有效的 Java - 尽管创建了多个实例，但方法调用时间相同

我正在学习《Effective Java》，在本书的第 5 条中，Joshua Bloch 谈到了避免创建不必要的对象。一个示例演示了可变的 Date 对象，一旦计算出它们的值就永远不会被修改。

这是“不好的做法”：

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;
}

isBabyBoomer 方法每次调用时都会不必要地创建一个新的 Calendar、TimeZone 和两个 Date 实例 - 这对我来说显然是有意义的。

这是改进的代码：

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 和 Date 实例仅在初始化时创建一次。 Bloch 解释说，如果该方法能够显着提高性能isBabyBoomer()被频繁调用。

在他的机器上：
错误版本：1000 万次调用需要 32,000 毫秒
改进版本：1000万次调用130ms

但是当我在我的系统上运行这些示例时，性能完全相同（14 毫秒）。 这是编译器功能，实例仅创建一次吗？

Edit:
这是我的基准：

    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);
}

干杯，马库斯

你的基准是错误的。通过最新的 Java 7 和适当的预热，我发现这两种方法之间存在显着差异：

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

这是完整的可运行代码：

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));

    }
}