Assertion in Java: Legal and illegal assert statements

Valid/Invalid Assertion expression

  1) assert (x > =0);
  2) assert (x > =0): "X is a negative number:"+x;
      exp1                exp2

The second expression can be anything that results in a value. Remember, the second expression is used to generate a String message that displays in the stack trace to give you a little more debugging information. It works much like System.out.println() in that you can pass it a primitive or an object, and it will convert it into a String representation. It must resolve to a value!

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class AssertTest {          // (for example)

  void noReturnValue() { }

  int aReturnValue() { return 1; }

  void assertTest() {

    int x = 1;

    boolean b = true;

    // the following six are legal assert statements

       assert(x == 1);

       assert(b);

       assert true;

       assert(x == 1) : x;

       assert(x == 1) : aReturnValue();

       assert(x == 1) : new ValidAssert();

    // the following six are ILLEGAL assert statements


       assert(x = 1);        // none of these are booleans
       assert(x);
       assert 0;
       assert(x == 1) : ;   // none of these return a value
       assert(x == 1) : noReturnValue();
       assert(x == 1) : ValidAssert va;

   }

}

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