diff --git a/c/vm_instance.c b/c/vm_instance.c
index 5ecbdc6..91a2c91 100644
--- a/c/vm_instance.c
+++ b/c/vm_instance.c
@@ -25,12 +25,12 @@ struct objectref * vm_instance_create(struct vm * vm, const char * class_name)
 struct objectref * vm_instance_string_from_constant(struct vm * vm, struct constant * constant)
 {
   int32_t count = constant->utf8.length;
-  struct arrayref * arrayref = prim_array_allocate(vm, 1, count);
+  struct arrayref * arrayref = prim_array_allocate(vm, 2, count);
   assert(arrayref != nullptr);
   arrayref->class_entry = nullptr; // byte[]
   arrayref->length = constant->utf8.length;
   for (int i = 0; i < constant->utf8.length; i++) {
-    arrayref->u8[i] = constant->utf8.bytes[i];
+    arrayref->u16[i] = constant->utf8.bytes[i];
   }
 
   struct objectref * objectref = vm_instance_create(vm, "java/lang/String");
diff --git a/classes/java/lang/Character.java b/classes/java/lang/Character.java
index 2892c5a..2a7e8e6 100644
--- a/classes/java/lang/Character.java
+++ b/classes/java/lang/Character.java
@@ -1,4 +1,144 @@
 package java.lang;
 
-class Character {
+class Character
+    implements Comparable<Character> {
+    public static final int MAX_RADIX = 36;
+
+    public static final int MAX_VALUE = '\uFFFF';
+
+    public static final int MIN_RADIX = 2;
+
+    public static final char MIN_VALUE = '\u0000';
+
+    public static final int SIZE = 16;
+
+    private final char value;
+
+    public Character(char value) {
+        this.value = value;
+    }
+
+    public char charValue() {
+        return value;
+    }
+
+    public int compareTo(Character anotherCharacter) {
+        return this.charValue() - anotherCharacter.charValue();
+    }
+
+    private static int _digit(char ch) {
+        if (Character.isDigit(ch))
+            return (char)((int)ch - (int)'0');
+        if (ch >= 'A' && ch <= 'Z')
+            return (char)((int)ch - (int)'A' + 10);
+        if (ch >= 'a' && ch <= 'z')
+            return (char)((int)ch - (int)'a' + 10);
+        return -1;
+    }
+
+    public static int digit(char ch, int radix) {
+        if (radix < MIN_RADIX || radix > MAX_RADIX)
+            return -1;
+        int value = _digit(ch);
+        return (value < radix) ? value : -1;
+    }
+
+    public boolean equals(Object obj) {
+        return obj instanceof Character && value == ((Character)obj).value;
+    }
+
+    public static char forDigit(int digit, int radix) {
+        if (digit < 0 || digit > MAX_RADIX)
+            return '\u0000';
+        if (digit < 10) {
+            return (char)((int)'0' + digit);
+        } else {
+            return (char)((int)'a' + digit - 10);
+        }
+    }
+
+    public int hashCode() {
+        return value;
+    }
+
+    public static boolean isDigit(char ch) {
+        return ch >= '0' && ch <= '9';
+    }
+
+    public static boolean isISOControl(char ch) {
+        return (ch >= '\u0000' && ch <= '\u001F') || (ch >= '\u007F' && ch <= '\u009F');
+    }
+
+    public static boolean isLowerCase(char ch) {
+        return
+            (ch >= 'a' && ch <= 'z') ||
+            (ch >= '\u00DF' && ch <= '\u00F6') ||
+            (ch >= '\u00F8' && ch <= '\u00FF');
+    }
+
+    public static boolean isSpaceChar(char ch) {
+        switch (ch) {
+        case ' ':  return true;
+        case '\t': return true;
+        case '\n': return true;
+        case '\r': return true;
+        case '\f': return true;
+        default: return false;
+        }
+    }
+
+    public static boolean isUpperCase(char ch) {
+        return
+            (ch >= 'A' && ch <= 'Z') ||
+            (ch >= '\u00C0' && ch <= '\u00D6') ||
+            (ch >= '\u00D8' && ch <= '\u00DE');
+    }
+
+    public static boolean isWhitespace(char ch) {
+        switch (ch) {
+        case ' ':      return true;
+        case '\t':     return true;
+        case '\n':     return true;
+        case '\u000B': return true;
+        case '\u000C': return true;
+        case '\r':     return true;
+        case '\u001C': return true;
+        case '\u001D': return true;
+        case '\u001E': return true;
+        case '\u001F': return true;
+        default: return false;
+        }
+    }
+
+    public static char toLowerCase(char ch) {
+        if (ch >= 'A' && ch <= 'Z')
+            return (char)((int)ch - (int)'A' + (int)'a');
+        if (ch >= '\u00C0' && ch <= '\u00D6')
+            return (char)((int)ch - (int)'\u00C0' + (int)'\u00E0');
+        if (ch >= '\u00D8' && ch <= '\u00DE')
+            return (char)((int)ch - (int)'\u00D8' + (int)'\u00F8');
+        return ch;
+    }
+
+    public String toString() {
+        return Character.toString(value);
+    }
+
+    public static String toString(char c) {
+        return new String(new char[] { c });
+    }
+
+    public static char toUpperCase(char ch) {
+        if (ch >= 'a' && ch <= 'z')
+            return (char)((int)ch - (int)'a' + (int)'A');
+        if (ch >= '\u00E0' && ch <= '\u00F6')
+            return (char)((int)ch - (int)'\u00E0' + (int)'\u00C0');
+        if (ch >= '\u00F8' && ch <= '\u00FE')
+            return (char)((int)ch - (int)'\u00F8' + (int)'\u00D8');
+        return ch;
+    }
+
+    public static Character valueOf(char c) {
+        return new Character(c);
+    }
 }
diff --git a/classes/java/lang/String.java b/classes/java/lang/String.java
index 78f77f0..41ee09c 100644
--- a/classes/java/lang/String.java
+++ b/classes/java/lang/String.java
@@ -1,30 +1,377 @@
 package java.lang;
 
-public class String {
-    private final byte[] value;
+
+public final class String
+    implements Comparable<String>, CharSequence {
+
+    private final char[] value;
 
     public String() {
-        this.value = new byte[0];
+        value = new char[0];
+    }
+
+    public String(byte[] bytes) {
+        this(bytes, 0, bytes.length);
+    }
+
+    public String(byte[] bytes, int offset, int length) {
+        value = new char[length];
+        int i = 0;
+        while (length > 0) {
+            value[i] = (char)bytes[offset];
+            i += 1;
+            offset += 1;
+            length -= 1;
+        }
+    }
+
+    public String(char[] value) {
+        this(value, 0, value.length);
+    }
+
+    public String(char[] value, int offset, int count) {
+        this.value = new char[count];
+        int i = 0;
+        while (count > 0) {
+            this.value[i] = value[offset];
+            i += 1;
+            offset += 1;
+            count -= 1;
+        }
     }
 
     public String(String original) {
-        this.value = original.value;
+        // shallow copy
+        value = original.value;
     }
 
-    public String(byte[] value) {
-        this.value = value;
+    public char charAt(int index) {
+        return value[index];
+    }
+
+    public int compareTo(String anotherString) {
+        int length = value.length < anotherString.value.length
+                   ? value.length
+                   : anotherString.value.length;
+
+        for (int k = 0; k < length; k++) {
+            int difference = value[k] - anotherString.value[k];
+            if (difference != 0)
+                return difference;
+        }
+
+        return value.length - anotherString.value.length;
+    }
+
+    public int compareToIgnoreCase(String str) {
+        int length = value.length < str.value.length
+                   ? value.length
+                   : str.value.length;
+
+        for (int k = 0; k < length; k++) {
+            char a = Character.toLowerCase(Character.toUpperCase(value[k]));
+            char b = Character.toLowerCase(Character.toUpperCase(str.value[k]));
+            int difference = a - b;
+            if (difference != 0)
+                return difference;
+        }
+
+        return value.length - str.value.length;
+    }
+
+    public String concat(String str) {
+        if (str.value.length == 0)
+            return this;
+        if (value.length == 0)
+            return str;
+        int length_a = value.length;
+        int length_b = str.value.length;
+        char[] newValue = new char[length_a + length_b];
+        for (int i = 0; i < length_a; i++) {
+            newValue[i] = value[i];
+        }
+        for (int i = 0; i < length_b; i++) {
+            newValue[length_a + i] = str.value[i];
+        }
+        return new String(newValue);
+    }
+
+    public boolean contains(CharSequence s) {
+        return indexOf(s.toString()) != -1;
+    }
+
+    public boolean contentEquals(CharSequence cs) {
+        return equals(cs.toString());
+    }
+
+    public static String copyValueOf(char[] data) {
+        return new String(data);
+    }
+
+    public static String copyValueOf(char[] data,
+                                     int offset,
+                                     int count) {
+        return new String(data, offset, count);
+    }
+
+    public boolean endsWith(String suffix) {
+        int fromIndex = value.length - suffix.value.length;
+        return regionMatches(false, fromIndex, suffix, 0, suffix.value.length);
+    }
+
+    public boolean equals(Object anObject) {
+        if (anObject == null)
+            return false;
+        if (!(anObject instanceof String))
+            return false;
+        String str = (String)anObject;
+        if (value == str.value)
+            return true;
+        if (value.length != str.value.length)
+            return false;
+        for (int i = 0; i < value.length; i++) {
+            if (value[i] != str.value[i])
+                return false;
+        }
+        return true;
+    }
+
+    public boolean equalsIgnoreCase(String anotherString) {
+        if (anotherString == null)
+            return false;
+        if (value == anotherString.value)
+            return true;
+        if (value.length != anotherString.value.length)
+            return false;
+        for (int i = 0; i < value.length; i++) {
+            char a = Character.toLowerCase(Character.toUpperCase(value[i]));
+            char b = Character.toLowerCase(Character.toUpperCase(anotherString.value[i]));
+            if (a != b)
+                return false;
+        }
+        return true;
     }
 
     public byte[] getBytes() {
-        return this.value;
+        byte[] bytes = new byte[value.length];
+        for (int i = 0; i < value.length; i++) {
+            bytes[i] = (byte)value[i];
+        }
+        return bytes;
+    }
+
+    public void getChars(int srcBegin,
+                         int srcEnd,
+                         char[] dst,
+                         int dstBegin) {
+        if (srcBegin < 0 || srcBegin > srcEnd || srcEnd > value.length ||
+            dstBegin < 0 || (dstBegin + (srcEnd - srcBegin)) > dst.length)
+            throw new IndexOutOfBoundsException();
+        for (int i = 0; i < srcEnd - srcBegin; i++) {
+            dst[dstBegin + i] = value[srcBegin + i];
+        }
+    }
+
+    public int hashCode() {
+        int code = 0;
+        for (int i = 0; i < value.length; i++) {
+            code = code * 31 + value[i];
+        }
+        return code;
+    }
+
+    public int indexOf(int ch) {
+        return indexOf(ch, 0);
+    }
+
+    public int indexOf(int ch, int fromIndex) {
+        if (ch > 0xffff)
+            return -1;
+        if (fromIndex < 0)
+            fromIndex = 0;
+        while (fromIndex < value.length) {
+            if (value[fromIndex] == ch)
+                return fromIndex;
+            fromIndex += 1;
+        }
+        return -1;
+    }
+
+    public int indexOf(String str) {
+        return indexOf(str, 0);
+    }
+
+    public int indexOf(String str, int fromIndex) {
+        if (fromIndex < 0)
+            fromIndex = 0;
+        int maxIndex = value.length - str.value.length;
+        while (fromIndex <= maxIndex) {
+            if (regionMatches(false, fromIndex, str, 0, str.value.length))
+                return fromIndex;
+            fromIndex += 1;
+        }
+        return -1;
+    }
+
+    public boolean isEmpty() {
+        return value.length == 0;
+    }
+
+    public int lastIndexOf(int ch) {
+        return lastIndexOf(ch, value.length - 1);
+    }
+
+    public int lastIndexOf(int ch, int fromIndex) {
+        if (ch > 0xffff)
+            return -1;
+        if (fromIndex >= value.length)
+            fromIndex = value.length - 1;
+        while (fromIndex >= 0) {
+            if (value[fromIndex] == ch)
+                return fromIndex;
+            fromIndex -= 1;
+        }
+        return -1;
+    }
+
+    public int lastIndexOf(String str) {
+        return indexOf(str, value.length - 1);
+    }
+
+    public int lastIndexOf(String str, int fromIndex) {
+        if (fromIndex >= value.length)
+            fromIndex = value.length - 1;
+        while (fromIndex >= 0) {
+            if (regionMatches(false, fromIndex, str, 0, str.value.length))
+                return fromIndex;
+            fromIndex -= 1;
+        }
+        return -1;
+    }
+
+    public int length() {
+        return value.length;
+    }
+
+    public boolean regionMatches(boolean ignoreCase,
+                                 int toffset,
+                                 String other,
+                                 int ooffset,
+                                 int len) {
+        if (toffset < 0 || ooffset < 0 || toffset + len > value.length || ooffset + len > other.value.length)
+            return false;
+
+        while (len > 0) {
+            len -= 1;
+            char a = value[toffset];
+            toffset += 1;
+            char b = other.value[ooffset];
+            ooffset += 1;
+
+            if (a != b) {
+                if (!ignoreCase)
+                    return false;
+                char a1 = Character.toLowerCase(Character.toUpperCase(a));
+                char b1 = Character.toLowerCase(Character.toUpperCase(b));
+                if (a1 != b1)
+                    return false;
+            }
+        }
+        return true;
+    }
+
+    public boolean regionMatches(int toffset,
+                                 String other,
+                                 int ooffset,
+                                 int len) {
+        return regionMatches(false, toffset, other, ooffset, len);
+    }
+
+    public String replace(char oldChar, char newChar) {
+        boolean replaced = false;
+        char[] newValue = new char[value.length];
+        for (int i = 0; i < value.length; i++) {
+            if (value[i] == oldChar) {
+                replaced = true;
+                newValue[i] = newChar;
+            } else {
+                newValue[i] = value[i];
+            }
+        }
+        if (replaced)
+            return new String(newValue);
+        else
+            return this;
+    }
+
+    public boolean startsWith(String prefix) {
+        return regionMatches(false, 0, prefix, 0, prefix.value.length);
+    }
+
+    public boolean startsWith(String prefix, int toffset) {
+        return regionMatches(false, toffset, prefix, 0, prefix.value.length);
+    }
+
+    public CharSequence subSequence(int beginIndex, int endIndex) {
+        return substring(beginIndex, endIndex);
+    }
+
+    public String substring(int beginIndex) {
+        return substring(beginIndex, value.length);
+    }
+
+    public String substring(int beginIndex, int endIndex) {
+        if (beginIndex < 0 || endIndex > value.length || beginIndex > endIndex)
+            throw new IndexOutOfBoundsException();
+        int length = endIndex - beginIndex;
+        return new String(value, beginIndex, length);
+    }
+
+    public char[] toCharArray() {
+        char[] array = new char[value.length];
+        for (int i = 0; i < value.length; i++) {
+            array[i] = value[i];
+        }
+        return array;
+    }
+
+    public String toLowerCase() {
+        char[] newValue = new char[value.length];
+        for (int i = 0; i < value.length; i++) {
+            newValue[i] = Character.toLowerCase(value[i]);
+        }
+        return new String(newValue);
     }
 
     public String toString() {
         return this;
     }
 
-    public int length() {
-        return value.length;
+    public String toUpperCase() {
+        char[] newValue = new char[value.length];
+        for (int i = 0; i < value.length; i++) {
+            newValue[i] = Character.toUpperCase(value[i]);
+        }
+        return new String(newValue);
+    }
+
+    public String trim() {
+        if (value.length == 0 || (value[0] > '\u0020' && value[value.length - 1] > '\u0020'))
+            return this;
+
+        int k = 0;
+        while (k < value.length - 1) {
+            if (value[k] > '\u0020')
+                break;
+            k += 1;
+        }
+        int m = value.length - 1;
+        while (m >= 0) {
+            if (value[m] > '\u0020')
+                break;
+            m -= 1;
+        }
+        return substring(k, m + 1);
     }
 
     public static String valueOf(boolean b) {
@@ -32,7 +379,25 @@ public class String {
     }
 
     public static String valueOf(char c) {
-        return new String(new byte[] { (byte)c });
+        return new String(new char[] { c });
+    }
+
+    public static String valueOf(char[] data) {
+        return new String(data, 0, data.length);
+    }
+
+    public static String valueOf(char[] data,
+                                 int offset,
+                                 int count) {
+        return new String(data, offset, count);
+    }
+
+    public static String valueOf(double d) {
+        return Double.toString(d);
+    }
+
+    public static String valueOf(float f) {
+        return Float.toString(f);
     }
 
     public static String valueOf(int i) {
@@ -43,14 +408,6 @@ public class String {
         return Long.toString(l);
     }
 
-    public static String valueOf(float f) {
-        return Float.toString(f);
-    }
-
-    public static String valueOf(double d) {
-        return Double.toString(d);
-    }
-
     public static String valueOf(Object obj) {
         return (obj == null) ? "null" : obj.toString();
     }