Java Puzzler on Arrays
Today, while working with arrays I came accross a Java puzzle which made me wonder WHY IS THIS PIECE OF CODE NOT WORKING?
Java Puzzle
Can you guess whether this Junit test case will pass?
@Test
public void testMapOfArray() {
Map<String[], String[]> map = new HashMap<String[], String[]>();
map.put(new String[] { "shekhar" },
new String[] { "I am Java Developer" });
map.put(new String[] { "rahul" }, new String[] { "I am C# Developer" });
String[] description = map.get(new String[] { "shekhar" });
assertThat(description[0], IsEqual.equalTo("I am Java Developer"));
}
Everytime I ran this test it failed with NullPointerException.
What's the reason for this unexpected behavior?
The problem is that String[] uses object identity for equals and hashcode methodsz. So,
String[] arr1 = new String[] { "shekhar" };
String[] arr2 = new String[] { "shekhar" };will not match with eachother in a HashMap.
Advice
Don't use arrays as HashMap keys. If you want to use an array as a key, then write a wrapper arround an array and override equals and the hashcode method.
(4 votes)
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Comments
Eric Jablow replied on Fri, 2010/08/06 - 4:41pm
Marco Rietveld replied on Fri, 2010/08/06 - 4:45pm
First there was a String constant pool.. and then a Long, and an Integer and probably even a Double constant pool, et cetera.
But there has never been an "array" constant pool because 1. Array's can't be autoboxed and 2. Even if you could, why would you autobox a collection? It's the content of the collection that counts!
Chad Retz replied on Fri, 2010/08/06 - 5:35pm
Um...I think it's pretty commonly known that
new String[] { "bob" }.hashCode() != new String[] { "bob" }.hashCode()
Just use eclipse's auto-generated hashCode and equals on a POJO containing your array and it will be fine (Arrays.hashCode, Arrays.equals, etc). Or just use a List or Set as your key which guarantee the equals and hashCode on common elementsThomas J. Clancy replied on Sat, 2010/08/07 - 12:17pm
Cosmin Mutu replied on Mon, 2010/08/09 - 12:48am
Pradeep Arumalla replied on Mon, 2010/08/09 - 10:22am
Try the below code,when ever you do a "new Somthing()", is a new object
Map<String[], String[]> map = new HashMap<String[], String[]>();
String[] key=new String[] { "shekhar" };
map.put(key,
new String[] { "I am Java Developer" });
map.put(new String[] { "rahul" }, new String[] { "I am C# Developer" });
String[] description = map.get(key);
//assertThat(description[0], "I am Java Developer".equals("I am Java Developer"));
System.out.println(description[0].toString());