Example. Testing contains with assert

[comp1110-1E]
  • Liam O'Connor, Fabian Muehlboeck

Take the contains function again from Example [comp1110-1A] and Example [comp1110-16]. Let us test it using assert:

void main() { int[] a1 = new int[]{1,2,3,4,5}; assert (contains(a1, 0) == false); assert (contains(a1,5)); assert (contains(a1,3)); int[] a2 = new int[]{-2, 0, 1000000}; assert (contains(a2, -2)); assert (!contains(a2,5)); }

Try to make each of these tests fail by modifying only the function contains.

Context

    Testing

    [comp1110-1B]
    • Liam O'Connor

    Construction. assertions in Java

    [comp1110-1C]
    • Liam O'Connor

    The assert command in Java looks like this:

    void main() { assert (1 == 2); }

    If you compile and run this program with the -ea (enable assertions) option, it will crash on the assert line and report an assertion violation, because 1 == 2 will evaluate to false.

    $ javac Assert.java $ java Assert # no -ea, nothing happens $ java -ea Assert Exception in thread "main" java.lang.AssertionError at Assert.main(Assert.java:2)

    If you give an assert an expression that evaluates to true, the program will continue executing as normal.

    You can use assertions to check that your preconditions are satisfied in a function — it's better to crash and stop the program if the environment doesn't match your expectation than continue to run and potentially do something wrong.

    Warning. 

    • Liam O'Connor
    Always remember to run your program with the -ea option, or you might be fooled into thinking your preconditions hold when they do not!

    For the first part of this course, we will use assert for testing.

    Warning. A word from E.W. Dijsktra

    [comp1110-1D]
    • Liam O'Connor

    Program testing can be used to show the presence of bugs, but never to show their absence!

    Whenever you run a test, that is one possible execution out of potentially millions, or even infinite. Try to be thorough with your testing, but be aware of its limitations!

    Example. Testing contains with assert

    [comp1110-1E]
    • Liam O'Connor, Fabian Muehlboeck

    Take the contains function again from Example [comp1110-1A] and Example [comp1110-16]. Let us test it using assert:

    void main() { int[] a1 = new int[]{1,2,3,4,5}; assert (contains(a1, 0) == false); assert (contains(a1,5)); assert (contains(a1,3)); int[] a2 = new int[]{-2, 0, 1000000}; assert (contains(a2, -2)); assert (!contains(a2,5)); }

    Try to make each of these tests fail by modifying only the function contains.

    Related

      Example. Adding specs to contains

      [comp1110-1A]
      • Liam O'Connor, Fabian Muehlboeck

      We can take the contains function from Example [comp1110-16] and annotate it with a specification. It holds no assumptions about the input parameters, so the function has no nontrivial preconditions. Its postcondition, however, captures the notion of correctness for this function:

      /** Returns if an integer is contained in an integer array @param haystack is an integer array @param needle is the integer we want to search for @return true iff there exists some j such that haystack[j] == needle */ boolean contains(int[] haystack, int needle) { int i = 0; while(i < haystack.length) { // i <= haystack.length && forall 0 <= j < i, haystack[j] != needle if(needle == haystack[i]) { return true; } i = i + 1; } // i == haystack.length // forall 0 <= j < haystack.length, haystack[j] != needle return false; }

      Note how the loop invariant, allows us to reason that our postcondition (the @return clause above) is satisfied by our function.

      Example. Searching for an Element in an Array, Generally

      [comp1110-16]
      • Liam O'Connor, Fabian Muehlboeck
       boolean contains(int[] haystack, int needle) { int i = 0; while(i < haystack.length) { // i <= haystack.length && forall 0 <= j < i, haystack[j] != needle if(needle == haystack[i]) { return true; } i = i + 1; } // i == haystack.length // forall 0 <= j < haystack.length, haystack[j] != needle return false; }

      Testing

      [comp1110-1B]
      • Liam O'Connor

      Construction. assertions in Java

      [comp1110-1C]
      • Liam O'Connor

      The assert command in Java looks like this:

      void main() { assert (1 == 2); }

      If you compile and run this program with the -ea (enable assertions) option, it will crash on the assert line and report an assertion violation, because 1 == 2 will evaluate to false.

      $ javac Assert.java $ java Assert # no -ea, nothing happens $ java -ea Assert Exception in thread "main" java.lang.AssertionError at Assert.main(Assert.java:2)

      If you give an assert an expression that evaluates to true, the program will continue executing as normal.

      You can use assertions to check that your preconditions are satisfied in a function — it's better to crash and stop the program if the environment doesn't match your expectation than continue to run and potentially do something wrong.

      Warning. 

      • Liam O'Connor
      Always remember to run your program with the -ea option, or you might be fooled into thinking your preconditions hold when they do not!

      For the first part of this course, we will use assert for testing.

      Warning. A word from E.W. Dijsktra

      [comp1110-1D]
      • Liam O'Connor

      Program testing can be used to show the presence of bugs, but never to show their absence!

      Whenever you run a test, that is one possible execution out of potentially millions, or even infinite. Try to be thorough with your testing, but be aware of its limitations!

      Example. Testing contains with assert

      [comp1110-1E]
      • Liam O'Connor, Fabian Muehlboeck

      Take the contains function again from Example [comp1110-1A] and Example [comp1110-16]. Let us test it using assert:

      void main() { int[] a1 = new int[]{1,2,3,4,5}; assert (contains(a1, 0) == false); assert (contains(a1,5)); assert (contains(a1,3)); int[] a2 = new int[]{-2, 0, 1000000}; assert (contains(a2, -2)); assert (!contains(a2,5)); }

      Try to make each of these tests fail by modifying only the function contains.

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