Practice Session 5 Java: Packages Collection Classes Iterators Generics Design by Contract Test Driven Development JUnit

Java Packages What is Package? – Way to group related class(es) and interface(s) into one unit – Package is basically a folder. Benefits? – Code organization All classes/interfaces related to one topic, are put under same package name. – Allows us to resolve conflicts between class names By splitting the same name classes into two different packages. – Controlling class visibility protected classes are visible to other classes inside the package only Not visible to other packages, when imported.

Java Packages Declaration: – Declared at beginning of class file. – Syntax: package PackageName; Example: Folder: Package MyTest – Sub-folder: Package Outs1 File: Class Print1.java File: Class Print2java – Sub-folder: Package Outs2 File: Class Print1.java MyTest Outs1 Print1.java Print2.java Print1.java Outs2

Java Packages How to use them? – Use of absolute path of class: Example: MyTest.Outs1.Print1 p1; – Using import keyword: Import class: import MyTest.Outs1.Print1; Import all classes under package: import MyTest.Outs1.*; Object declaration: Print1 p1;

Java Collection What is it? – A (huge)package of classes: import java.util.Collection; – Contains useful data structures: LinkedList Stack ArrayList Vector TreeSet PriorityQueue And much more… Full API:

Collection Iterator All java.util.Collection data structures return an iterator to their elements. Iterator commands: – boolean hasNext() Returns true if there is a next element – Object next() Returns the next object element/advances the iterator – void remove() Optional operator / not all iterators implement it Removes the element returned by next

Using the Iterator Example: – Print all Collection object elements: Code: static void printAll (Collection coll) { Iterator iter = coll.iterator( ); //returns iterator while (iter.hasNext( )) { //checks if has next System.out.println(iter.next( ) ); //returns object } }

Generics What are they? – Generic data structures are data structures that allows the use of any object type. – Explicitly declare what object type the data structure is going to use upon declaration, to help finding errors at compilation time. How? – java.util.Collection.Stack stk; Examples: – A Stack that holds Integer objects. java.util.Stack intStack; – A LinkedList holding Student objects: java.util.Collection.LinkedList studentList;

Generics Example Stack stackOfCows = new Stack () Stack stackOfFloat = new Stack () Cow cow = new Cow() stackOfCows.push(cow); stackOfFloat.push(cow);

Design by Contract(DBC) What? – An approach for designing software. – Expresses contract between specification and implementation. – An agreement between two parties: client and supplier. Why? – Trustworthy documentation. – Strong support for debugging. – Facilitate code reuse. Condition Types in DbC: – Preconditions Things that must be true before invoking a method No conditions on after invocation. – Postconditions Things that must be true after invoking a method No conditions on before invocation – Invariants things that must be true before and after any method is invoked Must be true immediately after construction.

Design by Contract Example public interface MyCollection { /** * Remove obj from the collection * !( obj == null ) * !contains( obj ) */ public void remove( Object obj ); Precondition disallows null argument. Postcondition varifies that method removes ALL occurrences of obj /** size() >= 0 */ public interface Queue { Assures queue size >= 0 always!

Design by Contract Principles Separate queries from commands Separate basic queries from derived queries. For each derived query, write a postcondition that specifies what result will be returned, in terms of one or more basic queries. For each command, write a postcondition that specifies the value of every basic query. For every query and command, decide on a suitable precondition. Write invariants to define unchanging properties of objects.

PRINCIPLE 1: separate commands and queries public int size(); public Object head(); public boolean isEmpty(); public void add( Object o ); public void remove(); //just a command - no result Commands Queries

PRINCIPLE 2: separate basic queries and derived queries public boolean isEmpty(); -- we can derive isEmpty() from size() public int size(); public Object head(); Basic queries Derived queries The choice of basic queries can change as we develop contracts

PRINCIPLE 3: specify derived queries in terms of basic queries /** * Is the queue empty? * return == ( size() == 0 ) */ public boolean isEmpty(); -- if we know the value of size(), we also know -- the value of isEmpty()

PRINCIPLE 4: specify postcondition for commands in terms of basic queries /** * The first element in the queue * size() >= 1 return == elements().get(0) */ public Object head(); postcondition uses basic query.get() for its implementation

PRINCIPLE 5: add preconditions where appropriate /** * The first element in the queue * size() >= 1 */ public Object head(); It wouldn’t make the software better to pretend that an empty queue has a head element.

PRINCIPLE 6: specify invariant properties /** size() >= 0 */ public interface Queue { Aim for properties that: - help readers build correct conceptual model

Test Driven Development (TDD) What? – Test cases are first implemented and only then the code necessary to pass these tests are implemented. Why? – Code is ensured to be working perfectly without any bugs! Main tool in java: JUnit

Junit - Assert methods Each assert method has parameters like these: message, expected-value, actual-value – assertTrue(String message, Boolean test) – assertFalse(String message, Boolean test) – assertNull(String message, Object object) – assertNotNull(String message, Object object) – assertEquals(String message, Object expected, Object actual) (uses equals method) – assertSame(String message, Object expected, Object actual) (uses == operator) – assertNotSame(String message, Object expected, Object actual) Assert methods dealing with floating point numbers get an additional argument, a tolerance. (for rounding) Each assert method has an equivalent version that does not take a message – however, this use is not recommended because: – messages helps documents the tests – messages provide additional information when reading failure logs

setUp/tearDown setUp()/tearDown() – Functions which will run before and after each test. – Example usage: setUp(): creating new objects tearDown(): clearing data, resetting data. – If self destruction is good enough, there is no need to implement it. setUpBeforeClass()/tearDownBeforeClass() – functions which will run exactly once before and after all the tests. – Can be used to initialize objects which will be used throughout the tests. – Changes done in first test, can be used as input data for the upcoming one.

Call order of the functions

JUnit tests for Counter class public class CounterTest { Counter counter1; public CounterTest() { } // default protected void setUp() { // creates a (simple) test fixture counter1 = new Counter(); public void testIncrement() { assertTrue(counter1.increment() == 1); assertTrue(counter1.increment() == 2); public void testDecrement() { assertTrue(counter1.decrement() == -1); } } Note that each test begins with a brand new counter This means you don’t have to worry about the order in which the tests are run

Test Driven Development Cycle Write the tests and for each Object Under Test (OUT): – Define interface – Define contract for each method – Specify invariant for the object, and for each method, pre and post- conditions. Write the code so it will pass the test Run tests Refactor! – Improve the code, more efficient, less repetition, better looking.. Repeat

Junit - Example Interface: package spl.util; /** * All objects are of type T. Contents are ordered in Last-In-First-Out order. */ public interface Stack { /** * add the object at the top of the stack. (This is a command.) obj * any non null T object to be added to the stack none. this.isEmpty()==false this.pop(); obj */ void push(T obj);

/** * remove the top object from the stack and returns it. the topmost object on the stack Exception in case the stack is empty this.isEmpty()== false; none. */ T pop() throws Exception; /** True if the Stack is empty, or False if the Stack contains at * least one Object}. none. none. */ boolean isEmpty(); }

Create Class StackImpl package spl.util; public class StackImpl implements Stack { public void push(T obj) { } public T pop() throws Exception { return null; } public boolean isEmpty() { return false; }

Creating a Test Case package spl.util; import static org.junit.Assert.*; import org.junit.After; import org.junit.Before; import org.junit.Test; public class StackImplTest { Stack public void setUp() throws Exception { this.stack = new StackImpl (); public void tearDown() throws Exception { } Before -> run before each test After-> run after each test

@Test public void testPush() { fail("Not yet implemented"); public void testPop() { fail("Not yet implemented"); public void testIsEmpty() { assertEquals(true, stack.isEmpty()); } What happens when we run the tests? All 3 tests fail: -testPush and testPop because the methods are not implemented. -testIsEmpty since isEmpty returns false, even though the stack is empty.

Add implementation to isEmpty() to pass the tests. Implement other tests and add the implementation of the methods they test to pass them. Change the interface to include public void remove() and public T top() in addition to pop(), and implement pop() using remove and top, to separate queries from commands. And so on….