api: Summarize EquivalentAddressGroup toString

api/src/main/java/io/grpc/EquivalentAddressGroup.java

@@ -34,6 +34,7 @@
  */
 @ExperimentalApi("https://github.com/grpc/grpc-java/issues/1770")
 public final class EquivalentAddressGroup {
+  private static final int MAX_ADDRESSES_TO_STRING = 100;
 
   /**
    * The authority to be used when constructing Subchannels for this EquivalentAddressGroup.
@@ -113,8 +114,24 @@ public Attributes getAttributes() {
 
   @Override
   public String toString() {
-    // TODO(zpencer): Summarize return value if addr is very large
-    return "[" + addrs + "/" + attrs + "]";
+    StringBuilder sb = new StringBuilder();
+    sb.append('[');
+    if (addrs.size() <= MAX_ADDRESSES_TO_STRING) {
+      sb.append(addrs);
+    } else {
+      sb.append('[');
+      for (int i = 0; i < MAX_ADDRESSES_TO_STRING; i++) {
+        if (i > 0) {
+          sb.append(", ");
+        }
+        sb.append(addrs.get(i));
+      }
+      sb.append(", ... ");
+      sb.append(addrs.size() - MAX_ADDRESSES_TO_STRING);
+      sb.append(" more]");
+    }
+    sb.append('/').append(attrs).append(']');
+    return sb.toString();
   }
 
   @Override

api/src/test/java/io/grpc/EquivalentAddressGroupTest.java

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2026 The gRPC 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 io.grpc;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import java.lang.reflect.Field;
+import java.net.SocketAddress;
+import java.util.ArrayList;
+import java.util.List;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
+
+/**
+ * Unit tests for {@link EquivalentAddressGroup}.
+ */
+@RunWith(JUnit4.class)
+public class EquivalentAddressGroupTest {
+
+  @Test
+  public void toString_summarizesLargeAddressList() {
+    int maxAddressesToString = maxAddressesToString();
+    List<SocketAddress> addrs = new ArrayList<>();
+    for (int i = 0; i <= maxAddressesToString; i++) {
+      addrs.add(new FakeSocketAddress("addr" + i));
+    }
+    EquivalentAddressGroup eag = new EquivalentAddressGroup(addrs);
+
+    StringBuilder expected = new StringBuilder();
+    expected.append('[').append('[');
+    for (int i = 0; i < maxAddressesToString; i++) {
+      if (i > 0) {
+        expected.append(", ");
+      }
+      expected.append(addrs.get(i));
+    }
+    expected.append(", ... 1 more]/{}]");
+    assertThat(eag.toString()).isEqualTo(expected.toString());
+  }
+
+  @Test
+  public void toString_doesNotSummarizeAtMaxAddressCount() {
+    int maxAddressesToString = maxAddressesToString();
+    List<SocketAddress> addrs = new ArrayList<>();
+    for (int i = 0; i < maxAddressesToString; i++) {
+      addrs.add(new FakeSocketAddress("addr" + i));
+    }
+    EquivalentAddressGroup eag = new EquivalentAddressGroup(addrs);
+
+    String expected = "[" + addrs + "/{}]";
+    assertThat(eag.toString()).isEqualTo(expected);
+  }
+
+  private static int maxAddressesToString() {
+    try {
+      Field field = EquivalentAddressGroup.class.getDeclaredField("MAX_ADDRESSES_TO_STRING");
+      field.setAccessible(true);
+      return (int) field.get(null);
+    } catch (NoSuchFieldException | IllegalAccessException e) {
+      throw new LinkageError("Unable to read MAX_ADDRESSES_TO_STRING", e);
+    }
+  }
+
+  private static final class FakeSocketAddress extends SocketAddress {
+
+    private final String name;
+
+    FakeSocketAddress(String name) {
+      this.name = name;
+    }
+
+    @Override
+    public String toString() {
+      return name;
+    }
+  }
+}