feature(RowMetadata)[gh-56]: add getColumnMetadata on RowMetadata

Replace Stream with forEach

replace foreach with for loop

remove unused import

change to Collection

Change to LinkedHashSet

r2dbc-spi-test/src/main/java/io/r2dbc/spi/test/MockRowMetadata.java

@@ -19,8 +19,12 @@
 import io.r2dbc.spi.ColumnMetadata;
 import io.r2dbc.spi.RowMetadata;
 
-import java.util.ArrayList;
+import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Collection;
+import java.util.Set;
+import java.util.Collections;
+import java.util.ArrayList;
 
 public final class MockRowMetadata implements RowMetadata {
 
@@ -50,6 +54,15 @@ public List<ColumnMetadata> getColumnMetadatas() {
         return this.columnMetadatas;
     }
 
+    @Override
+    public Collection<String> getColumnNames() {
+        Set<String> columnNames = new LinkedHashSet<>();
+        for(ColumnMetadata columnMetadata : columnMetadatas) {
+            columnNames.add(columnMetadata.getName());
+        }
+        return Collections.unmodifiableCollection(columnNames);
+    }
+
     @Override
     public String toString() {
         return "MockRowMetadata{" +

r2dbc-spi/src/main/java/io/r2dbc/spi/RowMetadata.java

@@ -16,6 +16,7 @@
 
 package io.r2dbc.spi;
 
+import java.util.Collection;
 import java.util.NoSuchElementException;
 
 /**
@@ -41,4 +42,14 @@ public interface RowMetadata {
      */
     Iterable<? extends ColumnMetadata> getColumnMetadatas();
 
+    /**
+     * Returns an unmodifiable collection of unique column names, and any attempts to modify the returned
+     * collection, whether direct or via its iterator, result in an {@link UnsupportedOperationException}.
+     * The order of the column names depends on the actual query and also column name lookup depends on the database
+     * sorting rules and some databases support escape characters to enforce a particular mode of comparison.
+     *
+     * @return the column names.
+     */
+    Collection<String> getColumnNames();
+
 }