JAVA-2636: Add getFirstKey method to BsonDocument

Author: jyemin

bson/src/main/org/bson/BsonDocument.java

@@ -771,6 +771,17 @@ public BsonDocument append(final String key, final BsonValue value) {
         return this;
     }
 
+    /**
+     * Gets the first key in the document.
+     *
+     * @return the first key in the document
+     * @throws java.util.NoSuchElementException if the document is empty
+     * @since 3.6
+     */
+    public String getFirstKey() {
+        return keySet().iterator().next();
+    }
+
     @Override
     public boolean equals(final Object o) {
         if (this == o) {

bson/src/main/org/bson/RawBsonDocument.java

@@ -38,6 +38,7 @@
 import java.nio.ByteOrder;
 import java.util.Collection;
 import java.util.Map;
+import java.util.NoSuchElementException;
 import java.util.Set;
 
 import static org.bson.assertions.Assertions.isTrueArgument;
@@ -243,6 +244,21 @@ public Set<String> keySet() {
         return toBsonDocument().keySet();
     }
 
+    @Override
+    public String getFirstKey() {
+        BsonBinaryReader bsonReader = createReader();
+        try {
+            bsonReader.readStartDocument();
+            try {
+                return bsonReader.readName();
+            } catch (BsonInvalidOperationException e) {
+                throw new NoSuchElementException();
+            }
+        } finally {
+            bsonReader.close();
+        }
+    }
+
     @Override
     public boolean containsKey(final Object key) {
         if (key == null) {

bson/src/test/unit/org/bson/BsonDocumentSpecification.groovy

@@ -324,6 +324,25 @@ class BsonDocumentSpecification extends Specification {
         thrown(BsonInvalidOperationException)
     }
 
+    def 'should get first key'() {
+        given:
+        def document = new BsonDocument('i', new BsonInt32(2))
+
+        expect:
+        document.getFirstKey() == 'i'
+    }
+
+    def 'getFirstKey should throw NoSuchElementException if the document is empty'() {
+        given:
+        def document = new BsonDocument()
+
+        when:
+        document.getFirstKey()
+
+        then:
+        thrown(NoSuchElementException)
+    }
+
     def 'should serialize and deserialize'() {
         given:
         def document = new BsonDocument('d', new BsonDocument().append('i2', new BsonInt32(1)))

bson/src/test/unit/org/bson/RawBsonDocumentSpecification.groovy

@@ -250,6 +250,22 @@ class RawBsonDocumentSpecification extends Specification {
         rawDocument << createRawDocumentVariants()
     }
 
+    def 'should get first key'() {
+        expect:
+        document.getFirstKey() == 'a'
+
+        where:
+        rawDocument << createRawDocumentVariants()
+    }
+
+    def 'getFirstKey should throw NoSuchElementException if the document is empty'() {
+        when:
+        emptyRawDocument.getFirstKey()
+
+        then:
+        thrown(NoSuchElementException)
+    }
+
     def 'toJson should return equivalent JSON'() {
         expect:
         new RawBsonDocumentCodec().decode(new JsonReader(rawDocument.toJson()), DecoderContext.builder().build()) == document