Java之创建对象>5.Avoid creating unnecessary objects

String s = new String("stringette"); // DON'T DO THIS!

The improved version is simply the following:

String s = "stringette";

根据生日来判断是否是婴儿潮时期出生的，isBabyBoomer()是一个糟糕的设计，每次调用这个方法都会创建Calendar,TimeZone以及2个Date对象实例，当此方法被频繁调用时将会非常地影响性能。

public class Person {

    private final Date birthDate;

    // 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;

    }

}

这个是优化后的方法，The improved version of the Person class creates Calendar, TimeZone, and Date instances only once

这些对象都是初始化后不会再被修改

class Person {

    private final Date birthDate;

    /**

    * 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;

    }

}

autoboxing(自动装箱)也会带来非常大的性能影响

 // Hideously slow program! Can you spot the object creation?

public static void main(String[] args) {

    Long sum = 0L;

    for (long i = 0; i < Integer.MAX_VALUE; i++) {

        sum += i;

    }

    System.out.println(sum);

}

巴特西

Java之创建对象>5.Avoid creating unnecessary objects

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