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How Immutability
Helps in OOP?
Yegor Bugayenko

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what is an object?

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“Each object looks
quite a bit like a
little computer — it
has a state, and it
has operations that
you can ask it to
perform”
- page 16

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“An object is some
memory that holds a
value of some type”
- page 40

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“A class is a
collection of data
fields that hold
values and methods
that operate on those
values”
- page 98

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“Objects may contain
data, in the form of
fields, often known
as attributes; and
code, in the form of
procedures, often
known as methods”

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class Calculator {
private int[] array;
public void calculate(int a, int b) {
this.array = new int[b - a];
for (int i = 0; i < this.array.length; ++i) {
this.array[i] = a + i;
}
}
public int[] numbers() {
return this.array;
}
}

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Calculator c = new Calculator();
c.calculate(5, 15);
int[] x = c.numbers();

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“who I am?”
vs.
“what I do?”

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class Range {
private final int a;
private final int b;
Range(int start, int finish) {
this.a = start;
this.b = finish;
}
public int[] numbers() {
int[] array = new int[this.b - this.a];
for (int i = 0; i < array.length; ++i) {
array[i] = this.a + i;
}
return array;
}
}

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Range r = new Range(5, 15);
int[] x = r.numbers();

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R range = new Range(5, 15);
int[] x = range.numbers();
Calculator c = new Calculator();
c.calculate(5, 15);
int[] x = c.numbers();

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R range = new OddOnly(
new Range(5, 15)
);
int[] x = range.numbers();
Calculator c = new Calculator();
c.setOddOnly(true);
c.calculate(5, 15);
int[] x = c.numbers();

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R range = new SquareAll(
new OddOnly(
new Range(5, 15)
)
);
int[] x = range.numbers();
Calculator c = new Calculator();
c.setOddOnly(true);
c.setSquareAll(true);
c.calculate(5, 15);
int[] x = c.numbers();

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R range = new Incremented(
new Squared(
new OddOnly(
new Range(5, 15)
)
),
100
);
int[] x = range.numbers();
Calculator c = new Calculator();
c.setOddOnly(true);
c.setSquared(true);
c.setIncremented(100);
c.calculate(5, 15);
int[] x = c.numbers();

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R range = new Logged(
new Incremented(
new Squared(
new OddOnly(
new Range(5, 15)
)
),
100
)
);
int[] x = range.numbers();
Calculator c = new Calculator();
c.setOddOnly(true);
c.setSquared(true);
c.setIncremented(100);
c.setLogged(true);
c.calculate(5, 15);
int[] x = c.numbers();

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R range = new Logged(
new Incremented(
new Squared(
new OddOnly(
new RangeFromFile(
new File(“a.txt”)
)
)
),
100
)
);
int[] x = range.numbers();
Calculator c = new Calculator();
c.setFromFile(“a.txt”); //???
c.setOddOnly(true);
c.setSquared(true);
c.setIncremented(100);
c.setLogged(true);
c.calculate(5, 15);
int[] x = c.numbers();

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what is an object?

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