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/io/IOException.java b/classes/java/io/IOException.java
new file mode 100644
index 0000000..2b5d6cb
--- /dev/null
+++ b/classes/java/io/IOException.java
@@ -0,0 +1,4 @@
+package java.io;
+
+public class IOException extends Exception {
+}
diff --git a/classes/java/lang/Appendable.java b/classes/java/lang/Appendable.java
new file mode 100644
index 0000000..5f70e5b
--- /dev/null
+++ b/classes/java/lang/Appendable.java
@@ -0,0 +1,11 @@
+package java.lang;
+
+import java.io.IOException;
+
+public interface Appendable {
+    Appendable append(char c) throws IOException;
+
+    Appendable append(CharSequence csq) throws IOException;
+
+    Appendable append(CharSequence csq, int start, int end) throws IOException;
+}
diff --git a/classes/java/lang/AutoCloseable.java b/classes/java/lang/AutoCloseable.java
new file mode 100644
index 0000000..b6fbe46
--- /dev/null
+++ b/classes/java/lang/AutoCloseable.java
@@ -0,0 +1,5 @@
+package java.lang;
+
+public interface AutoCloseable {
+    void close() throws Exception;
+}
diff --git a/classes/java/lang/Boolean.java b/classes/java/lang/Boolean.java
index 6985d20..eb954d6 100644
--- a/classes/java/lang/Boolean.java
+++ b/classes/java/lang/Boolean.java
@@ -1,4 +1,55 @@
 package java.lang;
 
 class Boolean {
+    public static final Boolean FALSE = new Boolean(false);
+
+    public static final Boolean TRUE = new Boolean(true);
+
+    public static final Class<Boolean> TYPE = (Class<Boolean>)Class.getPrimitiveClass("boolean");
+
+    private final boolean value;
+
+    public Boolean(boolean value) {
+        this.value = value;
+    }
+
+    public Boolean(String s) {
+        this(parseBoolean(s));
+    }
+
+    public boolean booleanValue() {
+        return value;
+    }
+
+    public int compareTo(Boolean b) {
+        return value == b.value ? 0 : (value ? 1 : -1);
+    }
+
+    public boolean equals(Object obj) {
+        return (obj instanceof Boolean) && (value == ((Boolean)obj).booleanValue());
+    }
+
+    public int hashCode() {
+        return value ? 1231 : 1237;
+    }
+
+    public static boolean parseBoolean(String s) {
+        return "true".equalsIgnoreCase(b) ? true : false;
+    }
+
+    public String toString() {
+        return String.valueOf(value);
+    }
+
+    public static String toString(boolean b) {
+        return String.valueOf(b);
+    }
+
+    public static Boolean valueOf(boolean b) {
+        return (b ? TRUE : FALSE);
+    }
+
+    public static Boolean valueOf(String s) {
+        return parseBoolean(s) ? TRUE : FALSE;
+    }
 }
diff --git a/classes/java/lang/CharSequence.java b/classes/java/lang/CharSequence.java
new file mode 100644
index 0000000..d6d1ceb
--- /dev/null
+++ b/classes/java/lang/CharSequence.java
@@ -0,0 +1,11 @@
+package java.lang;
+
+public interface CharSequence {
+    char charAt(int index);
+
+    int length();
+
+    CharSequence subSequence(int start, int end);
+
+    public String toString();
+}
diff --git a/classes/java/lang/Character.java b/classes/java/lang/Character.java
index 2892c5a..faafb40 100644
--- a/classes/java/lang/Character.java
+++ b/classes/java/lang/Character.java
@@ -1,4 +1,143 @@
 package java.lang;
 
 class 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 ch - '0';
+        if (ch >= 'A' && ch <= 'Z')
+            return ch - 'A' + 10;
+        if (ch >= 'a' && ch <= 'z')
+            return ch - '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) ? digit : -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 '0' + digit;
+        } else {
+            return '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 ch - 'A' + 'a';
+        if (ch >= '\u00C0' && ch <= '\u00D6')
+            return ch - '\u00C0' + '\u00E0';
+        if (ch >= '\u00D8' && ch <= '\u00DE')
+            return ch - '\u00D8' + '\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 ch - 'a' + 'A';
+        if (ch >= '\u00E0' && ch <= '\u00F6')
+            return ch - '\u00E0' + '\u00C0';
+        if (ch >= '\u00F8' && ch <= '\u00FE')
+            return ch - '\u00F8' + '\u00D8';
+        return ch;
+    }
+
+    public static Character valueOf(char c) {
+        return new Character(c);
+    }
 }
diff --git a/classes/java/lang/Cloneable.java b/classes/java/lang/Cloneable.java
new file mode 100644
index 0000000..d14aed4
--- /dev/null
+++ b/classes/java/lang/Cloneable.java
@@ -0,0 +1,4 @@
+package java.lang;
+
+public interface Cloneable {
+}
diff --git a/classes/java/lang/Comparable.java b/classes/java/lang/Comparable.java
new file mode 100644
index 0000000..650d5d1
--- /dev/null
+++ b/classes/java/lang/Comparable.java
@@ -0,0 +1,5 @@
+package java.lang;
+
+public interface Comparable<T> {
+    public int compareTo(T o);
+}
diff --git a/classes/java/lang/Iterable.java b/classes/java/lang/Iterable.java
new file mode 100644
index 0000000..e6b3b4e
--- /dev/null
+++ b/classes/java/lang/Iterable.java
@@ -0,0 +1,5 @@
+package java.lang;
+
+public interface Iterable<T> {
+    Iterator<T> iterator();
+}
diff --git a/classes/java/lang/Runnable.java b/classes/java/lang/Runnable.java
new file mode 100644
index 0000000..765ec99
--- /dev/null
+++ b/classes/java/lang/Runnable.java
@@ -0,0 +1,5 @@
+package java.lang;
+
+public interface Runnable {
+    void run();
+}
diff --git a/classes/java/lang/String.java b/classes/java/lang/String.java
index 78f77f0..3943a5e 100644
--- a/classes/java/lang/String.java
+++ b/classes/java/lang/String.java
@@ -1,22 +1,63 @@
 package java.lang;
 
 public class String {
-    private final byte[] value;
+    private final char[] value;
 
     public String() {
-        this.value = new byte[0];
+        this.value = new char[0];
+    }
+
+    public String(byte[] bytes) {
+        this(bytes, 0, bytes.length);
+    }
+
+    public String(byte[] bytes, int offset, int length) {
+        this.value = new char[length];
+        int i = 0;
+        while (length > 0) {
+            this.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 length) {
+        this.value = new char[length];
+        int i = 0;
+        while (length > 0) {
+            this.value[i] = value[offset];
+            i += 1;
+            offset += 1;
+            length -= 1;
+        }
     }
 
     public String(String original) {
+        // shallow copy
         this.value = original.value;
     }
 
-    public String(byte[] value) {
-        this.value = value;
+    public char charAt(int index) {
+        return this.value[index];
     }
 
-    public byte[] getBytes() {
-        return this.value;
+    public int compareTo(String anotherString) {
+        int length = this.length() < anotherString.length()
+                   ? this.length()
+                   : anotherString.length();
+
+        for (knt k = 0; k < length; k++) {
+            int difference = this.charAt(k) - anotherString.charAt(k);
+            if (difference != 0)
+                return difference;
+        }
+
+        return this.length() - anotherString.length();
     }
 
     public String toString() {
@@ -32,7 +73,7 @@ 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(int i) {