Add Builder in CompletionStats and refactor usage

Signed-off-by: Jean Kim <bgshhd95@gmail.com>

Author: pado0

server/src/main/java/org/opensearch/index/engine/CompletionStatsCache.java

@@ -116,7 +116,7 @@ CompletionStats get(String... fieldNamePatterns) {
                 }
             }
 
-            return new CompletionStats(sizeInBytes, new FieldMemoryStats(completionFields));
+            return new CompletionStats.Builder().sizeInBytes(sizeInBytes).fields(new FieldMemoryStats(completionFields)).build();
         });
 
         boolean success = false;
@@ -153,7 +153,7 @@ private static CompletionStats filterCompletionStatsByFieldName(String[] fieldNa
         } else {
             fieldMemoryStats = null;
         }
-        return new CompletionStats(fullCompletionStats.getSizeInBytes(), fieldMemoryStats);
+        return new CompletionStats.Builder().sizeInBytes(fullCompletionStats.getSizeInBytes()).fields(fieldMemoryStats).build();
     }
 
     @Override

server/src/main/java/org/opensearch/search/suggest/completion/CompletionStats.java

@@ -62,11 +62,26 @@ public class CompletionStats implements Writeable, ToXContentFragment {
 
     public CompletionStats() {}
 
+    /**
+     * Private constructor that takes a builder.
+     * This is the sole entry point for creating a new CompletionStats object.
+     * @param builder The builder instance containing all the values.
+     */
+    private CompletionStats(Builder builder) {
+        this.sizeInBytes = builder.sizeInBytes;
+        this.fields = builder.fields;
+    }
+
     public CompletionStats(StreamInput in) throws IOException {
         sizeInBytes = in.readVLong();
         fields = in.readOptionalWriteable(FieldMemoryStats::new);
     }
 
+    /**
+     * This constructor will be deprecated starting in version 3.4.0.
+     * Use {@link CompletionStats.Builder} instead.
+     */
+    @Deprecated
     public CompletionStats(long size, @Nullable FieldMemoryStats fields) {
         this.sizeInBytes = size;
         this.fields = fields;
@@ -84,6 +99,35 @@ public FieldMemoryStats getFields() {
         return fields;
     }
 
+    /**
+     * Builder for the {@link CompletionStats} class.
+     * Provides a fluent API for constructing a CompletionStats object.
+     */
+    public static class Builder {
+        private long sizeInBytes = 0;
+        private FieldMemoryStats fields = null;
+
+        public Builder() {}
+
+        public Builder sizeInBytes(long bytes) {
+            this.sizeInBytes = bytes;
+            return this;
+        }
+
+        public Builder fields(FieldMemoryStats fields) {
+            this.fields = fields;
+            return this;
+        }
+
+        /**
+         * Creates a {@link CompletionStats} object from the builder's current state.
+         * @return A new CompletionStats instance.
+         */
+        public CompletionStats build() {
+            return new CompletionStats(this);
+        }
+    }
+
     @Override
     public void writeTo(StreamOutput out) throws IOException {
         out.writeVLong(sizeInBytes);

server/src/test/java/org/opensearch/index/suggest/stats/CompletionsStatsTests.java

@@ -44,7 +44,7 @@ public class CompletionsStatsTests extends OpenSearchTestCase {
 
     public void testSerialize() throws IOException {
         FieldMemoryStats map = randomBoolean() ? null : FieldMemoryStatsTests.randomFieldMemoryStats();
-        CompletionStats stats = new CompletionStats(randomNonNegativeLong(), map);
+        CompletionStats stats = new CompletionStats.Builder().sizeInBytes(randomNonNegativeLong()).fields(map).build();
         BytesStreamOutput out = new BytesStreamOutput();
         stats.writeTo(out);
         StreamInput input = out.bytes().streamInput();