Re: typesafe java.util.Map construction and initialization

Lew <>
Sun, 24 Sep 2006 21:57:57 -0400

Tor Iver Wilhelmsen wrote:

No, there is no auto-fill constructor. Use

theMap = new java.util.HashMap<String,BarIF>();
assert(keys.length == bars.length);
for (int i = 0; i < keys.length; i++) {
    theMap.put(keys[i], bars[i]);

Lew wrote:

The assert is only useful if the keys and bars arrays are under the
control of the class, not parameters passed from the outside. Asserts
enforce provable conditions in an algorithm, and are not suitable for,
e.g., argument checking.

Josef Svitak wrote:

Given that the arrays _are_ under control of the class, is it true that
the assert should appear as early as possible in the object
construction? This class is actually a singleton (not as shown in OP)
so theMap must be initialized lazily during the call to getInstance, but
I suspect the assert should appear in the foo() ctor of which theMap is
a part.

The 'assert' should appear at a point in the code where you can prove that the
arrays are of equal length and before different lengths would mess up the
algorithm. It is possible that this particular assertion could occur at the
end of your constructor as a postcondition; without knowing your constructor
there is no way for me to know.

Generally, assertions should appear at points of invariant conditions in your
algorithm, such as preconditions and postconditions. As an example, if one
were calculating a square root of a value 'val', the assertion
   assert val >= 0;
should occur after the code that guarantees that val is non-negative and
before the square root calculation.

There are some tricky specialized uses of the 'assert' keyword as well.

Assertions are *not* a runtime check mechanism; that's the purview of
Exceptions, if (...) statements and so forth. Assertions are an
algorithm-proving mechanism.

Read the Sun article; it covers several important points, including that the
program cannot count on assertions being enabled at runtime. Check around for
general information on algorithm invariants also.


- Lew

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