diff --git a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/AbstractStringBuilder.java b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TAbstractStringBuilder.java
similarity index 57%
rename from teavm-classlib/src/main/java/org/teavm/classlib/java/lang/AbstractStringBuilder.java
rename to teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TAbstractStringBuilder.java
index 0e75edcd3..a45610a78 100644
--- a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/AbstractStringBuilder.java
+++ b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TAbstractStringBuilder.java
@@ -2,24 +2,25 @@ package org.teavm.classlib.java.lang;
 
 import org.teavm.classlib.java.lang.io.TSerializable;
 import org.teavm.classlib.java.util.TArrays;
+import org.teavm.javascript.ni.Rename;
 
 /**
  *
  * @author Alexey Andreev
  */
-class AbstractStringBuilder extends TObject implements TSerializable {
+class TAbstractStringBuilder extends TObject implements TSerializable {
     private char[] buffer;
     private int length;
 
-    public AbstractStringBuilder() {
+    public TAbstractStringBuilder() {
         this(16);
     }
 
-    public AbstractStringBuilder(int capacity) {
+    public TAbstractStringBuilder(int capacity) {
         buffer = new char[capacity];
     }
 
-    protected AbstractStringBuilder append(TString string) {
+    protected TAbstractStringBuilder append(TString string) {
         ensureCapacity(length + string.length());
         int j = length;
         for (int i = 0; i < string.length(); ++i) {
@@ -29,13 +30,20 @@ class AbstractStringBuilder extends TObject implements TSerializable {
         return this;
     }
 
-    protected AbstractStringBuilder append(int value) {
+    protected TAbstractStringBuilder append(int value) {
+        boolean positive = true;
         if (value < 0) {
-            append('-');
+            positive = false;
             value = -value;
         }
         if (value < 10) {
-            append((char)('0' + value));
+            if (!positive) {
+                ensureCapacity(length + 2);
+                buffer[length++] = '-';
+            } else {
+                ensureCapacity(length + 1);
+            }
+            buffer[length++] = (char)('0' + value);
         } else {
             int pos = 10;
             int sz = 1;
@@ -43,7 +51,14 @@ class AbstractStringBuilder extends TObject implements TSerializable {
                 pos *= 10;
                 ++sz;
             }
+            if (!positive) {
+                ++sz;
+            }
             ensureCapacity(length + sz);
+            if (!positive) {
+                buffer[length++] = '-';
+            }
+            pos /= 10;
             while (pos > 0) {
                 buffer[length++] = (char)('0' + value / pos);
                 value %= pos;
@@ -53,7 +68,7 @@ class AbstractStringBuilder extends TObject implements TSerializable {
         return this;
     }
 
-    protected AbstractStringBuilder append(char c) {
+    protected TAbstractStringBuilder append(char c) {
         ensureCapacity(length + 1);
         buffer[length++] = c;
         return this;
@@ -65,4 +80,10 @@ class AbstractStringBuilder extends TObject implements TSerializable {
         }
         buffer = TArrays.copyOf(buffer, capacity * 2 + 1);
     }
+
+    @Override
+    @Rename("toString")
+    public TString toString0() {
+        return new TString(buffer, 0, length);
+    }
 }
diff --git a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TMath.java b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TMath.java
new file mode 100644
index 000000000..387035c9b
--- /dev/null
+++ b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TMath.java
@@ -0,0 +1,14 @@
+package org.teavm.classlib.java.lang;
+
+/**
+ *
+ * @author Alexey Andreev
+ */
+public final class TMath extends TObject {
+    private TMath() {
+    }
+
+    public static int min(int a, int b) {
+        return a < b ? a : b;
+    }
+}
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 7474827ec..f494cb409 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
@@ -9,6 +9,7 @@ import org.teavm.javascript.ni.GeneratedBy;
  */
 public class TString extends TObject implements TSerializable {
     private char[] characters;
+    private transient int hashCode;
 
     public TString() {
         this.characters = new char[0];
@@ -47,6 +48,41 @@ public class TString extends TObject implements TSerializable {
         return new TStringBuilder().append(index).toString0();
     }
 
+    @Override
+    public boolean equals(TObject other) {
+        if (this == other) {
+            return true;
+        }
+        if (!(other instanceof TString)) {
+            return false;
+        }
+        TString str = (TString)other;
+        if (str.length() != length()) {
+            return false;
+        }
+        for (int i = 0; i < str.length(); ++i) {
+            if (charAt(i) != str.charAt(i)) {
+                return false;
+            }
+        }
+        return true;
+    }
+
+    @Override
+    public int hashCode() {
+        if (hashCode == 0) {
+            hashCode ^= 734262231;
+            for (char c : characters) {
+                hashCode = (hashCode << 4) | (hashCode >>> 28);
+                hashCode ^= 347236277 ^ c;
+                if (hashCode == 0) {
+                    ++hashCode;
+                }
+            }
+        }
+        return hashCode;
+    }
+
     @GeneratedBy(StringNativeGenerator.class)
     public static native TString wrap(String str);
 }
diff --git a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TStringBuilder.java b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TStringBuilder.java
index 50dae2324..33132c66f 100644
--- a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TStringBuilder.java
+++ b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TStringBuilder.java
@@ -4,7 +4,7 @@ package org.teavm.classlib.java.lang;
  *
  * @author Alexey Andreev
  */
-public class TStringBuilder extends AbstractStringBuilder {
+public class TStringBuilder extends TAbstractStringBuilder {
     @Override
     public TStringBuilder append(TString string) {
         super.append(string);
diff --git a/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TStringBuilderTests.java b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TStringBuilderTests.java
new file mode 100644
index 000000000..c84b43059
--- /dev/null
+++ b/teavm-classlib/src/main/java/org/teavm/classlib/java/lang/TStringBuilderTests.java
@@ -0,0 +1,17 @@
+package org.teavm.classlib.java.lang;
+
+import static org.junit.Assert.*;
+import org.junit.Test;
+
+/**
+ *
+ * @author Alexey Andreev
+ */
+class TStringBuilderTests {
+    @Test
+    public void integerAppended() {
+        TStringBuilder sb = new TStringBuilder();
+        sb.append(23);
+        assertEquals("23", sb.toString());
+    }
+}
diff --git a/teavm-classlib/src/main/java/org/teavm/classlibgen/ClasslibTestGenerator.java b/teavm-classlib/src/main/java/org/teavm/classlibgen/ClasslibTestGenerator.java
index d0d1e3e63..ac3d01776 100644
--- a/teavm-classlib/src/main/java/org/teavm/classlibgen/ClasslibTestGenerator.java
+++ b/teavm-classlib/src/main/java/org/teavm/classlibgen/ClasslibTestGenerator.java
@@ -30,7 +30,8 @@ public class ClasslibTestGenerator {
     private static Renderer renderer;
     private static List<MethodReference> testMethods = new ArrayList<>();
     private static Map<String, List<MethodReference>> groupedMethods = new HashMap<>();
-    private static String[] testClasses = { "java.lang.ObjectTests", "java.lang.SystemTests" };
+    private static String[] testClasses = { "java.lang.ObjectTests", "java.lang.SystemTests",
+            "java.lang.StringBuilderTests" };
 
     public static void main(String[] args) throws IOException {
         out = System.out;
diff --git a/teavm-core/src/main/java/org/teavm/dependency/DependencyChecker.java b/teavm-core/src/main/java/org/teavm/dependency/DependencyChecker.java
index a8f6c0db5..e5a454ebd 100644
--- a/teavm-core/src/main/java/org/teavm/dependency/DependencyChecker.java
+++ b/teavm-core/src/main/java/org/teavm/dependency/DependencyChecker.java
@@ -159,10 +159,20 @@ public class DependencyChecker {
         }
     }
 
-    private MethodGraph createMethodGraph(MethodReference methodRef) {
+    private MethodGraph createMethodGraph(final MethodReference methodRef) {
         initClass(methodRef.getClassName());
         ClassHolder cls = classSource.getClassHolder(methodRef.getClassName());
         MethodHolder method = cls.getMethod(methodRef.getDescriptor());
+        if (method == null) {
+            while (cls != null) {
+                method = cls.getMethod(methodRef.getDescriptor());
+                if (method != null) {
+                    return methodCache.map(new MethodReference(cls.getName(), methodRef.getDescriptor()));
+                }
+                cls = classSource.getClassHolder(cls.getParent());
+            }
+            throw new RuntimeException("Method not found: " + methodRef);
+        }
         ValueType[] arguments = method.getParameterTypes();
         int paramCount = arguments.length + 1;
         int varCount = Math.max(paramCount, method.getProgram().variableCount());
@@ -184,10 +194,15 @@ public class DependencyChecker {
                         method.getName() + MethodDescriptor.get(method) + ":RESULT");
             }
         }
-        MethodGraph graph = new MethodGraph(parameterNodes, paramCount, resultNode, this);
-        DependencyGraphBuilder graphBuilder = new DependencyGraphBuilder(this);
-        graphBuilder.buildGraph(method, graph);
-        achieveClass(methodRef.getClassName());
+        final MethodGraph graph = new MethodGraph(parameterNodes, paramCount, resultNode, this);
+        final MethodHolder currentMethod = method;
+        executor.submit(new Runnable() {
+            @Override public void run() {
+                DependencyGraphBuilder graphBuilder = new DependencyGraphBuilder(DependencyChecker.this);
+                graphBuilder.buildGraph(currentMethod, graph);
+                achieveClass(methodRef.getClassName());
+            }
+        });
         return graph;
     }
 
@@ -223,6 +238,9 @@ public class DependencyChecker {
     private void activateDependencyPlugin(MethodReference methodRef) {
         ClassHolder cls = classSource.getClassHolder(methodRef.getClassName());
         MethodHolder method = cls.getMethod(methodRef.getDescriptor());
+        if (method == null) {
+            return;
+        }
         AnnotationHolder depAnnot = method.getAnnotations().get(PluggableDependency.class.getName());
         if (depAnnot == null) {
             return;
diff --git a/teavm-core/src/main/java/org/teavm/model/ClassHolder.java b/teavm-core/src/main/java/org/teavm/model/ClassHolder.java
index 6f275dcf8..1924c3b85 100644
--- a/teavm-core/src/main/java/org/teavm/model/ClassHolder.java
+++ b/teavm-core/src/main/java/org/teavm/model/ClassHolder.java
@@ -98,5 +98,6 @@ public class ClassHolder extends ElementHolder {
                     field.getName() + " is not a member of " + getName());
         }
         fields.remove(field.getName());
+        field.setOwner(null);
     }
 }