the non-trivial java example ‘mouse in a maze’ · humboldt university berlin, university of...

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DAAD project “Joint Course on OOP using Java”

Humboldt University Berlin, University of Novi Sad, ‘Polytehnica’ University of Timisoara,University of Plovdiv, University of Belgrade

Author: Klaus Bothe Version: Sep. 29, 2004

The nonThe non--trivial Java example trivial Java example ‘‘Mouse in a MazeMouse in a Maze’’

A simple software development process from specification, to design,

implementation, and test

DAAD project „Joint Course on OOP using Java“ © 2

SourceSource

Based on an idea from S.N. Kamin, M.D. Mickunas, E.M. Reingold: „An introduction to computer science – Using Java“, McGraw-Hill, 1998

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Points of this Java sample Points of this Java sample programprogram

Introduce a non- trivial Java program

Illustrate a methodology howto present larger Java programs in lectures

Demonstrate theimportance of softwareengineeringprinciples forordinary Java programdevelopment

Interactive style of the lectureimproves learningeffects

Using Java API

For the lecturer

only


CourseCourse materialsmaterials

Lecture slides:• about 90 slides• takes 3 x 2 lecture hours at HU (interactive style)

Java sources

Sum: 378

Assignments:• modify, extend the program• implement as an applet (grafical interface)

For the lecturer

only

135SoftFrame

61Mouse

40MazeTest

23Easel

64MouseMaze

55Maze

LOCClasses

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DidacticalDidactical mistakemistake in in presentingpresenting complexcomplexJava Java programsprograms in in lectureslectures

The lecturer tries to explain each technical detail of program code

non-understandable in a lectureboring

The lecturer starts with a Java program too early

details of requirements specification still opendesign / class structure not well discussed

Finding a proper class structure may be more challengingthen implementing the Java program code.

For the lecturer

only


DidacticalDidactical principlesprinciples in in presentingpresentingcomplexcomplex Java Java programsprograms in in lectureslectures

Follow the software engineering process:• requirements analysis, design, implementation & test

Involve students into this process (interactivity)

Only outline the main ideas of the implementation

Assignments support detailed understanding thecode

For the lecturer

only

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Requirements analysis: Requirements analysis: ‚‚Mouse in a MazeMouse in a Maze‘‘

Task:Develop a program, that simulates the movement of a mouse in a maze (labyrinth) from an entry to an exit.

Next steps?What to do?


Facts concerning software Facts concerning software development phasesdevelopment phases

Most errors of software systems result from misunderstanding the problem to be solved.

Software development consists only of a small part of programming:

• Requirements analysis: 20 %• Design: 15 %• Implementation: 20 % • Test: 45 %

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Learning goalsLearning goals

This example should illustrate the importance of a complete and correct requirements specification for the project success. Before implementing, the next step is the design of the application: Which components belong to the program architecture?Starting with the implementation too early may lead to project’s failure.


DevelopmentDevelopment processprocess of of ‚‚Mouse in a Mouse in a MazeMaze‘‘

Requirements analysis

Design

Implementation and test

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Requirements analysis: Requirements analysis: ‚‚Mouse in a MazeMouse in a Maze‘‘

Task:Develop a program, that simulates the movement of a mouse in a maze (labyrinth) from an entry to an exit.

Open questions ?


Open Open questionsquestions

How does a maze look like?What is a mouse able to do (e.g. whichkinds of movements)?What is the initial state of the mouse?What, if there is no way from entry to exit?In which way should the solution bepresented?

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HowHow doesdoes a a mazemaze look look likelike??


ExamplesExamples of of ourour kindkind of of mazesmazes

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RequirementsRequirements specificationspecification (1)(1)

Develop a program, that simulates the movement of a mouse in a maze (labyrinth) from an entry to an exit.

1. The Maze

A maze is a collection of quadratic rooms arranged in a rectangle. Two adjacent rooms are separated by a wall or opening (gap). The maze is surrounded by a wall which is open at two locations (entry and exit). The size of the maze (length and height) is variable.Examples:


HowHow doesdoes a a mousemouse look look likelike??

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OurOur mousemouse


Mouse Mouse confrontedconfronted withwith different different mazesmazes

entry

exit

entry

exit

This will become hard

No problem ☺

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WhichWhich kindskinds of of movementsmovements isis ourourmousemouse ableable to do?to do?

step forwardturn leftturn right


PossiblePossible movementsmovements of of thethe mousemouse

entry

exit

May turn left or right by 90 degrees

May move one stepforward into the

adjacent room in theline of vision

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entry

exit

TwoTwo implementedimplemented solutionssolutions

entry

exit

Proper solution for the wrong problem.In each case: the shortest path has been found.

Requirements specification is incomplete

What may be suspicious with these

solutions?


RequirementsRequirements specificationspecification isisincompleteincomplete

entry

exit

In each case: the shortest path has been found

Only possible if the mouse knows the whole maze (view from above) This is not the characteristic of maze problems.

entry

exit

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PerceptionsPerceptions of of thethe mousemouse

entry

exit

Line of vision onlystraight forward

May see only inside of one room (not into the

adjacent room)

May decide: Do I face a wall or an opening (gap)? May decide: Am I inside

or outside the maze?


FieldField of of viewview of a of a mousemouse

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TheThe problemproblem maymay bebe hugehuge ☺☺


RequirementsRequirements SpecificationSpecification (2)(2)

2. The Mouse:The mouse has no general overview of the maze. a) The mouse may move in the following way:

turn left, turn right (by 90 degrees), move forwardinto the adjacent room.

b) The mouse is located either in a fixed room inside themaze or just next to the entry or exit. In addition, ithas a fixed line of vision.

c) The mouse can see only in the line of vision. It candecide wether it faces a wall or not.

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RequirementsRequirements specificationspecification: : openopen problemsproblems??

Description of the initial state of the mouseDescription of the final state of the mouseDescription of the taskSpecial casesUser interface


3. Desription of the initial state of the mouse:The mouse is initially placed in front of the entrance to the maze (the next step forward takes her to the maze).

Example:


entry

exit

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4. Desription of the final state of the mouse and of the task:We a looking for a sequence of movements (turn left, turn right, step forward) leading the mouse from theentry to the exit.

Example:


entry

exit


5. Special case:If there is no exit or if there is no way from the entry to the exit, then the mouse should leave the mazethrough the entry.

Example:


entry

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6. User interfacea) Textual output of the solution:

Example: step forward, turn right, …

b) Graphical user interface:The sequence of movements will be displayed withthe help of a graphical applet, i.e. the steps of themouse are visualized. The user can interactivelytrigger the next step.


SolvabilitySolvability of of thethe problemproblem

Is there an algorithm at all that solves the problem for arbitrary mazes taking into account the characteristic properties of the mouse?

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AlgorithmAlgorithm: : basicbasic principleprinciple

Principle:Have the mouse walk with its right side on the wall of the maze.

Still not an algorithm

An algorithm has to determine the sequence of mouse movements.


AlgorithmAlgorithm: : as a as a pseudopseudo codecode programprogram

if mouse is still inside the maze:turn right;if (facing a wall?):then

turn left and test if (facing a wall?);if (facing a wall?) then turn left and test ...

else

step forward

only one step:

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Strong pseudo code: Strong pseudo code: one stepone step

IF (NOT outside the maze?)BEGIN /*do next step*/

turn right;WHILE (facing a wall?) DO

turn left;ENDWHILEstep forward;

END

Impossible:


StrongStrong pseudopseudo codecode: : wholewhole algorithmalgorithm

step forward; /* enter the maze */WHILE(NOT outside the maze?)

BEGIN /*do next step*/turn right;WHILE (facing a wall?) DO

turn left;ENDWHILEstep forward;

ENDENDWHILE

Think about: The algorithm fulfills the principle.

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Mouse Mouse movementsmovements accordingaccording to to thethealgorithmalgorithm: : firstfirst exampleexample

entry

exit


Mouse Mouse movementsmovements accordingaccording to to thethealgorithmalgorithm in detail: in detail: firstfirst exampleexample

entry

exit

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Mouse Mouse movementsmovements accordingaccording to to thethealgorithmalgorithm: second : second exampleexample

entry

exit


Mouse Mouse movementsmovements accordingaccording to to thethealgorithmalgorithm in detail: second in detail: second exampleexample

entry

exit

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Mouse Mouse movementsmovements: : graphicalgraphical and and textualtextual outputoutput

entry

exit

step forwardturn rightstep forwardturn rightturn leftturn leftstep forwardturn rightturn leftturn leftstep forwardturn rightstep forward


Textual output of the solutionTextual output of the solution

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xx xx xx xx xx xx xx xx xx xx xx xx x x xxxxxxxxxxxxxx

x x xx x xx x xx x xx x xx x xx x xx x xx x xx x xx x xx x x

x x x x x x x x xx x x xx x x xx x x xx x x xx x x xx x x xx x x xx x x xx x x xx x x xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

step forward

turn to the right

step forward

turn to the right

turn to the left

turn to the left

turn to the left

step forward

…

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Mouse in a maze Mouse in a maze –– only a nice game?only a nice game?

Sample of finding algorithms for robots dealing with different tasks:

• Playing soccer• Robot moving on the moon• Rescue robots




Design


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TaskTask

Requirements specification

Next step ?

Start with programming?

If so, • with which part/component?• ???


Design: develop the software architectureDesign: develop the software architecture

SW architecture description languages: UML

Our task: define an object-oriented architecture

Main problem: How to find classes?

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Software development: Software development: Phases and resultsPhases and results

Analysis & DefinitionRequirements specification

DesignSoftware architecture

ImplementationProgram

TestTest cases


As a matter of fact As a matter of fact ……

Finding a proper class structure maybe more challenging thenimplementing the Java program code.

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Software architectureSoftware architecture

Structure of the software:Which components exist?Which relations are between them?

Stack:

Push(e)Pop()Top()


Architecture specification languagesArchitecture specification languages

Graphical languages for the specification of software architectures

Industry standard: UMLUnified Modelling Language

Now: examples of one graphical element of UML:class diagram

What programming languages mean to the implementation phase, are architecture specification languages to the design phase.

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UML classes: the structureUML classes: the structure

Time

- hour : int- minute : int

Time (hour: int, minute : int)addMinutes (Min : int)printTime ( )

Name

Attributes: state

Operations: behaviour

hidden


UML classes as interfacesUML classes as interfaces

Time



UML classes provide an interface: Which information is visible from outside ?

Why are hidden data – which are not visible from outside – part of UML class?

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The usable interface of a class: The usable interface of a class: only visible featuresonly visible features

Time



Hidden data support the understanding of the interface. They are part of the class model.

The only way to interact with objects

of this class is to call its methods


Relations between classes: Relations between classes: Associations, inheritance, Associations, inheritance, ……

Class 1 Class 2

Class 1 Class 2

Association

Directed association: objects of class one know objects of class

two – and not vice versa

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How to find classes?How to find classes?

By separation of the problem to sub-problems• Sub-problems may become classes

From the objects of the problem area• Investigate the requirements specification to

find objects of the problem area

Principles ?


Case study:Case study:‘‘Mouse in a MazeMouse in a Maze’’

Which objects of the problem area should be implemented as component / class of the system?

MouseMaze

Algorithm for mouse movement

User interface /output

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Which relations exist between the Which relations exist between the components?components?

MouseMaze



Which relations ?

Who needs whom ?


Which relation: Who needs whom?Which relation: Who needs whom?

Mouse Maze


?

?

?

e.g. test if in front of a wall

no

Alg. has not to know the maze


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Class diagram for a mouseClass diagram for a mouse

Mouse

?

?

Which data and which operations characterize the mouse?


Interface of the mouse: Interface of the mouse: Which data, which operations? Which data, which operations?

(first approach)(first approach)

Mouse

- Location : Point- Direction : int

stepForward ( )turnLeft ( )turnRight ( )facingWall ( ) : booleanoutsideMaze ( ) : boolean

State: place / direction in a

maze

Behaviour: abilities of the

mouse (movements & perceptions)

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Detailed interface of the mouseDetailed interface of the mouse

Mouse

- Location : Point- Direction : int+ started : boolean- theMaze : Maze

Mouse (Maze m)getLocation ( ) : PointstepForward ( )turnLeft ( )turnRight ( )facingWall ( ) : booleanoutsideMaze ( ) : boolean

Create a mouse in relation to a

particular maze

Current position of the mouse

Visible attribute


Interface of the mazeInterface of the maze

Maze

?

?

Class diagram of the maze: data and operations?

Interface of the maze: Which information is necessary for a user of this class (user = objects of class “Mouse”)?

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Interface of the mazeInterface of the maze(first approach)(first approach)

Maze

outside (pos : Point) : booleancheckWall (direction : int, pos : Point) : boolean

getStartLocation ( ) : Point

Where to place the mouse initially? (position & direction)

Is there a wall at position pos in this direction?

Is the current position outside of

the maze?


Interface of the maze: attributes ?Interface of the maze: attributes ?

Maze

outside (pos : Point) : booleancheckWall (direction : int, pos : Point) : boolean

getStartLocation ( ) : ?

The representation of the maze is too complex to be included as attributes (array)

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Detailed interface of the mazeDetailed interface of the maze

Maze

- height : int- width : int

getStartLocation ( ) : PointgetStartDirection ( ) : intgetSize ( ) : PointcheckWall (direction : int, pos : Point) : booleancheckWall (direction : int, col : int, row : int) : booleanoutside (pos : Point) : boolean

To find class diagrams is an iterative process: start with a simple solution and extend and modify if necessary.

Detailed maze data are too complex to be included in a class diagram.

The height and width of the maze are useful to draw the maze

Height and width as a point.


Software Architecture: Software Architecture: the whole view (textual output)the whole view (textual output)

Mouse



Maze




main ( )

Textual output of the maze

printMaze ( )

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Problems of the development of a Problems of the development of a software architecturesoftware architecture

Software architecture:• Not unique (many good and many bad solutions)

Design of a software architecture does not succeed for the first timeLong process:• Software architecture develops stepwise

Principle: • Start with one preliminary architecture • The usability of the methods finally turns out only

during the implementation.




Design


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Java sourcesJava sources

135SoftFrame

61Mouse

40MazeTest

23Easel

64MouseMaze

55Maze

LOCClasses

Sum: 378


Information to Information to thethe audienceaudience

This presentation gives only an outline of the principle points of the implementation.Several crucial technical details will beexplained.However, not each implementation detail will be presented. This would be boring in a lecture.During the self-study and withassignments, everybody should explainoneself open questions.

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Planning of implementation steps Planning of implementation steps

UML class diagram:• Dependencies order of implementation

Mouse Maze


User interface:Textual output of the

maze

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3a3b

In which order should the classes be implemented?


Software Architecture: Software Architecture: the whole viewthe whole view

Mouse



Maze




main ( )

Textual output of the maze

printMaze ( )

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3a3b

In the lecture: 1, 2, 3b (main ideas) Self-study: 3a; 1, 2, 3b (details)

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Implementation of the mazeImplementation of the maze

Maze



Starting from the class diagram


Representation of the maze as Representation of the maze as a data structurea data structure

How to represent the mazeby Java data structures?

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(5,4)(4,4)(3,4)(2,4)(1,4)(0,4)

(5,3)(4,3)(3,3)(2,3)(1,3)(0,3)

(5,2)(4,2)(3,2)(2,2)(1,2)(0,2)

(5,1)(4,1)(3,1)(2,1)(1,1)(0,1)

(5,0)(4,0)(3,0)(2,0)(1,0)(0,0)

Position in a maze Position in a maze

Position = coordinate (x,y) = (number of column, number of line)


Movement path of the mouseMovement path of the mouse

Path: (0,2), (1,2), (1,3), (1,2), (1,1), …, (4,0)

(5,4)(4,4)(3,4)(2,4)(1,4)(0,4)

(5,3)(4,3)(3,3)(2,3)(1,3)(0,3)

(5,2)(4,2)(3,2)(2,2)(1,2)(0,2)

(5,1)(4,1)(3,1)(2,1)(1,1)(0,1)

(5,0)(4,0)(3,0)(2,0)(1,0)(0,0)

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Movement path of the mouse: Movement path of the mouse: Long formLong form

Detailed description:(0,2), step forward to (1,2), turn to right, step forward to (1,3), turn

to right, turn to left, turn to left, turn to left, step forward …

(5,4)(4,4)(3,4)(2,4)(1,4)(0,4)

(5,3)(4,3)(3,3)(2,3)(1,3)(0,3)

(5,2)(4,2)(3,2)(2,2)(1,2)(0,2)

(5,1)(4,1)(3,1)(2,1)(1,1)(0,1)

(5,0)(4,0)(3,0)(2,0)(1,0)(0,0)


Representation of the maze: Representation of the maze: store the wallsstore the walls

For each relevant position:

• Is there a wall to the south boolean [][] sWall

• Is there a wall to the eastboolean [][] eWall

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Representation of the maze: Representation of the maze: store the wallsstore the walls

(5,4)(4,4)(3,4)(2,4)(1,4)(0,4)

(5,3)(4,3)(3,3)(2,3)(1,3)(0,3)

(5,2)(4,2)(3,2)(2,2)(1,2)(0,2)

(5,1)(4,1)(3,1)(2,1)(1,1)(0,1)

(5,0)(4,0)(3,0)(2,0)(1,0)(0,0)

boolean [][] sWall

boolean [][] eWall


boolean [][] sWall ={{true, true, true, false},{false, false, false, true},{false, false, false, false},{true, true, true, true}}

South walls of the maze: exampleSouth walls of the maze: example

(5,4)(4,4)(3,4)(2,4)(1,4)(0,4)

(5,3)(4,3)(3,3)(2,3)(1,3)(0,3)

(5,2)(4,2)(3,2)(2,2)(1,2)(0,2)

(5,1)(4,1)(3,1)(2,1)(1,1)(0,1)

(5,0)(4,0)(3,0)(2,0)(1,0)(0,0)

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South walls of the maze: the arraySouth walls of the maze: the array

boolean [][] sWall ={{true, true, true, false},{false, false, false, true},{false, false, false, false},{true, true, true, true}}

truetruetruetrue3

falsefalsefalsefalse2

truefalsefalsefalse1

falsetruetruetrue0

3210


Method Method ‘‘outsideoutside’’

API:Point pos : (x , y)

public boolean outside (Point pos){return ((pos.x < 1) //left ...|| (pos.x > width) //right ...|| (pos.y < 1) //over ...|| (pos.y > height) //under ...); // the maze

}

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Method Method ‘‘checkWallcheckWall’’: technical detail: technical detail

(5,4)(4,4)(3,4)(2,4)(1,4)(0,4)

(5,3)(4,3)(3,3)(2,3)(1,3)(0,3)

(5,2)(4,2)(3,2)(2,2)(1,2)(0,2)

(5,1)(4,1)(3,1)(2,1)(1,1)(0,1)

(5,0)(4,0)(3,0)(2,0)(1,0)(0,0)

Boolean checkWall(int dir, int col, int row)

{switch (dir){

case NORTH: return sWall[row-1][col-1];

case SOUTH: return sWall[row][col-1];

...


Components: Components: separate implementation & separate testseparate implementation & separate test

Component test:• Test of the component independent of the rest

of the system.

Integration test:• Later on, the collaboration with other

components will be tested.

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ComponentComponent test of test of thethe mazemaze

Componentto be tested

Testframe forclass Maze

class Maze {…}

class MazeTest {…}


Test frame: Test frame: basic principles for test outputbasic principles for test output

% java MazeTestStart location is (0,2): [x=0,y=2]Start direction is EAST = 1: 1Outside true : trueNot outside -> false : falseNot outside -> false : falseWall -> true : trueNo wall -> true : trueWall -> true : true

Expected value

Actual value

Semantic interpretation

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Implementation of the mouseImplementation of the mouseStarting from the class diagram

Mouse




Communication between the objects: Communication between the objects: send a messagesend a message

public boolean facingWall() {return

theMaze.checkWall (direction, location);}

Operation of the mouse

Operation of the maze

The mouse sends a message to the maze: ‘If I have a certain location and direction in the maze, is then a wall in front of me?’

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Methods: Methods: ‘‘turnLeftturnLeft’’ and and ‘‘turnRightturnRight’’

public void turnLeft() {printoutMove(“turn to the left”);direction = (direction + 3) % 4;

}

public void turnRight() {printoutMove(“turn to the right”);direction = (direction + 1) % 4;

}

Think about these formulas.


Method: Method: ‘‘stepForwardstepForward’’

public void stepForward() {switch (direction) {

case NORTH: location.y--; break;case EAST: location.x++; break; case SOUTH: location.y++; break; case WEST: location.x--; break;

}printoutMove(“step forward”);

}

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Search algorithm:Search algorithm:class class ‘‘MouseMazeMouseMaze’’ (1)(1)

public static void main ( ... ) {Maze theMaze = new Maze(); Mouse littleMouse = new Mouse(theMaze);printMaze(theMaze);

//move the mouse step by stepdo{

makeStep(littleMouse);}while (!littleMouse.outsideMaze());

}

Moves the mouse one

step forward, if necessary with turns


Search algorithm:Search algorithm:class class ‘‘MouseMazeMouseMaze’’ (2)(2)

private static void makestep(Mouse m){if (m.started){

if (!m.outsideMaze()){m.turnRight();while (m.facingWall()){

m.turnLeft();}m.stepForward();

}} else {

m.stepForward();m.started = true;

}}

Moves the mouse one

step forward, if necessary with turns

Visible attribute

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Critics of the implementationCritics of the implementation

Constant NORTH = 0, … repeatedly defined in three classessource of errorbetter solution: define them in an interface only once

Variable ‘started’ visible outsideinstead of this: - private + additional access operation- modify the algorithm

Data representation of the maze is error-prone• true/false sequencies correct ?• Variables height, width, size may be in contradiction to eWall, sWall

better let them compute

Strategy of the mouse may be a part of the mouse:Other mice may have other strategies: move at the left-hand wall(mice = plural of mouse)

the non-trivial java example ‘mouse in a maze’ · humboldt university berlin, university of...

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