Week 5

Practice Session 5
• Java:
Collection Classes
• 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.
• Declaration:
Java Packages
– 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
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:
And much more…
Full API: http://docs.oracle.com/javase/6/docs/api/java/util/Collection.html
Collection Iterator
• All java.util.Collection data structures return an iterator to their
• 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<Integer> coll) {
Iterator<Integer> iter = coll.iterator( ); //returns iterator
while (iter.hasNext( )) { //checks if has next
System.out.println(iter.next( ) ); //returns object
• What are they?
– Generic data structures are data structures that allows the use of any object
– 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<Object-Type> stk;
• Examples:
– A Stack that holds Integer objects.
• java.util.Stack<Integer> intStack;
– A LinkedList holding Student objects:
• java.util.Collection.LinkedList<Student> studentList;
Generics Example
Stack<Cow> stackOfCows = new Stack<Cow>()
Stack<Float> stackOfFloat = new Stack<Float>()
Cow cow = new 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 {
Precondition disallows null argument.
* Remove obj from the collection
* @require !( obj == null )
* @ensure !contains( obj )
public void remove( Object obj );
Postcondition varifies that method
removes ALL occurrences of obj
* @invariant 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
public void add( Object o );
public void remove(); //just a command - no result
public int size();
public Object head();
public boolean isEmpty();
PRINCIPLE 2: separate basic
queries and derived queries
Basic queries
public int size();
public Object head();
The choice of basic
queries can change
as we develop
Derived queries
public boolean isEmpty();
-- we can derive isEmpty() from size()
PRINCIPLE 3: specify derived queries in
terms of basic queries
* Is the queue empty?
* @ensure 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
* @require size() >= 1
* @ensure return == elements().get(0)
public Object head();
postcondition uses
basic query .get() for
its implementation
PRINCIPLE 5: add preconditions where
* The first element in the queue
* @require 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
* @invariant 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()
– 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 constructor
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 postconditions.
• 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<T> {
* add the object at the top of the stack. (This is a command.)
* @param obj
any non null T object to be added to the stack
* @pre: none.
* @post: this.isEmpty()==false
* @post: this.pop(); [email protected](this)
* @post: this.pop()== @param obj
void push(T obj);
* remove the top object from the stack and returns it.
* @return the topmost object on the stack
* @throws Exception in case the stack is empty
* @pre: this.isEmpty()== false;
* @post: none.
T pop() throws Exception;
* @return True if the Stack is empty, or False if the Stack contains at
least one [email protected] Object}.
* @pre: none.
* @post: none.
boolean isEmpty();
Create Class StackImpl
package spl.util;
public class StackImpl<T> implements Stack<T> {
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<Integer> stack;
Before -> run before each test
After-> run after each test
public void setUp() throws Exception {
this.stack = new StackImpl<Integer>();
public void tearDown() throws Exception {
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
• 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….

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