Question

Yet another technique used in combination with console output, flags, and graphical debugging is Asserts. This technique allows you to convert Class Invariants and Preconditions/Postconditions into actual code that will fail fast if we find anything wrong. While asserts are terribly useful in many programming languages, they can incur runtime overhead (depending on implementation) and so are disabled by default in Java. When you run your code on the command line, you invoke the java virtual machine using "java.exe". If we provide the VM with the command line argument “-ea" (as in "java.exe -ea"), then we can make use of assertions in our code. 1. Download the AssertDemo.java driver and run it It should run with no problems, but this is incorrect. We must enable asserts in our IDEs. 2. If you compile using the command line, then use "java.exe-ea Yourclass.java" For BlueJ: a. b. i. http://www.cs.uwlax.edu/riley/CS120F09/Handouts/7_assert_in_BlueJ.pdf C. For Eclipse: i. Click on "Run Configurations" in Eclipse. (a little arrow next to the play button) ii. Choose the current project (AssertDemo.java) from the list to the left iii. Choose the second tab "x-Arguments" iv. In the textarea labeled "VM arguments", add "-ea" "-ea" tells "java.exe" to "enable assertions" 1. Click Ok or Apply and rerun AssertsDemo.java v. For any other editor (NetBeans, JGrasp, etc), simply Google the name of the editor and "how to enable asserts" for instructions. d. Now with asserts enabled ("-ea"), your code should fail in the first two lines of asserts Look at and explain the first two asserts in main; which of the two is breaking the program? 3. 4. Comment out the offending assert line and rerun your code a. In main, start by commenting out everything except what you're working on Find the function warmUpAsserts() and look at the assert examples in the code 5. 6. At the end of this function, find the TODO and build two new assert code a. Next, look at the function called assertWithPrimitives() and uncomment 7.

import java.util.ArrayList;
import java.util.Scanner;

public class AssertDemo {

   /* Work on this in a piecewise fashion by uncommenting and focusing on one section at a time
   * in isolation rather than running everything at once.
   */
   public static void main(String[] args) {
       assert(true);
       assert(false);
      
       warmUpAsserts();
      
       assertWithPrimitives();
      
       assertWithObjects();
      
       homeworkRelatedAsserts();
   }

   /*
   * Just a few warm-up asserts here.
   * Add two new assert() statements after the TODO below
   */
   private static void warmUpAsserts() {  
           assert( 5 > 1 );
          
           int a = 30;
           assert(a != 0);
  
           assert(null == null);   //could this ever be false?
           assert(true == true);   //a bad day indeed if this could be false
           //TODO: craft two more asserts and place them here. If they're false, they'll crash the program.
   }

   /*
   * Using asserts in conjunction with primitive types is familiar to you;
   * just as in a loop or if, you want to form a true/false (boolean) expression
   * by employing the relational operators.
   */
   private static void assertWithPrimitives() {
       //assert below to ensure a Fraction's denominator is never 0
       Scanner keys = new Scanner(System.in);
       System.out.println("Enter an integer numerator:");
       int num = keys.nextInt();
              
       System.out.println("Enter an int denominator, not 0:");
       int denom = keys.nextInt();
              
       assert(denom != 0);
  
       //assert that all ArrayLists start empty
       ArrayList<String> emptyList = new ArrayList<String>();
       assert(emptyList.size() == 0);
       //TODO: build two more asserts that use primitives and relational operators here
   }
  
   /*
   * Asserts work with both primitives and objects. Just as you
   * use "==" with primitives and ".equals()" with objects, so too
   * will you use ".equals()" in asserts that deal with object equality.
   */
   private static void assertWithObjects() {
       AssertDemo ad = new AssertDemo();
       ad.checkAddress(ad);      
       //guess what .equals() does if you don't create one yourself? (hint ==)
       assert( ad.equals(ad) );
       //TODO: make a few objects from any previous lab and test them with assert
       //example: make two Point objects at the origin and assert they are equal
       //assert(p1.equals(p2); //example
   }

   /*
   * This function compares the address at "this" to the address of the object handed
   * into the function.
   */
   public void checkAddress(Object input) {
       System.out.println("Address of this :" + this);
       System.out.println("Address of input:" + input);
       //how many aliases for the one "new" object created in main exist in this scope?
       //1? 2? 3? Which are they?
       assert(this == input); //== does an address check for objects, which is frequently NOT what we want
   }

   /*  
   * Asserts are a useful tool for transforming postconditions and class invariants into code.
   * Lets build a few asserts that work with your current Bill & Money assignment.
   * Change the value of paidDate and cents to trip the asserts and observe the asserted error message.
   */
   private static void homeworkRelatedAsserts() {
       Object paidDate = new Object(); //really, a Date
       assert( paidDate != null);     //perhaps one rule is that paidDate shouldn't be null after calling setPaidDate()
       int cents = 0;
       assert( cents >= 0 && cents <=99); //another class invariant is written as an assert here.
       //TODO: craft 2 more asserts that you could use with any assignment
   }
}

Answer 1

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import java.util.ArrayList;
import java.util.Scanner;

public class AssertDemo {

    public static void main(String[] args) {
        assert(true);
        //assert(false);

        warmUpAsserts();

        assertWithPrimitives();

        assertWithObjects();

        homeworkRelatedAsserts();
    }

    private static void warmUpAsserts() {
        assert( 5 > 1 );

        int a = 30;
        assert(a != 0);

        assert(null == null);  
        assert(true == true);  
        assert(4 == 8/2);
        assert('k' == 'k');
    }

    private static void assertWithPrimitives() {
        Scanner keys = new Scanner(System.in);
        System.out.println("Enter an integer numerator:");
        int num = keys.nextInt();

        System.out.println("Enter an int denominator, not 0:");
        int denom = keys.nextInt();

        assert(denom != 0);

        ArrayList<String> emptyList = new ArrayList<String>();
        assert(emptyList.size() == 0);
        assert(2 > 1);
        int i = 0;
        int j = 0;
        assert(i == j);
    }

    private static void assertWithObjects() {
        AssertDemo ad = new AssertDemo();
        ad.checkAddress(ad);
        assert( ad.equals(ad) );
    }

  
    public void checkAddress(Object input) {
        System.out.println("Address of this :" + this);
        System.out.println("Address of input:" + input);

        assert(this == input);
    }

    private static void homeworkRelatedAsserts() {
        Object paidDate = new Object();
        assert( paidDate != null);    
        int cents = 0;
        assert( cents >= 0 && cents <=99);
        Box a = new Box(1,2,3,4);
        Box b = new Box(1,2,3,4);
        Box c = new Box(a.getWidth(), a.getHeight(),a.getDepth(),a.getGrade());
        Box d = a;
        assert(a.equals(b));
        assert(d.equals(a));
        assert(d == a);
        assert(a.equals(c));
    }
}

-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
public class Box{

    private int width, depth;
    private int height, grade;

    public Box(int width, int height, int depth, int grade)
    {
        this.width = width;
        this.height = height;
        this.depth = depth;
        this.grade = grade;
    }

    public boolean equals(Box b)
    {
        return this.getVolume() == b.getVolume() && this.getGrade() == b.getGrade();
    }

   public Box larger(Box b)
    {
        if(b.getVolume() > this.getVolume())
            return b;
        else
            return this;
    }

    public int getGrade()
    {
        return grade;
    }

    public int getVolume()
    {
        return this.width * this.height * this.depth * this.grade;
    }

    public int getWidth()
    {
        return this.width;
    }

    public int getHeight()
    {
        return this.height;
    }

    public int getDepth()
    {
        return this.depth;
    }
}

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