diff --git a/user/super/com/google/gwt/emul/java/io/PrintStream.java b/user/super/com/google/gwt/emul/java/io/PrintStream.java
index a47fb8c98b3..49dffb4ceb9 100644
--- a/user/super/com/google/gwt/emul/java/io/PrintStream.java
+++ b/user/super/com/google/gwt/emul/java/io/PrintStream.java
@@ -19,7 +19,7 @@
  * See <a href="http://java.sun.com/j2se/1.5.0/docs/api/java/io/PrintStream.html">the official Java
  * API doc</a> for details.
  */
-public class PrintStream extends FilterOutputStream {
+public class PrintStream extends FilterOutputStream implements Appendable {
 
   /** Indicates whether or not this PrintStream has incurred an error. */
   private boolean ioError = false;
@@ -195,4 +195,22 @@ protected void clearError() {
   private void newline() {
     print('\n');
   }
+
+  @Override
+  public Appendable append(CharSequence csq) {
+    print(csq);
+    return this;
+  }
+
+  @Override
+  public Appendable append(CharSequence csq, int start, int end) {
+    print(csq.subSequence(start, end));
+    return this;
+  }
+
+  @Override
+  public Appendable append(char c) {
+    print(c);
+    return this;
+  }
 }
diff --git a/user/super/com/google/gwt/emul/java/io/Reader.java b/user/super/com/google/gwt/emul/java/io/Reader.java
index 63216ad3a92..db1b0c65935 100644
--- a/user/super/com/google/gwt/emul/java/io/Reader.java
+++ b/user/super/com/google/gwt/emul/java/io/Reader.java
@@ -18,7 +18,7 @@
 /**
  * Reads a stream of characters.
  */
-public abstract class Reader {
+public abstract class Reader implements Closeable {
   /**
    * The maximum buffer size to incrementally read in {@link #skip}.
    */
@@ -27,6 +27,7 @@ public abstract class Reader {
   /**
    * Closes the reader, and releases any associated resources.
    */
+  @Override
   public abstract void close() throws IOException;
 
   /**
diff --git a/user/super/com/google/gwt/emul/java/lang/Boolean.java b/user/super/com/google/gwt/emul/java/lang/Boolean.java
index 4aafa9c1c66..188e08b72d2 100644
--- a/user/super/com/google/gwt/emul/java/lang/Boolean.java
+++ b/user/super/com/google/gwt/emul/java/lang/Boolean.java
@@ -18,13 +18,14 @@
 import static javaemul.internal.InternalPreconditions.checkNotNull;
 
 import java.io.Serializable;
+import java.lang.constant.Constable;
 import javaemul.internal.JsUtils;
 import jsinterop.annotations.JsMethod;
 
 /**
  * Wraps native <code>boolean</code> as an object.
  */
-public final class Boolean implements Comparable<Boolean>, Serializable {
+public final class Boolean implements Comparable<Boolean>, Serializable, Constable {
 
   public static final Boolean FALSE = false;
   public static final Boolean TRUE = true;
diff --git a/user/super/com/google/gwt/emul/java/lang/Byte.java b/user/super/com/google/gwt/emul/java/lang/Byte.java
index d5f0f1f93d7..d40fbd3ab77 100644
--- a/user/super/com/google/gwt/emul/java/lang/Byte.java
+++ b/user/super/com/google/gwt/emul/java/lang/Byte.java
@@ -15,10 +15,12 @@
  */
 package java.lang;
 
+import java.lang.constant.Constable;
+
 /**
  * Wraps native <code>byte</code> as an object.
  */
-public final class Byte extends Number implements Comparable<Byte> {
+public final class Byte extends Number implements Comparable<Byte>, Constable {
 
   public static final byte MIN_VALUE = (byte) 0x80;
   public static final byte MAX_VALUE = (byte) 0x7F;
diff --git a/user/super/com/google/gwt/emul/java/lang/Class.java b/user/super/com/google/gwt/emul/java/lang/Class.java
index c59eed0b6cc..8bda1112fb6 100644
--- a/user/super/com/google/gwt/emul/java/lang/Class.java
+++ b/user/super/com/google/gwt/emul/java/lang/Class.java
@@ -17,6 +17,7 @@
 
 import com.google.gwt.core.client.JavaScriptObject;
 
+import java.lang.constant.Constable;
 import java.lang.reflect.Type;
 import javaemul.internal.annotations.DoNotInline;
 
@@ -29,7 +30,7 @@
  *
  * @param <T> the type of the object
  */
-public final class Class<T> implements Type {
+public final class Class<T> implements Type, Constable {
 
   private static final int PRIMITIVE = 0x00000001;
   private static final int INTERFACE = 0x00000002;
diff --git a/user/super/com/google/gwt/emul/java/lang/Enum.java b/user/super/com/google/gwt/emul/java/lang/Enum.java
index b044a57b6cb..861eeed3a0e 100644
--- a/user/super/com/google/gwt/emul/java/lang/Enum.java
+++ b/user/super/com/google/gwt/emul/java/lang/Enum.java
@@ -21,6 +21,7 @@
 import com.google.gwt.core.client.JavaScriptObject;
 
 import java.io.Serializable;
+import java.lang.constant.Constable;
 import jsinterop.annotations.JsIgnore;
 import jsinterop.annotations.JsNonNull;
 import jsinterop.annotations.JsType;
@@ -31,7 +32,7 @@
  * @param <E>
  */
 @JsType
-public abstract class Enum<E extends Enum<E>> implements Comparable<E>, Serializable {
+public abstract class Enum<E extends Enum<E>> implements Comparable<E>, Serializable, Constable {
 
   @JsIgnore
   public static <T extends Enum<T>> T valueOf(Class<T> enumType, String name) {
diff --git a/user/super/com/google/gwt/emul/java/lang/Float.java b/user/super/com/google/gwt/emul/java/lang/Float.java
index 4e5ec5ec02b..39d5f71b2c7 100644
--- a/user/super/com/google/gwt/emul/java/lang/Float.java
+++ b/user/super/com/google/gwt/emul/java/lang/Float.java
@@ -15,12 +15,14 @@
  */
 package java.lang;
 
+import java.lang.constant.Constable;
+import java.lang.constant.ConstantDesc;
 import javaemul.internal.JsUtils;
 
 /**
  * Wraps a primitive <code>float</code> as an object.
  */
-public final class Float extends Number implements Comparable<Float> {
+public final class Float extends Number implements Comparable<Float>, Constable, ConstantDesc {
   public static final float MAX_VALUE = 3.4028235e+38f;
   public static final float MIN_VALUE = 1.4e-45f;
   public static final int MAX_EXPONENT = 127;
diff --git a/user/super/com/google/gwt/emul/java/lang/Integer.java b/user/super/com/google/gwt/emul/java/lang/Integer.java
index d349bdab092..60f179fbcc5 100644
--- a/user/super/com/google/gwt/emul/java/lang/Integer.java
+++ b/user/super/com/google/gwt/emul/java/lang/Integer.java
@@ -15,13 +15,15 @@
  */
 package java.lang;
 
+import java.lang.constant.Constable;
+import java.lang.constant.ConstantDesc;
 import javaemul.internal.JsUtils;
 import javaemul.internal.annotations.HasNoSideEffects;
 
 /**
  * Wraps a primitive <code>int</code> as an object.
  */
-public final class Integer extends Number implements Comparable<Integer> {
+public final class Integer extends Number implements Comparable<Integer>, Constable, ConstantDesc {
 
   public static final int MAX_VALUE = 0x7fffffff;
   public static final int MIN_VALUE = 0x80000000;
diff --git a/user/super/com/google/gwt/emul/java/lang/Long.java b/user/super/com/google/gwt/emul/java/lang/Long.java
index 33622aa6b4f..886984b52fc 100644
--- a/user/super/com/google/gwt/emul/java/lang/Long.java
+++ b/user/super/com/google/gwt/emul/java/lang/Long.java
@@ -15,11 +15,13 @@
  */
 package java.lang;
 
+import java.lang.constant.Constable;
+import java.lang.constant.ConstantDesc;
 import javaemul.internal.LongUtils;
 import javaemul.internal.annotations.HasNoSideEffects;
 
 /** Wraps a primitive <code>long</code> as an object. */
-public final class Long extends Number implements Comparable<Long> {
+public final class Long extends Number implements Comparable<Long>, Constable, ConstantDesc {
 
   /** Use nested class to avoid clinit on outer. */
   static class BoxedValues {
diff --git a/user/super/com/google/gwt/emul/java/lang/String.java b/user/super/com/google/gwt/emul/java/lang/String.java
index 67b1cff1107..126cc180677 100644
--- a/user/super/com/google/gwt/emul/java/lang/String.java
+++ b/user/super/com/google/gwt/emul/java/lang/String.java
@@ -24,6 +24,8 @@
 
 import java.io.Serializable;
 import java.io.UnsupportedEncodingException;
+import java.lang.constant.Constable;
+import java.lang.constant.ConstantDesc;
 import java.nio.charset.Charset;
 import java.nio.charset.UnsupportedCharsetException;
 import java.util.Comparator;
@@ -55,7 +57,7 @@
 // Needed to have constructors not fail compilation internally at Google
 @SuppressWarnings({ "ReturnValueIgnored", "unusable-by-js" })
 public final class String implements Comparable<String>, CharSequence,
-    Serializable {
+    Serializable, Constable, ConstantDesc {
   /* TODO(jat): consider whether we want to support the following methods;
    *
    * <ul>
diff --git a/user/super/com/google/gwt/emul/java/lang/StringBuffer.java b/user/super/com/google/gwt/emul/java/lang/StringBuffer.java
index 91f154a436e..4fb2d838871 100644
--- a/user/super/com/google/gwt/emul/java/lang/StringBuffer.java
+++ b/user/super/com/google/gwt/emul/java/lang/StringBuffer.java
@@ -23,7 +23,8 @@
  * This class is an exact clone of {@link StringBuilder} except for the name.
  * Any change made to one should be mirrored in the other.
  */
-public final class StringBuffer extends AbstractStringBuilder {
+public final class StringBuffer extends AbstractStringBuilder
+    implements Comparable<StringBuffer> {
 
   public StringBuffer() {
     super("");
@@ -187,4 +188,9 @@ public StringBuffer reverse() {
     reverse0();
     return this;
   }
+
+  @Override
+  public int compareTo(StringBuffer other) {
+    return toString().compareTo(other.toString());
+  }
 }
diff --git a/user/super/com/google/gwt/emul/java/lang/StringBuilder.java b/user/super/com/google/gwt/emul/java/lang/StringBuilder.java
index da705efb0a6..5c8d37ae8ea 100644
--- a/user/super/com/google/gwt/emul/java/lang/StringBuilder.java
+++ b/user/super/com/google/gwt/emul/java/lang/StringBuilder.java
@@ -23,7 +23,8 @@
  * This class is an exact clone of {@link StringBuffer} except for the name.
  * Any change made to one should be mirrored in the other.
  */
-public final class StringBuilder extends AbstractStringBuilder {
+public final class StringBuilder extends AbstractStringBuilder
+    implements Comparable<StringBuilder> {
 
   public StringBuilder() {
     super("");
@@ -187,4 +188,9 @@ public StringBuilder reverse() {
     reverse0();
     return this;
   }
+
+  @Override
+  public int compareTo(StringBuilder other) {
+    return toString().compareTo(other.toString());
+  }
 }
diff --git a/user/super/com/google/gwt/emul/java/lang/constant/Constable.java b/user/super/com/google/gwt/emul/java/lang/constant/Constable.java
new file mode 100644
index 00000000000..49a8d2975d3
--- /dev/null
+++ b/user/super/com/google/gwt/emul/java/lang/constant/Constable.java
@@ -0,0 +1,19 @@
+/*
+ * Copyright 2025 GWT Project Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package java.lang.constant;
+
+public interface Constable {
+}
diff --git a/user/super/com/google/gwt/emul/java/lang/constant/ConstantDesc.java b/user/super/com/google/gwt/emul/java/lang/constant/ConstantDesc.java
new file mode 100644
index 00000000000..f51d69dbcda
--- /dev/null
+++ b/user/super/com/google/gwt/emul/java/lang/constant/ConstantDesc.java
@@ -0,0 +1,19 @@
+/*
+ * Copyright 2026 GWT Project Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+package java.lang.constant;
+
+public interface ConstantDesc {
+}
diff --git a/user/super/com/google/gwt/emul/java/util/BitSet.java b/user/super/com/google/gwt/emul/java/util/BitSet.java
index 1fa15d55677..0455a86f4de 100644
--- a/user/super/com/google/gwt/emul/java/util/BitSet.java
+++ b/user/super/com/google/gwt/emul/java/util/BitSet.java
@@ -34,7 +34,7 @@
  * public static BitSet valueOf(ByteBuffer)
  * public static BitSet valueOf(LongBuffer)
  */
-public class BitSet {
+public class BitSet implements Cloneable {
   private static final int WORD_MASK = 0x7fffffff;
   private static final int BITS_PER_WORD = 31;
 
diff --git a/user/super/com/google/gwt/emul/java/util/EnumMap.java b/user/super/com/google/gwt/emul/java/util/EnumMap.java
index 00ef1b98e95..949c3398043 100644
--- a/user/super/com/google/gwt/emul/java/util/EnumMap.java
+++ b/user/super/com/google/gwt/emul/java/util/EnumMap.java
@@ -28,7 +28,7 @@
  * @param <K> key type
  * @param <V> value type
  */
-public class EnumMap<K extends Enum<K>, V> extends AbstractMap<K, V> {
+public class EnumMap<K extends Enum<K>, V> extends AbstractMap<K, V> implements Cloneable {
 
   private final class EntrySet extends AbstractSet<Entry<K, V>> {
 
diff --git a/user/super/com/google/gwt/emul/java/util/EnumSet.java b/user/super/com/google/gwt/emul/java/util/EnumSet.java
index 9d3c1472268..aa48fce4845 100644
--- a/user/super/com/google/gwt/emul/java/util/EnumSet.java
+++ b/user/super/com/google/gwt/emul/java/util/EnumSet.java
@@ -31,7 +31,7 @@
  *
  * @param <E> enumeration type
  */
-public abstract class EnumSet<E extends Enum<E>> extends AbstractSet<E> {
+public abstract class EnumSet<E extends Enum<E>> extends AbstractSet<E> implements Cloneable {
 
   /**
    * Implemented via sparse array since the set size is finite. Iteration takes
diff --git a/user/super/com/google/gwt/emul/java/util/Random.java b/user/super/com/google/gwt/emul/java/util/Random.java
index 4a7da061541..acb44db3c37 100644
--- a/user/super/com/google/gwt/emul/java/util/Random.java
+++ b/user/super/com/google/gwt/emul/java/util/Random.java
@@ -37,6 +37,7 @@
 import static javaemul.internal.InternalPreconditions.checkCriticalArgument;
 import static javaemul.internal.InternalPreconditions.checkNotNull;
 
+import java.util.random.RandomGenerator;
 import javaemul.internal.JsUtils;
 
 /**
@@ -46,7 +47,7 @@
  *
  * This emulated version of Random is not serializable.
  */
-public class Random {
+public class Random implements RandomGenerator {
 
   private static final double multiplierHi = 0x5de;
   private static final double multiplierLo = 0xece66d;
@@ -132,23 +133,12 @@ public Random(long seed) {
     setSeed(seed);
   }
 
-  /**
-   * Returns the next pseudo-random, uniformly distributed {@code boolean} value
-   * generated by this generator.
-   *
-   * @return a pseudo-random, uniformly distributed boolean value.
-   */
+  @Override
   public boolean nextBoolean() {
     return nextInternal(1) != 0;
   }
 
-  /**
-   * Modifies the {@code byte} array by a random sequence of {@code byte}s
-   * generated by this random number generator.
-   *
-   * @param buf non-null array to contain the new random {@code byte}s.
-   * @see #next
-   */
+  @Override
   public void nextBytes(byte[] buf) {
     checkNotNull(buf);
 
@@ -165,25 +155,13 @@ public void nextBytes(byte[] buf) {
     }
   }
 
-  /**
-   * Generates a normally distributed random {@code double} number between 0.0
-   * inclusively and 1.0 exclusively.
-   *
-   * @return a random {@code double} in the range [0.0 - 1.0)
-   * @see #nextFloat
-   */
+  @Override
   public double nextDouble() {
     return nextInternal(26) * twoToTheMinus26 +
       nextInternal(27) * twoToTheMinus53;
   }
 
-  /**
-   * Generates a normally distributed random {@code float} number between 0.0
-   * inclusively and 1.0 exclusively.
-   *
-   * @return float a random {@code float} number between [0.0 and 1.0)
-   * @see #nextDouble
-   */
+  @Override
   public float nextFloat() {
     return (float) (nextInternal(24) * twoToTheMinus24);
   }
@@ -222,28 +200,12 @@ public synchronized double nextGaussian() {
     return v1 * norm;
   }
 
-  /**
-   * Generates a uniformly distributed 32-bit {@code int} value from the random
-   * number sequence.
-   *
-   * @return a uniformly distributed {@code int} value.
-   * @see java.lang.Integer#MAX_VALUE
-   * @see java.lang.Integer#MIN_VALUE
-   * @see #next
-   * @see #nextLong
-   */
+  @Override
   public int nextInt() {
     return (int) nextInternal(32);
   }
 
-  /**
-   * Returns a new pseudo-random {@code int} value which is uniformly
-   * distributed between 0 (inclusively) and the value of {@code n}
-   * (exclusively).
-   *
-   * @param n the exclusive upper border of the range [0 - n).
-   * @return a random {@code int}.
-   */
+  @Override
   public int nextInt(int n) {
     checkCriticalArgument(n > 0);
 
@@ -258,17 +220,7 @@ public int nextInt(int n) {
     return (int) val;
   }
 
-  /**
-   * Generates a uniformly distributed 64-bit integer value from the random
-   * number sequence.
-   *
-   * @return 64-bit random integer.
-   * @see java.lang.Integer#MAX_VALUE
-   * @see java.lang.Integer#MIN_VALUE
-   * @see #next
-   * @see #nextInt()
-   * @see #nextInt(int)
-   */
+  @Override
   public long nextLong() {
     return ((long) nextInternal(32) << 32) + (long) nextInternal(32);
   }
diff --git a/user/super/com/google/gwt/emul/java/util/random/RandomGenerator.java b/user/super/com/google/gwt/emul/java/util/random/RandomGenerator.java
new file mode 100644
index 00000000000..b096eb46555
--- /dev/null
+++ b/user/super/com/google/gwt/emul/java/util/random/RandomGenerator.java
@@ -0,0 +1,100 @@
+/*
+ * Copyright 2026 GWT Project Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License"); you may not
+ * use this file except in compliance with the License. You may obtain a copy of
+ * the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations under
+ * the License.
+ */
+
+package java.util.random;
+
+public interface RandomGenerator {
+
+    /**
+     * Returns the next pseudo-random, uniformly distributed {@code boolean} value
+     * generated by this generator.
+     *
+     * @return a pseudo-random, uniformly distributed boolean value.
+     */
+    boolean nextBoolean();
+
+    /**
+     * Generates a uniformly distributed 32-bit {@code int} value from the random
+     * number sequence.
+     *
+     * @return a uniformly distributed {@code int} value.
+     * @see java.lang.Integer#MAX_VALUE
+     * @see java.lang.Integer#MIN_VALUE
+     * @see #nextLong
+     */
+    int nextInt();
+
+    /**
+     * Returns a new pseudo-random {@code int} value which is uniformly
+     * distributed between 0 (inclusively) and the value of {@code n}
+     * (exclusively).
+     *
+     * @param n the exclusive upper border of the range [0 - n).
+     * @return a random {@code int}.
+     */
+    int nextInt(int n);
+
+    /**
+     * Generates a uniformly distributed 64-bit integer value from the random
+     * number sequence.
+     *
+     * @return 64-bit random integer.
+     * @see java.lang.Integer#MAX_VALUE
+     * @see java.lang.Integer#MIN_VALUE
+     * @see #nextInt()
+     * @see #nextInt(int)
+     */
+    long nextLong();
+
+    /**
+     * Generates a normally distributed random {@code float} number between 0.0
+     * inclusively and 1.0 exclusively.
+     *
+     * @return float a random {@code float} number between [0.0 and 1.0)
+     * @see #nextDouble
+     */
+    float nextFloat();
+
+    /**
+     * Generates a normally distributed random {@code double} number between 0.0
+     * inclusively and 1.0 exclusively.
+     *
+     * @return a random {@code double} in the range [0.0 - 1.0)
+     * @see #nextFloat
+     */
+    double nextDouble();
+
+    /**
+     * Pseudo-randomly generates (approximately) a normally distributed {@code
+     * double} value with mean 0.0 and a standard deviation value of {@code 1.0}
+     * using the <i>polar method<i> of G. E. P. Box, M. E. Muller, and G.
+     * Marsaglia, as described by Donald E. Knuth in <i>The Art of Computer
+     * Programming, Volume 2: Seminumerical Algorithms</i>, section 3.4.1,
+     * subsection C, algorithm P.
+     *
+     * @return a random {@code double}
+     * @see #nextDouble
+     */
+    double nextGaussian();
+
+    /**
+     * Modifies the {@code byte} array by a random sequence of {@code byte}s
+     * generated by this random number generator.
+     *
+     * @param buf non-null array to contain the new random {@code byte}s.
+     */
+    void nextBytes(byte[] buf);
+}
\ No newline at end of file
diff --git a/user/test/com/google/gwt/emultest/java/io/PrintStreamTest.java b/user/test/com/google/gwt/emultest/java/io/PrintStreamTest.java
index bdfeb4921cb..3b10d8a4f13 100644
--- a/user/test/com/google/gwt/emultest/java/io/PrintStreamTest.java
+++ b/user/test/com/google/gwt/emultest/java/io/PrintStreamTest.java
@@ -38,7 +38,7 @@ public class PrintStreamTest extends GWTTestCase {
 
   private static class MockPrintStream extends PrintStream {
 
-    public MockPrintStream(OutputStream os) {
+    MockPrintStream(OutputStream os) {
       super(os);
     }
 
@@ -281,4 +281,11 @@ public void testClearError() {
     assertFalse(ps.checkError());
     ps.close();
   }
+
+  public void testAppend() {
+    ps.append('h');
+    ps.append("owd");
+    ps.append("xyz", 1, 2);
+    assertEquals("howdy", new String(baos.toByteArray(), UTF_8));
+  }
 }
diff --git a/user/test/com/google/gwt/emultest/java/io/StringReaderTest.java b/user/test/com/google/gwt/emultest/java/io/StringReaderTest.java
index c9d79de9338..191f83d8d5c 100644
--- a/user/test/com/google/gwt/emultest/java/io/StringReaderTest.java
+++ b/user/test/com/google/gwt/emultest/java/io/StringReaderTest.java
@@ -102,6 +102,14 @@ public void testSkip_longString() throws Exception {
     assertEquals(-1, reader.read());
   }
 
+  public void testClose() throws Exception {
+    char[] out = new char[3];
+    try (StringReader reader = new StringReader("foo")) {
+      assertEquals(3, reader.read(out));
+    }
+    assertEquals("foo", new String(out));
+  }
+
   private static String repeat(char c, int count) {
     char[] arr = new char[count];
     Arrays.fill(arr, c);
diff --git a/user/test/com/google/gwt/emultest/java/lang/StringBufferTest.java b/user/test/com/google/gwt/emultest/java/lang/StringBufferTest.java
index 72ce24508c8..a7b78692999 100644
--- a/user/test/com/google/gwt/emultest/java/lang/StringBufferTest.java
+++ b/user/test/com/google/gwt/emultest/java/lang/StringBufferTest.java
@@ -18,6 +18,8 @@
 import com.google.gwt.junit.client.GWTTestCase;
 
 import java.util.Locale;
+import java.util.TreeSet;
+import java.util.stream.Collectors;
 
 /**
  * This class tests classes StringBuffer and StringBuilder.
@@ -552,4 +554,30 @@ public void testSubstring() {
     assertEquals("bc", "abcdef".substring(1, 3));
     assertEquals("bcdef", "abcdef".substring(1));
   }
+
+  public void testCompareToBuffer() {
+    assertTrue(new StringBuffer("b").compareTo(new StringBuffer("a")) > 0);
+    assertTrue(new StringBuffer("b").compareTo(new StringBuffer("c")) < 0);
+    assertEquals(0, new StringBuffer("b").compareTo(new StringBuffer("b")));
+
+    TreeSet<StringBuffer> sorted = new TreeSet<>();
+    sorted.add(new StringBuffer("b"));
+    sorted.add(new StringBuffer("c"));
+    sorted.add(new StringBuffer("d"));
+    sorted.add(new StringBuffer("a"));
+    assertEquals("abcd", sorted.stream().map(Object::toString).collect(Collectors.joining()));
+  }
+
+  public void testCompareToBuilder() {
+    assertTrue(new StringBuilder("b").compareTo(new StringBuilder("a")) > 0);
+    assertTrue(new StringBuilder("b").compareTo(new StringBuilder("c")) < 0);
+    assertEquals(0, new StringBuilder("b").compareTo(new StringBuilder("b")));
+
+    TreeSet<StringBuilder> sorted = new TreeSet<>();
+    sorted.add(new StringBuilder("b"));
+    sorted.add(new StringBuilder("c"));
+    sorted.add(new StringBuilder("d"));
+    sorted.add(new StringBuilder("a"));
+    assertEquals("abcd", sorted.stream().map(Object::toString).collect(Collectors.joining()));
+  }
 }