CMSC 202 — Spring 2013

Constructor Preconditions

Precondition #1

In many cases, your code will not work with certain inputs. A frequent example: How will your method behave if it is passed a null pointer? Have a look at the javadoc of the BoundedArray constructor. It clearly mentions the precondition as 'The input array to the constructor cannot be null.' You will find the following code in the constructor checking this precondition.

	//Precondition #1 The input array to the constructor cannot be null.
	if(array==null){
		throw new NullPointerException("Constructor precondition " +
				"not met: Input array cannot be null");
	}

A unit test case for this precondition is provided in the main method:

	case 1:
	{
		// 1:  Constructor array=null
		System.out.println("Passing null array. Expecting error statement.");
		int[] testArray=null;
		BoundedArray b=new BoundedArray(testArray, 0, 100);
	}
	break;

Run the program, and type in the number 1 (to run test case #1). A null array is passed in the constructor and an error message is expected which you will find in the output console.


Precondition #2

The BoundedArray class in its constructor takes in an array and two integers, minVal and maxVal. This class has the following two class invariants:

1. minVal must be less than or equal to maxVal.
2. All values of the array must be in the range minVal and maxVal, INCLUSIVE.

The following code is present in the existing constructor. This checks 'precondition #2' which ensures that the 'class invariant #1' is not yet violated.

	//Precondition #2 'minVal' must be less than or equal to 'maxVal'
	if(maxVal < minVal){
		throw new RuntimeException("Constructor precondition " +
			"not met: 'minVal' IS NOT less than or equal to 'maxVal'.");
	}

The test case to check this precondition can be found in the main method

	case 2:
	{
		// 2:  Constructor minVal>maxVal
		System.out.println("Passing minVal>maxVal. Expecting error statement.");
		int[] testArray={1,2,3,4,5};
		BoundedArray b=new SearchArray(testArray1, 10, 9);
	}
	break;

Run the code, and type in the number 2 to run testcase number 2. The argument minVal is passed as 10 and maxVal as 9, clearly violating the second precondition. An error message is expected in this test case and you should see an exception thrown in the console. This just tells you that the precondition checking code is in place and working.


Precondition #3
Objective 1 for this lab

Your first objective today is to write code in the constructor which ensures that third precondition is not violated. We need to make sure that every element of the array is greater than or equal to 'minVal' and less that or equal to 'maxVal'
Insert your code in the following section of the constructor:

	//
	// Precondition #3 All values of the array must be in the range
	//                  'minVal' and 'maxVal' INCLUSIVE

	// INSERT CODE HERE

You should then write two test cases in main() to check that. Your test case should go in the following sections of main():

	case 3:
	{
		// 3:  Constructor array elements < minVal
		// INSERT CODE HERE
	}
	break;
	case 4:
	{
		// 4:  Constructor array elements > maxVal
		// INSERT CODE HERE
	}
	break;