Validate that headers do not contain duplicates.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/csv/trunk@1508618 13f79535-47bb-0310-9956-ffa450edef68

Author: garydgregory

src/main/java/org/apache/commons/csv/CSVFormat.java

@@ -17,11 +17,11 @@
 
 package org.apache.commons.csv;
 
+import static org.apache.commons.csv.Constants.BACKSLASH;
 import static org.apache.commons.csv.Constants.COMMA;
 import static org.apache.commons.csv.Constants.CR;
 import static org.apache.commons.csv.Constants.CRLF;
 import static org.apache.commons.csv.Constants.DOUBLE_QUOTE_CHAR;
-import static org.apache.commons.csv.Constants.BACKSLASH;
 import static org.apache.commons.csv.Constants.LF;
 import static org.apache.commons.csv.Constants.TAB;
 
@@ -30,6 +30,8 @@
 import java.io.Serializable;
 import java.io.StringWriter;
 import java.util.Arrays;
+import java.util.HashSet;
+import java.util.Set;
 
 /**
  * Specifies the format of a CSV file and parses input.
@@ -530,6 +532,14 @@ void validate() throws IllegalStateException {
         if (escape == null && quotePolicy == Quote.NONE) {
             throw new IllegalStateException("No quotes mode set but no escape character is set");
         }
+        
+        if (header != null) {
+            Set<String> set = new HashSet<String>(header.length);
+            set.addAll(Arrays.asList(header));
+            if (set.size() != header.length) {
+                throw new IllegalStateException("The header contains duplicate names: " + Arrays.toString(header));
+            }
+        }
     }
 
     /**

src/test/java/org/apache/commons/csv/CSVFormatTest.java

@@ -57,6 +57,11 @@ public void testDelimiterSameAsEscapeThrowsException() {
         CSVFormat.DEFAULT.withDelimiter('!').withEscape('!').validate();
     }
 
+    @Test(expected = IllegalStateException.class)
+    public void testDuplicateHeaderElements() {
+        CSVFormat.DEFAULT.withHeader("A", "A").validate();
+    }
+
     @Test
     public void testEquals() {
         final CSVFormat right = CSVFormat.DEFAULT;