Invariants What is an invariant?

Transcription

1 Invariants What is an invariant? Objects are manipulated and change state, but some properties are always true whatever the state A well-formed state is when all invariants are true An example for a doubly linked list

2 Preserving Invariants Document the invariant with tag that names the invariant method

3 Assertions Syntax: assert Assertion; Assertions are commonly applied to preconditions, postconditions, and invariants public Object head() { assert!isempty(); // precondition Object result = (head!= null? head.item : null); assert result == element(0); // postcondition return result;

4 public void insert(object item, int i) { assert item!= null && i >= 0 && i <= size(); // assert the pre-condition assert _wellformed(); // assert the invaraint // the object in pre-state, used in the post-conditions int size_pre = size(); LinkedList this_pre = null; try { this_pre = (LinkedList) clone(); catch (CloneNotSupportedException e) { // begin insertion if (i <= 0) { inserthead(item); else if (i >= count) { insertlast(item); else { // i > 0 && i < count; Node n = head; for (int j = 0; n!= null && j < i - 1; j++) { n = n.next; Node node = new Node(); node.item = item; node.next = n.next; node.prev = n; node.next.prev = node; n.next = node; count++; // end insertion Detailed Example - 1

5 // assert the post-condition int size_post = size(); assert size_post == size_pre + 1; // assert the post-condition Detailed Example 2 // the quantified experssion is translated into a for-loop boolean insertok = true; for (int k = 0; insertok && k < size(); k++) { if (k < i) { insertok = (this_pre.element(k) == element(k)); else if (k == i) { insertok = (item == element(k)); else { insertok = (this_pre.element(k - 1) == element(k)); assert insertok; // assert the invariant assert _wellformed();

6 Design By Contract Contracts are specified by preconditions and postconditions Each method should have a contract

7 Design of a Canonical Class Every class in Java is an Object; this means you may override some higher level methods

8 No Argument Constructor Normal creation of classes The class and its constructors are known The new is used to create an instance Dynamic creation of classes at runtime In some applications, particularly when processing is distributed between machines, the class itself may not be known until runtime The JVM can still create instances of the class provided a no argument constructor is provided This ability to load classes and create instances at runtime is a very powerful feature in Java

9 Object Equality Conditions to be satisfied by an equals( ) method public boolean equals(object other) { if (this == other) return true; if (other instanceof C) { C otherobj = (C) other; -- compare each field and return false if not equal return true; else return false;

10 public boolean equals(object other) { if (other!= null && other instanceof LinkedList) { LinkedList otherlist = (LinkedList) other; if (this.size() == otherlist.size()) { Node thisnode = this.head; Node othernode = otherlist.head; while (thisnode!= null && othernode!= null) { if (!thisnode.item.equals(othernode.item)) return false; thisnode = thisnode.next; othernode = othernode.next; return true; return false; Equals for the Linked List

11 Overriding hashcode If equals is overriden, hashcode should also be overriden A hashcode for two equal objects must return the same value; here is the hashcode for LinkedList

12 Calculating a Hashcode Common ways to combine hash values

13 Cloning Objects 1

14 Cloning Objects - 2 Which structure would result from the previous cloning method?

15 Using Clones With Assertions Create a copy of the original object before insert LinkedList this_pre = null; try { this_pre = (LinkedList) clone(); catch (CloneNotSupportedException e) { At the end, check that the insertion was correct boolean insertok = true; for (int k = 0; insertok && k < size(); k++) { if (k < i) { insertok = (this_pre.element(k) == element(k)); else if (k == i) { insertok = (item == element(k)); else { insertok = (this_pre.element(k - 1) == element(k)); assert insertok;

16 Implementing tostring The tostring method is defined for every object For complex objects, the default is to print the class name The programmer can override the tostring method to make a more detailed printout of information How would you override tostring for a linked list?

17 Implementing Serialization What is serialization? Transform an object to and from a byte stream Important if the object is to be sent over a network to a remote site or to be stored in a file If an object is to be serialized, the class should implement the java.io.seralizable interface to custom define the methods writeobject( ) and readobject( ) Java provides a default implementation for objects This implementation is adequate for our LinkedList example

18 Class Organization What do each of these category names mean?

19 Levels of Testing In this chapter we concentrate on unit testing We show a simple test first Then we show a more complex test We introduce JUnit, a unit testing tool We introduce Ant, a build tool for java that, in addition to compilation, can run all JUnit tests

20 Testing Your Programs Testing can be hierarchical from the unit level up to the program level as components are integrated As a minimum, unit testing should include Using these criteria, how would you design a testing strategy for the linked list class

21 package test; import mylist.*; Test Utilities - 1 public class TestUtil { /** * An auxiliary method that tests whether the contents of a list match the * contents of an array of objects. * It returns true if the list and the array are of the same length and each * object in the list equals to the object in the array at the same position. */ public static boolean match(list list, Object[] array) { boolean result = false; if (list!= null && array!= null) { int n = list.size(); if (n == array.length) { for (int i = 0; i < n; i++) { Object item = list.element(i); if (item!= null) { if (!item.equals(array[i])) { return false; else { if (array[i]!= null) { return false; result = true; else if (list == null && array == null) { result = true; return result;

22 Test Utilities 2 /** * An auxiliary method that converts an array of integer values to an array of integer objects */ public static Object[] tointegerarray(int[] intarray) { if (intarray!= null) { int n = intarray.length; Object[] resultarray = new Object[n]; for (int i = 0; i < n; i++) { resultarray[i] = new Integer(intArray[i]); return resultarray; else { return null;

23 JUnit Testing Tool As implied by the name, JUnit tests are performed at the unit level; what does this mean? Documentation about JUnit can be found at Similar unit testing is provided for other languages, including C++ Ant, a build tool we will discuss shortly, also supports JUnit This means you can specify in Ant that it should run all your JUnit tests

24 package unittest; import junit.framework.*; import mylist.*; import test.testutil; public class LinkedListUnitTest extends TestCase { public LinkedListUnitTest(String name) { super(name); public void testinsert() { LinkedList l = new LinkedList(); l.inserthead(new Integer(1)); l.inserthead(new Integer(2)); l.insertlast(new Integer(3)); l.insertlast(new Integer(4)); l.insert(new Integer(5), 3); l.insert(new Integer(6), 3); l.insert(new Integer(7), 3); asserttrue(testutil.match(l, TestUtil.toIntegerArray(new int[] {2, 1, 3, 7, 6, 5, 4 ))); public void testremove() { LinkedList l = new LinkedList(); for (int i = 1; i <= 7; i++) { l.insertlast(new Integer(i)); l.removehead(); l.removelast(); l.remove(2); asserttrue(testutil.match(l, TestUtil.toIntegerArray(new int[] {2, 3, 5, 6))); JUnit Test - 1

25 JUnit Test - 2 public void testclone() throws CloneNotSupportedException { LinkedList l1 = new LinkedList(); for (int i = 1; i <= 7; i++) { l1.insertlast(new Integer(i)); int[] ia1 = {1, 2, 3, 4, 5, 6, 7; int[] ia2 = {2, 3, 4, 5, 6, 7; LinkedList l2 = (LinkedList) l1.clone(); asserttrue("clone is not identity", l2!= l1); asserttrue("clone equals to original", l1.equals(l2)); asserttrue("match 1", TestUtil.match(l1, TestUtil.toIntegerArray(ia1))); asserttrue("match 2", TestUtil.match(l2, TestUtil.toIntegerArray(ia1))); l2.removehead(); asserttrue("match 3", TestUtil.match(l1, TestUtil.toIntegerArray(ia1))); asserttrue("match 4", TestUtil.match(l2, TestUtil.toIntegerArray(ia2))); asserttrue("not equal",!l1.equals(l2)); public static Test suite() { return new TestSuite(LinkedListUnitTest.class);

26 JUnit Test Runner There is also a GUI interface to JUnit is you prefer such an interface

27 Ant A Build Tool Ant directives are stored in a XML file <project name = project-name default = default-target-name > property definitions target definitions </project> <property name= prop-name value= prop-value /> //names are unique example <property name= junit value= path to junit tests /> <target name = target-name depends = list of dependencies task definitions </target> example <target name = compile depends = init <javac srcdir = ${src destdir = ${classes includes = ${mylist/* source = 1.4 /> </target>

28 Common Tasks and Targets Running: ant target-name Ant is available from jakarta.apache.org Common Tasks Typical Targets