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Java Puzzler on Arrays

· Java Zone

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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.

 

 

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