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How Much Immutability Is

EnoughYegor Bugayenko

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object is a representative

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File f = new File(“picture.jpg”); if (f.length() > 1000) { f.delete(); }

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immutability means loyalty

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class Book { private int isbn; private String title; public setIsbn(int isbn) { this.isbn = isbn; } public setTitle(String title) { this.title = title; } }

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class Book { private final int isbn; private final String title; public Book(int isbn, String title) { this.isbn = isbn; this.title = title; } }

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thread-safe

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List<String> list = new ArrayList<>(); list.add(“x”); // thread A list.add(“y”); // thread B

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failure atomicity

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class Numbers { private int[] array; private int position; void add(int x) { this.position++; if (x < 0) { throw new Exception(“only positive”); } this.array[this.position] = x; } }

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no identity mutability

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Map<Book, Boolean> map = new HashMap<>(); book.setId(1); map.put(book, true); book.setId(2); map.put(book, true);

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side effect free

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void print(Book book) { System.out.println( “book ID is: ” + book.getId() ); book.setId(123); }

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no temporal coupling

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request.setURI(“http://google.com”); request.setMethod(“GET”); request.addHeader(“Accept”, “text/html”); String body = request.fetch();

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trust

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monster objects

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reader = new Reader(); reader.setFile(“/tmp/names.csv”); reader.setFormat(“CSV”); reader.setColumn(3); reader.setAllCaps(true); reader.setCache(true); List<String> x = reader.read();

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object composition

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List<String> x = new CachedList<>( new AllCaps( // List<String> new OneColumn( // List<String> new CSV( // List<List<String>> new LinesFromStream( // List<String> new FileInputStream( new File(“/tmp/names.csv”) ) ) ), 3 ) ) );

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