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# Wednesday, 22 November 2017

We've run into following java function that models some variation of NULL arithmetic:

public static Long add(Long value1, Long value2)
{
  return value1 == null ? value2 : value2 == null ? value1 : value1 + value2;
}

When this function runs the outcome is different from what we have expected.

Here is a quiz:

  1. What is outcome of add(1, 2):

    1. 3;
    2. null;
    3. other.
  2. What is outcome of add(3, null):

    1. 3;
    2. null;
    3. other.
  3. What is outcome of add(null, 4):

    1. null;
    2. 4;
    3. other.
  4. What is outcome of add(null, null):

    1. null;
    2. 0;
    3. other.

Our assumptions were:

  1. add(1, 2) == 3;
  2. add(3, null) == 3;
  3. add(null, 4) == 4;
  4. add(null, null) == null;

Java works differently:

  1. add(1, 2) == 3;
  2. add(3, null) throws NullPointerException;
  3. add(null, 4) throws NullPointerException;
  4. add(null, null) throws NullPointerException;

The problem is with compile time type of ternary ?: operator. Compiler decides it's long, while we intuitively expected Long. Java casts null to long (which results into NPE), and then to Long.

Correct code would be:

public static Long add(Long value1, Long value2)
{
  if (value1 == null)
  {
    return value2;
  }
  else if (value2 == null)
  {
    return value;
  }
  else
  {
    return value1 + value2;
  }
}

This version does not cast anything to long, and works as we originally expected.

Honestly, we're a bit anexious about this subtle difference of if-then-else and ?: operator.

Wednesday, 22 November 2017 12:00:23 UTC  #    Comments [0] -
Java | Tips and tricks
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