Hashtable - Iterator on keySet, values and entrySet is fail-safe or fail-fast?
The iterators returned by the iterator() method of the collections returned by all three Map's “collection view methods" are fail-fast. Means any structural modification made to Hashtable like adding or removing elements during Iteration will throw java.util.ConcurrentModificationException.
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import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.Map.Entry;
/** Copyright (c), AnkitMittal JavaMadeSoEasy.com */
public class HashtableTest {
public static void main(String args[]){
Map<Integer,String> hashtable=new Hashtable<Integer,String>();
hashtable.put(11, "ankit");
hashtable.put(21, "javaMadeSoEasy");
System.out.println("\n---1. Iterate on keys, by obtaining iterator on keySet---");
//fail-fast
Iterator<Integer> keyIterator=hashtable.keySet().iterator();
while(keyIterator.hasNext()){
hashtable.put(4,"newEle1");//unComment to avoid ConcurrentModificationException
System.out.println(keyIterator.next());
}
System.out.println("\n---2. Iterate on values, by obtaining iterator on values---");
//fail-fast
Iterator<String> valueIterator=hashtable.values().iterator();
while(valueIterator.hasNext()){
hashtable.put(4,"newEle1");//unComment to avoid ConcurrentModificationException
System.out.println(valueIterator.next());
}
System.out.println("\n---3. Iterate on entry, by obtaining iterator on entrySet---");
//fail-fast
Iterator<Entry<Integer, String>> entryIterator=hashtable.entrySet().iterator();
while(entryIterator.hasNext()){
hashtable.put(4,"newEle1");//unComment to avoid ConcurrentModificationException
System.out.println(entryIterator.next());
}
}
}
/*OUTPUT
---1. Iterate on keys, by obtaining iterator on keySet---
Exception in thread "main" java.util.ConcurrentModificationException
at java.util.Hashtable$Enumerator.next(Unknown Source)
at hashtable_iterator_fail.HashtableTest.main(HashtableTest.java:24)
*/
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