32_t pos) const;
+    virtual void doSubstitution(int64_t /*number*/, UnicodeString& /*toInsertInto*/, int32_t /*_pos*/) const {}
+    virtual int64_t transformNumber(int64_t /*number*/) const { return 0; }
+    virtual double transformNumber(double number) const { return number - uprv_floor(number); }
     
-    UBool doParse(const UnicodeString& text,
+    virtual UBool doParse(const UnicodeString& text,
         ParsePosition& parsePosition,
         double baseValue,
         double upperBound,
         UBool lenientParse,
         Formattable& result) const;
     
-    double composeRuleValue(double newRuleValue, double oldRuleValue) const { return newRuleValue + oldRuleValue; }
-    double calcUpperBound(double /*oldUpperBound*/) const { return 0.0; }
-    UChar tokenChar() const { return (UChar)0x003e; } // '>'
+    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const { return newRuleValue + oldRuleValue; }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const { return 0.0; }
+    virtual UChar tokenChar() const { return (UChar)0x003e; } // '>'
 private:
     static const char fgClassID;
     
@@ -454,11 +454,11 @@ public:
         UErrorCode& status)
         : NFSubstitution(_pos, _ruleSet, formatter, description, status) {}
     
-    int64_t transformNumber(int64_t number) const { return number >= 0 ? number : -number; }
-    double transformNumber(double number) const { return uprv_fabs(number); }
-    double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const { return -newRuleValue; }
-    double calcUpperBound(double /*oldUpperBound*/) const { return DBL_MAX; }
-    UChar tokenChar() const { return (UChar)0x003e; } // '>'
+    virtual int64_t transformNumber(int64_t number) const { return number >= 0 ? number : -number; }
+    virtual double transformNumber(double number) const { return uprv_fabs(number); }
+    virtual double composeRuleValue(double newRuleValue, double /*oldRuleValue*/) const { return -newRuleValue; }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const { return DBL_MAX; }
+    virtual UChar tokenChar() const { return (UChar)0x003e; } // '>'
 private:
     static const char fgClassID;
     
@@ -482,12 +482,12 @@ public:
         ldenominator = util64_fromDouble(denominator);
     }
     
-    UBool operator==(const NFSubstitution& rhs) const;
+    virtual UBool operator==(const NFSubstitution& rhs) const;
     
-    int64_t transformNumber(int64_t number) const { return number * ldenominator; }
-    double transformNumber(double number) const { return uprv_round(number * denominator); }
+    virtual int64_t transformNumber(int64_t number) const { return number * ldenominator; }
+    virtual double transformNumber(double number) const { return uprv_round(number * denominator); }
     
-    UBool doParse(const UnicodeString& text, 
+    virtual UBool doParse(const UnicodeString& text, 
         ParsePosition& parsePosition,
         double baseValue,
         double upperBound,
@@ -499,9 +499,9 @@ public:
         // it SERIOUSLY messes up the algorithm
         return NFSubstitution::doParse(text, parsePosition, baseValue, upperBound, FALSE, result);
     }
-    double composeRuleValue(double newRuleValue, double oldRuleValue) const { return newRuleValue / oldRuleValue; }
-    double calcUpperBound(double /*oldUpperBound*/) const { return denominator; }
-    UChar tokenChar() const { return (UChar)0x003c; } // '<'
+    virtual double composeRuleValue(double newRuleValue, double oldRuleValue) const { return newRuleValue / oldRuleValue; }
+    virtual double calcUpperBound(double /*oldUpperBound*/) const { return denominator; }
+    virtual UChar tokenChar() const { return (UChar)0x003c; } // '<'
 private:
     static const char fgClassID;
     
@@ -519,22 +519,22 @@ public:
         UErrorCode& status)
         : NFSubstitution(_pos, _ruleSet, formatter, description, status) {}
     
-    void toString(UnicodeString& /*result*/) const {}
-    void doSubstitution(double /*number*/, UnicodeString& /*toInsertInto*/, int32_t /*_pos*/) const {}
-    void doSubstitution(int64_t /*number*/, UnicodeString& /*toInsertInto*/, int32_t /*_pos*/) const {}
-    int64_t transformNumber(int64_t /*number*/) const { return 0; }
-    double transformNumber(double /*number*/) const { return 0; }
-    UBool doParse(const UnicodeString& /*text*/,
+    virtual void toString(UnicodeString& /*result*/) const {}
+    virtual void doSubstitution(double /*number*/, UnicodeString& /*toInsertInto*/, int32_t /*_pos*/) const {}
+    virtual void doSubstitution(int64_t /*number*/, UnicodeString& /*toInsertInto*/, int32_t /*_pos*/) const {}
+    virtual int64_t transformNumber(int64_t /*number*/) const { return 0; }
+    virtual double transformNumber(double /*number*/) const { return 0; }
+    virtual UBool doParse(const UnicodeString& /*text*/,
                 ParsePosition& /*parsePosition*/, 
                 double baseValue,
                 double /*upperBound*/,
                 UBool /*lenientParse*/,
                 Formattable& result) const
             { result.setDouble(baseValue); return TRUE; }
-    double composeRuleValue(double /*newRuleValue*/, double /*oldRuleValue*/) const { return 0.0; } // never called
-    double calcUpperBound(double /*oldUpperBound*/) const { return 0; } // never called
-    UBool isNullSubstitution() const { return TRUE; }
-    UChar tokenChar() const { return (UChar)0x0020; } // ' ' never called
+    virtual double composeRuleValue(double /*newRuleValue*/, double /*oldRuleValue*/) const { return 0.0; } // never called
+    virtual double calcUpperBound(double /*oldUpperBound*/) const { return 0; } // never called
+    virtual UBool isNullSubstitution() const { return TRUE; }
+    virtual UChar tokenChar() const { return (UChar)0x0020; } // ' ' never called
 private:
     static const char fgClassID;
     
diff --git a/icu4c/source/i18n/timezone.cpp b/icu4c/source/i18n/timezone.cpp
index 12d86ec79c..54f051e502 100644
--- a/icu4c/source/i18n/timezone.cpp
+++ b/icu4c/source/i18n/timezone.cpp
@@ -788,11 +788,11 @@ public:
         return new TZEnumeration(*this);
     }
 
-    int32_t count(UErrorCode& status) const {
+    virtual int32_t count(UErrorCode& status) const {
         return U_FAILURE(status) ? 0 : len;
     }
 
-    const UnicodeString* snext(UErrorCode& status) {
+    virtual const UnicodeString* snext(UErrorCode& status) {
         if (U_SUCCESS(status) && pos < len) {
             getID((map == 0) ? pos : map[pos]);
             ++pos;
@@ -801,7 +801,7 @@ public:
         return 0;
     }
 
-    void reset(UErrorCode& /*status*/) {
+    virtual void reset(UErrorCode& /*status*/) {
         pos = 0;
     }
 
diff --git a/icu4c/source/i18n/unicode/coleitr.h b/icu4c/source/i18n/unicode/coleitr.h
index ffdb7a64e4..e2add4a24a 100644
--- a/icu4c/source/i18n/unicode/coleitr.h
+++ b/icu4c/source/i18n/unicode/coleitr.h
@@ -143,7 +143,7 @@ public:
     * Destructor
     * @stable ICU 2.0
     */
-    ~CollationElementIterator();
+    virtual ~CollationElementIterator();
 
     // CollationElementIterator public methods ----------------------------------
 
diff --git a/icu4c/source/i18n/unicode/dtfmtsym.h b/icu4c/source/i18n/unicode/dtfmtsym.h
index 4ec91a5e05..3546ab093d 100644
--- a/icu4c/source/i18n/unicode/dtfmtsym.h
+++ b/icu4c/source/i18n/unicode/dtfmtsym.h
@@ -150,7 +150,7 @@ public:
      * subclassed.
      * @stable ICU 2.0
      */
-    ~DateFormatSymbols();
+    virtual ~DateFormatSymbols();
 
     /**
      * Return true if another object is semantically equal to this one.