The document discusses the concept of immutability in programming, emphasizing its importance for thread safety and reducing side effects. It compares mutable and immutable object designs and highlights issues such as failure atomicity and trust in object interactions. It also showcases practical examples of code implementations to demonstrate these principles.

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How Much
Immutability Is
Enough
Yegor 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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