diff --git a/teavm-classlib/src/main/java/org/teavm/classlib/impl/charset/UTF16Helper.java b/teavm-classlib/src/main/java/org/teavm/classlib/impl/charset/UTF16Helper.java
index 44ad12c12..12fc86a3a 100644
--- a/teavm-classlib/src/main/java/org/teavm/classlib/impl/charset/UTF16Helper.java
+++ b/teavm-classlib/src/main/java/org/teavm/classlib/impl/charset/UTF16Helper.java
@@ -51,8 +51,8 @@ public class UTF16Helper {
     }
 
     public static int buildCodePoint(char a, char b) {
-        return ((a & SURROGATE_BIT_INV_MASK) << MEANINGFUL_SURROGATE_BITS) |
-                (b & SURROGATE_BIT_INV_MASK) + SUPPLEMENTARY_PLANE;
+        return (((a & SURROGATE_BIT_INV_MASK) << MEANINGFUL_SURROGATE_BITS) | (b & SURROGATE_BIT_INV_MASK)) +
+                SUPPLEMENTARY_PLANE;
     }
 
     public static boolean isSurrogate(char c) {
diff --git a/teavm-classlib/src/main/java/org/teavm/classlib/impl/charset/UTF8Charset.java b/teavm-classlib/src/main/java/org/teavm/classlib/impl/charset/UTF8Charset.java
index 84e595d6a..8485cede1 100644
--- a/teavm-classlib/src/main/java/org/teavm/classlib/impl/charset/UTF8Charset.java
+++ b/teavm-classlib/src/main/java/org/teavm/classlib/impl/charset/UTF8Charset.java
@@ -35,7 +35,7 @@ public class UTF8Charset extends Charset {
                 dest.put((byte)(0x80 | (ch & 0x3F)));
             } else if (UTF16Helper.isHighSurrogate(ch)) {
                 char low = source.get();
-                if (!UTF16Helper.isLowSurrogate(ch)) {
+                if (!UTF16Helper.isLowSurrogate(low)) {
                     source.back(1);
                     dest.put((byte)'?');
                 } else {
diff --git a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TCharacter.java b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TCharacter.java
index 252f28ff0..d64b1e07c 100644
--- a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TCharacter.java
+++ b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TCharacter.java
@@ -198,11 +198,10 @@ public class TCharacter extends TObject implements TComparable<TCharacter> {
     }
 
     public static int codePointBefore(TCharSequence seq, int index) {
-        if (index == 1 || !UTF16Helper.isLowSurrogate(seq.charAt(index - 2)) ||
-                !UTF16Helper.isHighSurrogate(seq.charAt(index - 2))) {
+        if (index == 1 || !isLowSurrogate(seq.charAt(index - 1)) || !isHighSurrogate(seq.charAt(index - 2))) {
             return seq.charAt(index - 1);
         }
-        return UTF16Helper.buildCodePoint(seq.charAt(index - 2), seq.charAt(index - 1));
+        return toCodePoint(seq.charAt(index - 2), seq.charAt(index - 1));
     }
 
     public static int codePointBefore(char[] a, int index) {
@@ -329,7 +328,7 @@ public class TCharacter extends TObject implements TComparable<TCharacter> {
     }
 
     public static int codePointCount(TCharSequence seq, int beginIndex, int endIndex) {
-        int count = endIndex;
+        int count = endIndex - beginIndex;
         --endIndex;
         for (int i = beginIndex; i < endIndex; ++i) {
             if (UTF16Helper.isHighSurrogate(seq.charAt(i)) && UTF16Helper.isLowSurrogate(seq.charAt(i + 1))) {
diff --git a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TString.java b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TString.java
index 0f4320526..5ac53c3be 100644
--- a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TString.java
+++ b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TString.java
@@ -95,7 +95,7 @@ public class TString extends TObject implements TSerializable, TComparable<TStri
         this.characters = new char[sb.length()];
         ByteBuffer source = new ByteBuffer(bytes, offset, offset + length);
         char[] destChars = new char[TMath.max(8, TMath.min(length * 2, 1024))];
-        CharBuffer dest = new CharBuffer(destChars, 0, length * 2);
+        CharBuffer dest = new CharBuffer(destChars, 0, destChars.length);
         while (!source.end()) {
             charset.decode(source, dest);
             sb.append(destChars, 0, dest.position());
@@ -269,7 +269,7 @@ public class TString extends TObject implements TSerializable, TComparable<TStri
             char lo = UTF16Helper.lowSurrogate(ch);
             for (int i = fromIndex; i >= 1; --i) {
                 if (characters[i] == lo && characters[i - 1] == hi) {
-                    return i;
+                    return i - 1;
                 }
             }
             return -1;