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Asked: 2026-05-31T22:03:47+00:00

Why Java decided to allow ‘null’ as a key in HashMap? As we know

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Why Java decided to allow ‘null’ as a key in HashMap? As we know hashcode() can’t be calculated on null value. Is there any design consideration for this decision to allow null key in HashMap which is not there in Hashtable(Other than considering as a default value)?

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  1. Editorial Team

    From the JDK 1.2 Java Collections API Change Summary (not sure where to find the official version on Oracle’s website):

    Added null-key support to HashMap. This was done for consistency with
    TreeMap and the late, unlamented ArrayMap, and because customers
    requested it. Now all of our general-purpose collection
    implementations accept null keys, values and elements.

    Joshua Bloch and Doug Lea disagreed on this, and this caused problems for concurrent hash maps.

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