Perhaps well interpret consecutive pairs of characters as subranges, so that the string rep acgg is interpreted as two range pairs, [a-c] and [g-g], and therefore represents the set {a,b,c,g}. HoidDap347.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.in As an Amazon Associate we earn from qualifying purchases. HT-T534R0 B#= mjbgkh L endstream endobj 1416 0 obj 47 endobj 1417 0 obj << /Length 1416 0 R /Filter /FlateDecode >> stream #raisemyhouse #houser, Awesome project under way on James St * Find tweets written by a particular user. #rais, Before & After Shot We can achieve 100% abstraction using interfaces. 0000027572 00000 n

HT-T534R0 B#= mbngikh endstream endobj 1422 0 obj 45 endobj 1423 0 obj << /Length 1422 0 R /Filter /FlateDecode >> stream Notice that an immutable rep is particularly easy to argue for safety from rep exposure.

0000009213 00000 n Except where otherwise noted, content on this wiki is licensed under the following license: Clean Code: A Handbook of Agile Software Craftsmanship.

// author and text are Strings, so are guaranteed immutable; // timestamp is a mutable Date, so Tweet() constructor and getTimestamp(). This work is licensed under, /** You may object that this seems wasteful. // all Strings in followersOf are Twitter usernames, // (i.e., nonempty strings of letters, digits, underscores), // no user follows themselves, i.e. This is a comment that examines each part of the rep, looks at the code that handles that part of the rep (particularly with respect to parameters and return values from clients, because that is where rep exposure occurs), and presents a reason why the code doesnt expose the rep. Heres an example of Tweet with its rep invariant, abstraction function, and safety from rep exposure fully documented: Notice that we dont have any explicit rep invariant conditions on timestamp (aside from the conventional assumption that timestamp!=null, which we have for all object references). 0000007560 00000 n HT-T534R0 B#= mbdkh X endstream endobj 1426 0 obj 47 endobj 1427 0 obj << /Length 1426 0 R /Filter /FlateDecode >> stream In 6.005, the preconditions and postconditions of our methods implicitly require that objects and arrays be non-null. Immutability is one crucial invariant that weve already encountered: once created, an immutable object should always represent the same value, for its entire lifetime. Ready for change. 0000007710 00000 n 0000008761 00000 n 0000007410 00000 n

During this step, you can begin collecting accurate quotes for constructions. The reader of the code has to find out that the first four lines of the loop body belong together. 0000024356 00000 n %PDF-1.4 % , v]eYLQ5W@ Tn endstream endobj 1490 0 obj 403 endobj 1387 0 obj << /Type /Page /Parent 1379 0 R /Resources << /Font << /F0 1392 0 R >> /XObject 1388 0 R /ProcSet 1488 0 R >> /Contents 1389 0 R /MediaBox [ 0 0 613 793 ] /CropBox [ 0 0 613 793 ] /Rotate 0 /Thumb 1337 0 R >> endobj 1388 0 obj << /im1 1405 0 R /im2 1405 0 R /im3 1405 0 R /im4 1405 0 R /im5 1405 0 R /im6 1405 0 R /im7 1405 0 R /im8 1405 0 R /im9 1465 0 R /im10 1405 0 R /im11 1467 0 R /im12 1469 0 R /im13 1471 0 R /im14 1405 0 R /im15 1405 0 R /im16 1405 0 R /im17 1473 0 R /im18 1475 0 R /im19 1477 0 R /im20 1479 0 R /im21 1481 0 R /im22 1405 0 R /im23 1405 0 R /im24 1405 0 R /im25 1405 0 R /im26 1405 0 R /im27 1405 0 R /im28 1405 0 R /im29 1405 0 R /im30 1483 0 R /im31 1485 0 R /im32 1487 0 R /im33 1406 0 R >> endobj 1389 0 obj [ 1393 0 R 1395 0 R 1397 0 R 1399 0 R 1401 0 R 1403 0 R 1407 0 R 1411 0 R 1413 0 R 1415 0 R 1417 0 R 1419 0 R 1421 0 R 1423 0 R 1425 0 R 1427 0 R 1429 0 R 1431 0 R 1433 0 R 1435 0 R 1437 0 R 1439 0 R 1441 0 R 1443 0 R 1445 0 R 1447 0 R 1449 0 R 1451 0 R 1453 0 R 1455 0 R 1457 0 R 1459 0 R 1461 0 R 1463 0 R ] endobj 1390 0 obj 43 endobj 1391 0 obj << /Type /FontDescriptor /FontName /Arial /Flags 32 /FontBBox [ -250 -204 1067 907 ] /MissingWidth 648 /StemV 71 /StemH 71 /ItalicAngle 0 /CapHeight 907 /XHeight 634 /Ascent 907 /Descent -204 /Leading 148 /MaxWidth 889 /AvgWidth 389 >> endobj 1392 0 obj << /Type /Font /Subtype /TrueType /Name /F0 /BaseFont /Arial /FirstChar 31 /LastChar 255 /Widths [ 750 278 278 355 556 556 889 667 191 333 333 389 584 278 333 278 278 556 556 556 556 556 556 556 556 556 556 278 278 584 584 584 556 1015 667 667 722 722 667 611 778 722 278 500 667 556 833 722 778 667 778 722 667 611 722 667 944 667 667 611 278 278 278 469 556 333 556 556 500 556 556 278 556 556 222 222 500 222 833 556 556 556 556 333 500 278 556 500 722 500 500 500 334 260 334 584 750 556 750 222 556 333 1000 556 556 333 1000 667 333 1000 750 750 750 750 222 222 333 333 350 556 1000 333 1000 500 333 944 750 750 667 278 333 556 556 556 556 260 556 333 737 370 556 584 333 737 552 400 549 333 333 333 576 537 278 333 333 365 556 834 834 834 611 667 667 667 667 667 667 1000 722 667 667 667 667 278 278 278 278 722 722 778 778 778 778 778 584 778 722 722 722 722 667 667 611 556 556 556 556 556 556 889 500 556 556 556 556 278 278 278 278 556 556 556 556 556 556 556 549 611 556 556 556 556 500 556 500 ] /Encoding /WinAnsiEncoding /FontDescriptor 1391 0 R >> endobj 1393 0 obj << /Length 1390 0 R /Filter /FlateDecode >> stream The design company will help you with this if need be and will provide the relevant town planning advice if its required for your project. Invariants must be established by creators and producers, and preserved by observers and mutators. Its less obvious why the choice of both spaces doesnt determine AF and RI. Then we could define AF, as above, to interpret the arrays elements as the elements of the set. HT-T534R0 B#= mhagkd I endstream endobj 1446 0 obj 47 endobj 1447 0 obj << /Length 1446 0 R /Filter /FlateDecode >> stream HT-T534R0 B#= mfngk m endstream endobj 1402 0 obj 46 endobj 1403 0 obj << /Length 1402 0 R /Filter /FlateDecode >> stream * Make a new RatNum == (n / d). 0000002476 00000 n 0000010864 00000 n Rather than deciding, as we did above, that the strings have no duplicates, we could instead allow duplicates, but at the same time require that the characters be sorted, appearing in nondecreasing order. Recall from the specs reading that null values are troublesome and unsafe, so much so that we try to remove them from our programming entirely. 0000002579 00000 n A further indicator for a missing method is the combination of a blank line, a comment and a block of code. HT-T534R0 B#= ml`gkd D endstream endobj 1436 0 obj 45 endobj 1437 0 obj << /Length 1436 0 R /Filter /FlateDecode >> stream Rep invariants and abstraction functions explicate the meaning of a data types representation, and how it relates to its abstraction. HT-T534R0 B#= mbj b$*g( endstream endobj 1424 0 obj 44 endobj 1425 0 obj << /Length 1424 0 R /Filter /FlateDecode >> stream 0000006041 00000 n * This immutable data type represents a tweet from Twitter. 0000016775 00000 n 0000031381 00000 n Why cant we just solve this problem by a carefully written specification, like this? You will receive a link and will create a new password via email. ]-\0bd>B>Di}\=c|ZG5B] 0000027190 00000 n HT-T534R0 B#=mlghkd w endstream endobj 1442 0 obj 46 endobj 1443 0 obj << /Length 1442 0 R /Filter /FlateDecode >> stream H+T534R0 B#= mnfgnk endstream endobj 1394 0 obj 46 endobj 1395 0 obj << /Length 1394 0 R /Filter /FlateDecode >> stream For the latter there is no syntactical grouping. 0000010113 00000 n Sometimes this is not achievable without other drawbacks but certainly large loop bodies can be considered a smell. Perfectly reasonable client code created a subtle bug. If an invariant of an abstract data type is. 0000004510 00000 n AR$\H xs}BomJ9jFp`F@C&4E43iR#1aN?W,0nA, 0000008140 00000 n Bulimba project underway So when that date object is mutated by d.setHours(), this affects the date in t as well, as shown in the snapshot diagram. zBkw3s nf`T4BA]s829QQR2 V?^8[3 vvmaVm*xA3/KK HT-T534R0 B#= mnbgdk M endstream endobj 1396 0 obj 46 endobj 1397 0 obj << /Length 1396 0 R /Filter /FlateDecode >> stream HT-T534R0 B#= mjngak ] endstream endobj 1414 0 obj 46 endobj 1415 0 obj << /Length 1414 0 R /Filter /FlateDecode >> stream 2. // All fields are private, and all types in the rep are immutable. Comment( in reference to Blue J (JAVA)). For example, instead of a spec like this, with an elaborate precondition: We can instead use an ADT that captures the desired property: This is easier to understand, because the name of the ADT conveys all the programmer needs to know. 0000010991 00000 n preserved by mutators, and observers; and. The abstraction function maps a concrete representation to the abstract value it represents. 0000010842 00000 n Consider this (again perfectly reasonable) client code: This code intends to advance a single Date object through the 24 hours of a day, creating a tweet for every hour. HT-T534R0 B#= mgakl endstream endobj 1460 0 obj 43 endobj 1461 0 obj << /Length 1460 0 R /Filter /FlateDecode >> stream 0000019714 00000 n But we still need to include timestamp in the rep exposure safety argument, because the immutability property of the whole type depends on all the fields remaining unchanged. By default, in 6.005, the rep invariant implicitly includes x != null for every reference x in the rep that has object type (including references inside arrays or lists). 0000007194 00000 n Because of that in such a case extracting a new class is the next step in adhering to the principle. 0000008912 00000 n HT-T534R0 B#= mfigdk X endstream endobj 1400 0 obj 45 endobj 1401 0 obj << /Length 1400 0 R /Filter /FlateDecode >> stream Heres a specific example: How do we guarantee that these Tweet objects are immutable that, once a tweet is created, its author, message, and date can never be changed? Suppose C is an abstract data type whose representation has two String fields: Suppose Louis Reasoner has created CharSet with the following rep: But Louis unfortunately neglects to write down the abstraction function (AF) and rep invariant (RI). 0000017264 00000 n 0000009790 00000 n 0000024380 00000 n - In other words, RI tells us whether a given rep value is well-formed. A set of characters could equally be represented as a string, as above, or as a bit vector, with one bit for each possible character. x is not in followersOf.get(x), // represents the follower graph where Twitter user x is followed by user y, // if and only if followersOf.get(x).contains(y), /** Get all the sides of the triangle. 0000017288 00000 n But theres no a priori reason to let the rep decide the interpretation. & endstream endobj 1407 0 obj << /Length 1404 0 R /Filter /FlateDecode >> stream The trivial or the non-essentials units are not displayed to the user. Heres what the AF and RI would look like for that representation: The essential point is that designing an abstract type means not only choosing the two spaces the abstract value space for the specification and the rep value space for the implementation but also deciding what rep values to use and how to interpret them. Alternatively the parameters could be converted to fields of the class. All statements of a method should belong to the same level of abstraction. Heres an example of an abstract data type for rational numbers. The downside here is that you get immutability at runtime, but not at compile time. HT-T534R0 B#= mnhgnk qW endstream endobj 1398 0 obj 46 endobj 1399 0 obj << /Length 1398 0 R /Filter /FlateDecode >> stream Rep exposure like this threatens not only invariants, but also representation independence. A rep invariant that maps rep values to booleans: For a rep value r, RI(r) is true if and only if r is mapped by AF. An invariant is a property that is always true of an ADT object instance, for the lifetime of the object. This is better readable as no mental grouping is necessary. Consider the following problematic datatype: We now take a deeper look at the theory underlying abstract data types. For example, an abstract type for unbounded integers might have the mathematical integers as its abstract value space; the fact that it might be implemented as an array of primitive (bounded) integers, say, is not relevant to the user of the type.

The properties and behaviors of an object differentiate it from other objects of similar type and also help in classifying/grouping the objects. 1385 0 obj << /Linearized 1 /O 1387 /H [ 2579 522 ] /L 1375995 /E 31989 /N 18 /T 1348175 >> endobj xref 1385 106 0000000016 00000 n Switching between levels of abstraction makes code harder to read. 0000009191 00000 n 0000009662 00000 n 0000009490 00000 n It would be completely reasonable to design another implementation of this same ADT with a more permissive RI. Weve been doing that above. Later in this reading, well see many interesting invariants. Furthermore the two methods are still separately understandable (PSU) so no mental inlining is necessary and if you don't care about the details of the toDto method, you can just read and understand buildResult without being distracted by unnecessary detail. First there is the loop which acts upon the whole result set and second there is the loop body which converts a single entity to a DTO. The rep invariant specifies legal values of the representation, and should be checked at runtime with. 0000011162 00000 n

0000007216 00000 n HT-T534R0 B#= mjdgikh 4} endstream endobj 1418 0 obj 46 endobj 1419 0 obj << /Length 1418 0 R /Filter /FlateDecode >> stream 0000010714 00000 n In the absence of compelling arguments to the contrary, its almost always worth it for an abstract data type to guarantee its own invariants, and preventing rep exposure is essential to that. While reading the code you have to mentally construct the missing abstractions by trying to find groups of statements which belong together (mental grouping). 0000007238 00000 n Why is checkRep private? So the following code is better: Now there are two smaller methods each of which is written in terms of a single level of abstraction. In most of the cases the code block should go to a new private method. & endstream endobj 1406 0 obj << /Type /XObject /Subtype /Image /Name /im33 /Filter /CCITTFaxDecode /Width 2550 /Height 36 /BitsPerComponent 1 /Decode [ 1 0 ] /ColorSpace /DeviceGray /Length 1410 0 R /DecodeParms << /K -1 /Columns 2550 >> >> stream In simple cases, an abstract type will be implemented as a single object, but more commonly a small network of objects is needed, so this value is actually often something rather complicated. Stating the rep invariant explicitly, and checking it at runtime with checkRep(), catches misunderstandings and bugs earlier, rather than continuing on with a corrupt data structure. But if s is not otherwise checked by your rep invariant, then assert s != null explicitly. The rep invariant isnt just a neat mathematical idea. Consider this perfectly reasonable client code that uses Tweet: retweetLater takes a tweet and should return another tweet with the same message (called a retweet) but sent an hour later. Each method should be written in terms of a single level of abstraction. HT-T534R0 B=3 egkl endstream endobj 1462 0 obj 1640 endobj 1463 0 obj << /Length 1462 0 R /Filter /FlateDecode >> stream All of them were also mentioned in the reading above. 0000006362 00000 n

Whats the problem here? 2022 Raise My House. 0000006490 00000 n In java, abstraction is achieved by interfaces and abstract classes. 0000006213 00000 n 0000016798 00000 n 0000007839 00000 n - 0000009962 00000 n The space of representation values (or rep values for short) consists of the values of the actual implementation entities. In thinking about an abstract type, it helps to consider the relationship between two spaces of values. Alternatively, you can think of RI as a set: its the subset of rep values on which AF is defined.

The RI requires that numerator/denominator pairs be in reduced form (i.e., lowest terms), so pairs like (2,4) and (18,12) above should be drawn as outside the RI. - The principle is maybe older, though. Why? 0000014554 00000 n A good ADT preserves its own invariants, so that those invariants are less vulnerable to bugs in the ADTs clients, and violations of the invariants can be more easily isolated within the implementation of the ADT itself.

HW6[ fUl i`m5dnH&)SN "slv1g 2i(e4ko 0000010415 00000 n

HT-T534R0 B#=m`gfkd 5 endstream endobj 1444 0 obj 46 endobj 1445 0 obj << /Length 1444 0 R /Filter /FlateDecode >> stream #raisemyhouse #houserais, Another one going up 0000008289 00000 n 0000029874 00000 n HT-T534R0 B#= mlng`kh _ endstream endobj 1430 0 obj 47 endobj 1431 0 obj << /Length 1430 0 R /Filter /FlateDecode >> stream Designed to accommodate change without rewriting. Doing so will result in smaller methods. 0000008162 00000 n The rep invariant will make it easier to catch bugs caused by a corrupted data structure. 0000006063 00000 n 0000003338 00000 n The getTimestamp call returns a reference to the same date object referenced by tweet t. HT-T534R0 B#= mlglkl F endstream endobj 1458 0 obj 45 endobj 1459 0 obj << /Length 1458 0 R /Filter /FlateDecode >> stream What happens if different implementers disagree about the meaning of the rep? Lets focus on immutability for now. Heres a method for RatNum that tests its rep invariant: You should certainly call checkRep() to assert the rep invariant at the end of every operation that creates or mutates the rep in other words, creators, producers, and mutators. Correct today and correct in the unknown future. 0000004217 00000 n Lost your password? We can fix this problem too by using judicious defensive copying, this time in the constructor: In general, you should carefully inspect the argument types and return types of all your ADT operations. Loops should ideally contain a single statement (usually a method call). So you can construct a list using mutators, then seal it up in an unmodifiable wrapper (and throw away your reference to the original mutable list), and get an immutable list. An abstraction function that maps rep values to the abstract values they represent: The arcs in the diagram show the abstraction function. Collaboratively authored with contributions from: Saman Amarasinghe, Adam Chlipala, Srini Devadas, Michael Ernst, Max Goldman, John Guttag, Daniel Jackson, Rob Miller, Martin Rinard, and Armando Solar-Lezama. The Java collections classes offer an interesting compromise: immutable wrappers. So if your rep is: then its rep invariant automatically includes s != null, and you dont need to state it in a rep invariant comment. Another piece of documentation that 6.005 asks you to write is a rep exposure safety argument.

- We extend that prohibition to the reps of abstract data types. Representation exposure threatens both representation independence and invariant preservation. Observer methods dont normally need to call checkRep(), but its good defensive practice to do so anyway. So we describe it by giving two things: 1. 0000010264 00000 n 0000008588 00000 n Often the body of a loop can be extracted resulting in a separate private method. 0000009362 00000 n Often that check comes for free from Java, because checking other parts of your rep invariant will throw an exception if s is null. But thats still better than nothing, so using unmodifiable lists, maps, and sets can be a very good way to reduce the risk of bugs. Don't Mix Different Levels of Abstractions. The problem is that Tweet leaked out a reference to a mutable object that its immutability depended on. 0000027167 00000 n 0000007688 00000 n

Youll engage a private certifier to work with you and assess the application on behalf of the Council. If we had used an immutable date object, like java.time.ZonedDateTime, instead of the mutable java.util.Date, then we would have ended this section after talking about public and private. 0000009512 00000 n Sometimes extracting the method would result in the new method having a large number of parameters. Lorem ipsum dolor sit amet, consectetur adipiscing elit.Morbi adipiscing gravdio, sit amet suscipit risus ultrices eu.Fusce viverra neque at purus laoreet consequa.Vivamus vulputate posuere nisl quis consequat. HT-T534R0 B#= mjiglk \ endstream endobj 1412 0 obj 46 endobj 1413 0 obj << /Length 1412 0 R /Filter /FlateDecode >> stream