是独立于语言环境的Java字符串哈希码

Question

Java String.hashcode()完全独立于语言环境吗?换句话说，如果有人篡改了默认的Locale，我们能100%确定这不会影响哈希码吗?

我们知道这种扰动影响toUpperCase()和toLowerCase()。

Answer 1

区域设置不影响字符串的hashCode(直接)。它完全基于存储在String中的字符。hashCode是由

生成的

char[] val;
for (int i = 0; i < len; i++) {
    h = 31*h + val[off++];
}

，但问题是如何生成字符串。例如，如果是依赖于Locale的toUpperCase的结果，那么显然结果String依赖于Locale, hashCode也是如此。

Answer 2

好问题，我运行了一个快速测试，似乎改变默认区域设置不会(谢天谢地)改变哈希码…

import java.util.Locale;
public class HashCodeTester {
    public static void main(String[] args) {
        String test = "test";
        int hashCode = test.hashCode();
        System.out.println("hashcode [" + hashCode + "] - locale [" + Locale.getDefault() + "]");
        Locale[] availableLocales = Locale.getAvailableLocales();
        for(int i=0; i<availableLocales.length; i++) {          
            Locale.setDefault(availableLocales[i]);
            System.out.println("hashcode [" + test.hashCode() + "] - locale [" + Locale.getDefault() + "]");
        }
    }
}

输出

hashcode [3556498] - locale [en_IE]
hashcode [3556498] - locale [ja_JP]
hashcode [3556498] - locale [es_PE]
hashcode [3556498] - locale [en]
hashcode [3556498] - locale [ja_JP_JP]
hashcode [3556498] - locale [es_PA]
hashcode [3556498] - locale [sr_BA]
hashcode [3556498] - locale [mk]
hashcode [3556498] - locale [es_GT]
hashcode [3556498] - locale [ar_AE]
hashcode [3556498] - locale [no_NO]
hashcode [3556498] - locale [sq_AL]
hashcode [3556498] - locale [bg]
hashcode [3556498] - locale [ar_IQ]
hashcode [3556498] - locale [ar_YE]
hashcode [3556498] - locale [hu]
hashcode [3556498] - locale [pt_PT]
hashcode [3556498] - locale [el_CY]
hashcode [3556498] - locale [ar_QA]
hashcode [3556498] - locale [mk_MK]
hashcode [3556498] - locale [sv]
hashcode [3556498] - locale [de_CH]
hashcode [3556498] - locale [en_US]
hashcode [3556498] - locale [fi_FI]
hashcode [3556498] - locale [is]
hashcode [3556498] - locale [cs]
hashcode [3556498] - locale [en_MT]
hashcode [3556498] - locale [sl_SI]
hashcode [3556498] - locale [sk_SK]
hashcode [3556498] - locale [it]
hashcode [3556498] - locale [tr_TR]
hashcode [3556498] - locale [zh]
hashcode [3556498] - locale [th]
hashcode [3556498] - locale [ar_SA]
hashcode [3556498] - locale [no]
hashcode [3556498] - locale [en_GB]
hashcode [3556498] - locale [sr_CS]
hashcode [3556498] - locale [lt]
hashcode [3556498] - locale [ro]
hashcode [3556498] - locale [en_NZ]
hashcode [3556498] - locale [no_NO_NY]
hashcode [3556498] - locale [lt_LT]
hashcode [3556498] - locale [es_NI]
hashcode [3556498] - locale [nl]
hashcode [3556498] - locale [ga_IE]
hashcode [3556498] - locale [fr_BE]
hashcode [3556498] - locale [es_ES]
hashcode [3556498] - locale [ar_LB]
hashcode [3556498] - locale [ko]
hashcode [3556498] - locale [fr_CA]
hashcode [3556498] - locale [et_EE]
hashcode [3556498] - locale [ar_KW]
hashcode [3556498] - locale [sr_RS]
hashcode [3556498] - locale [es_US]
hashcode [3556498] - locale [es_MX]
hashcode [3556498] - locale [ar_SD]
hashcode [3556498] - locale [in_ID]
hashcode [3556498] - locale [ru]
hashcode [3556498] - locale [lv]
hashcode [3556498] - locale [es_UY]
hashcode [3556498] - locale [lv_LV]
hashcode [3556498] - locale [iw]
hashcode [3556498] - locale [pt_BR]
hashcode [3556498] - locale [ar_SY]
hashcode [3556498] - locale [hr]
hashcode [3556498] - locale [et]
hashcode [3556498] - locale [es_DO]
hashcode [3556498] - locale [fr_CH]
hashcode [3556498] - locale [hi_IN]
hashcode [3556498] - locale [es_VE]
hashcode [3556498] - locale [ar_BH]
hashcode [3556498] - locale [en_PH]
hashcode [3556498] - locale [ar_TN]
hashcode [3556498] - locale [fi]
hashcode [3556498] - locale [de_AT]
hashcode [3556498] - locale [es]
hashcode [3556498] - locale [nl_NL]
hashcode [3556498] - locale [es_EC]
hashcode [3556498] - locale [zh_TW]
hashcode [3556498] - locale [ar_JO]
hashcode [3556498] - locale [be]
hashcode [3556498] - locale [is_IS]
hashcode [3556498] - locale [es_CO]
hashcode [3556498] - locale [es_CR]
hashcode [3556498] - locale [es_CL]
hashcode [3556498] - locale [ar_EG]
hashcode [3556498] - locale [en_ZA]
hashcode [3556498] - locale [th_TH]
hashcode [3556498] - locale [el_GR]
hashcode [3556498] - locale [it_IT]
hashcode [3556498] - locale [ca]
hashcode [3556498] - locale [hu_HU]
hashcode [3556498] - locale [fr]
hashcode [3556498] - locale [en_IE]
hashcode [3556498] - locale [uk_UA]
hashcode [3556498] - locale [pl_PL]
hashcode [3556498] - locale [fr_LU]
hashcode [3556498] - locale [nl_BE]
hashcode [3556498] - locale [en_IN]
hashcode [3556498] - locale [ca_ES]
hashcode [3556498] - locale [ar_MA]
hashcode [3556498] - locale [es_BO]
hashcode [3556498] - locale [en_AU]
hashcode [3556498] - locale [sr]
hashcode [3556498] - locale [zh_SG]
hashcode [3556498] - locale [pt]
hashcode [3556498] - locale [uk]
hashcode [3556498] - locale [es_SV]
hashcode [3556498] - locale [ru_RU]
hashcode [3556498] - locale [ko_KR]
hashcode [3556498] - locale [vi]
hashcode [3556498] - locale [ar_DZ]
hashcode [3556498] - locale [vi_VN]
hashcode [3556498] - locale [sr_ME]
hashcode [3556498] - locale [sq]
hashcode [3556498] - locale [ar_LY]
hashcode [3556498] - locale [ar]
hashcode [3556498] - locale [zh_CN]
hashcode [3556498] - locale [be_BY]
hashcode [3556498] - locale [zh_HK]
hashcode [3556498] - locale [ja]
hashcode [3556498] - locale [iw_IL]
hashcode [3556498] - locale [bg_BG]
hashcode [3556498] - locale [in]
hashcode [3556498] - locale [mt_MT]
hashcode [3556498] - locale [es_PY]
hashcode [3556498] - locale [sl]
hashcode [3556498] - locale [fr_FR]
hashcode [3556498] - locale [cs_CZ]
hashcode [3556498] - locale [it_CH]
hashcode [3556498] - locale [ro_RO]
hashcode [3556498] - locale [es_PR]
hashcode [3556498] - locale [en_CA]
hashcode [3556498] - locale [de_DE]
hashcode [3556498] - locale [ga]
hashcode [3556498] - locale [de_LU]
hashcode [3556498] - locale [de]
hashcode [3556498] - locale [es_AR]
hashcode [3556498] - locale [sk]
hashcode [3556498] - locale [ms_MY]
hashcode [3556498] - locale [hr_HR]
hashcode [3556498] - locale [en_SG]
hashcode [3556498] - locale [da]
hashcode [3556498] - locale [mt]
hashcode [3556498] - locale [pl]
hashcode [3556498] - locale [ar_OM]
hashcode [3556498] - locale [tr]
hashcode [3556498] - locale [th_TH_TH]
hashcode [3556498] - locale [el]
hashcode [3556498] - locale [ms]
hashcode [3556498] - locale [sv_SE]
hashcode [3556498] - locale [da_DK]
hashcode [3556498] - locale [es_HN]

Answer 3

String的equals方法明确指出，字符串只有在表示相同的字符序列时才相等(也就是说，这里没有进行转换)。

虽然这不能保证hashcode不使用语言环境信息(通常可能)，但Oracle JVM中的实现是这样的:

public int hashCode() {
    int h = hash;
        int len = count;
    if (h == 0 && len > 0) {
        int off = offset;
        char val[] = value;
            for (int i = 0; i < len; i++) {
                h = 31*h + val[off++];
            }
            hash = h;
        }
        return h;
    }

只使用字符而不使用区域信息

Answer 4

给定String对象的哈希码不依赖于语言环境。从您所链接的javadoc中，这应该是显而易见的。

然而，任何在字符串中产生不同字符的转换都将导致不同的(不相等的)字符串和不同的哈希码。例如，使用不同的默认字符编码将一堆字节转换为字符串可以产生不同的字符。

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总结，更改Locale并不直接影响String哈希码，但它可能导致应用程序产生不同的String值，而这将影响它们的哈希码。