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Effective Java - method call time at the same time, despite creating multiple instances

I study effective Java and in paragraph 5 of the book, Joshua Bloch talks about avoiding creating unnecessary objects. The example demonstrates mutable Date objects that never change after calculating their values.

Here's the "bad practice":

public Person(Date birthDate) {
    this.birthDate = new Date(birthDate.getTime());
}

// DON'T DO THIS!
public boolean isBabyBoomer() {
    // Unnecessary allocation of expensive object
    Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
    gmtCal.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
    Date boomStart = gmtCal.getTime();
    gmtCal.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
    Date boomEnd = gmtCal.getTime();
    return birthDate.compareTo(boomStart) >= 0
            && birthDate.compareTo(boomEnd) < 0;
}

The isBabyBoomer method unnecessarily creates a new Calendar, TimeZone and two Date instances each time it is called, and this clearly makes sense to me.

And here is the improved code:

public Person(Date birthDate) {
    this.birthDate = new Date(birthDate.getTime());
}

/**
 * The starting and ending dates of the baby boom.
 */
private static final Date BOOM_START;
private static final Date BOOM_END;

static {
    Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
    gmtCal.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
    BOOM_START = gmtCal.getTime();
    gmtCal.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
    BOOM_END = gmtCal.getTime();
}

public boolean isBabyBoomer() {
    return birthDate.compareTo(BOOM_START) >= 0
            && birthDate.compareTo(BOOM_END) < 0;
}

Calendar, TimeZone, and Date instances are created only once when they are initialized. Bloch explains that this leads to a significant increase in productivity if the method is often used isBabyBoomer().

:
: 32 000 10
: 130 10

, (14 ). , ?

Edit:
:

    public static void main(String[] args) {
    Calendar cal = Calendar.getInstance();
    cal.set(1960, Calendar.JANUARY, 1, 1, 1, 0);
    Person p = new Person(cal.getTime());
    long startTime = System.nanoTime();
    for (int i = 0; i < 10000000; i++) {
        p.isBabyBoomer();
    }
    long stopTime = System.nanoTime();
    long elapsedTime = stopTime - startTime;
    double mseconds = (double) elapsedTime / 1000000.0;
    System.out.println(mseconds);
}

,

+2
2

. Java 7 :

Person::main: estimatedSeconds 1 = '8,42'
Person::main: estimatedSeconds 2 = '0,01'

:

import java.util.Calendar;
import java.util.Date;
import java.util.TimeZone;

public class Person {
    private Date birthDate;
    static Date BOOM_START;
    static Date BOOM_END;

    public Person(Date birthDate) {
        this.birthDate = new Date(birthDate.getTime());
    }

    static {
        Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
        gmtCal.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
        BOOM_START = gmtCal.getTime();
        gmtCal.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
        BOOM_END = gmtCal.getTime();
    }

    public boolean isBabyBoomerWrong() {
        // Unnecessary allocation of expensive object
        Calendar gmtCal = Calendar.getInstance(TimeZone.getTimeZone("GMT"));
        gmtCal.set(1946, Calendar.JANUARY, 1, 0, 0, 0);
        Date boomStart = gmtCal.getTime();
        gmtCal.set(1965, Calendar.JANUARY, 1, 0, 0, 0);
        Date boomEnd = gmtCal.getTime();
        return birthDate.compareTo(boomStart) >= 0
                && birthDate.compareTo(boomEnd) < 0;
    }

    public boolean isBabyBoomer() {
        return birthDate.compareTo(BOOM_START) >= 0
                && birthDate.compareTo(BOOM_END) < 0;
    }

    public static void main(String[] args) {
        Person p = new Person(new Date());

        for (int i = 0; i < 10_000_000; i++) {
            p.isBabyBoomerWrong();
            p.isBabyBoomer();
        }

        long startTime = System.nanoTime();

        for (int i = 0; i < 10_000_000; i++) {
            p.isBabyBoomerWrong();
        }

        double estimatedSeconds = (System.nanoTime() - startTime) / 1000000000.0;
        System.out.println(String.format("Person::main: estimatedSeconds 1 = '%.2f'", estimatedSeconds));

        startTime = System.nanoTime();

        for (int i = 0; i < 10_000_000; i++) {
            p.isBabyBoomer();
        }

        estimatedSeconds = (System.nanoTime() - startTime) / 1000000000.0;
        System.out.println(String.format("Person::main: estimatedSeconds 2 = '%.2f'", estimatedSeconds));

    }
}
+4

.

( ) JVM, . .

, , , , , - . , :

:

( , , Point , getDistanceFrom() ):

public class Point {
    private int x, y;
    public Point(int x, int y) {
        this.x = x; this.y = y;
    }
    public Point(Point p) { this(p.x, p.y); }
    public int getX() { return x; }
    public int getY() { return y; }
}

public class Component {
    private Point location;
    public Point getLocation() { return new Point(location); }
    public double getDistanceFrom(Component other) {
        Point otherLocation = other.getLocation();
        int deltaX = otherLocation.getX() - location.getX();
        int deltaY = otherLocation.getY() - location.getY();
        return Math.sqrt(deltaX*deltaX + deltaY*deltaY);
    }
}

getLocation() , Point ; , , , getLocation() . getDistanceFrom() ; Point , .

JVM , . -, getLocation() getX() getY(), getDistanceFrom() :

(, getDistanceFrom())

public double getDistanceFrom(Component other) {
    Point otherLocation = new Point(other.x, other.y);
    int deltaX = otherLocation.x - location.x;
    int deltaY = otherLocation.y - location.y;
    return Math.sqrt(deltaX*deltaX + deltaY*deltaY);
}

, , getDistanceFrom() . ( , , .) , Point -, , , , , , :

, getDistanceFrom():

public double getDistanceFrom(Component other) {
    int tempX = other.x, tempY = other.y;
    int deltaX = tempX - location.x;
    int deltaY = tempY - location.y;
    return Math.sqrt(deltaX*deltaX + deltaY*deltaY);
}

, , , , ( ).

+1

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