了解CaseInsensitiveComparator中的逻辑 [英] Understanding logic in CaseInsensitiveComparator

问题描述

任何人都可以从 String.java 解释以下代码，具体为什么有三个 if 语句（我已标记 // 1 ， // 2 和 // 3 ）

Can anyone explain the following code from String.java, specifically why there are three if statements (which I've marked //1, //2 and //3)?

private static class CaseInsensitiveComparator
                     implements Comparator<String>, java.io.Serializable {
// use serialVersionUID from JDK 1.2.2 for interoperability
private static final long serialVersionUID = 8575799808933029326L;

    public int compare(String s1, String s2) {
        int n1=s1.length(), n2=s2.length();
        for (int i1=0, i2=0; i1<n1 && i2<n2; i1++, i2++) {
            char c1 = s1.charAt(i1);
            char c2 = s2.charAt(i2);
            if (c1 != c2) {/////////////////////////1
                c1 = Character.toUpperCase(c1);
                c2 = Character.toUpperCase(c2);
                if (c1 != c2) {/////////////////////////2
                    c1 = Character.toLowerCase(c1);
                    c2 = Character.toLowerCase(c2);
                    if (c1 != c2) {/////////////////////////3
                        return c1 - c2;
                    }
                }
            }
        }
        return n1 - n2;
    }
}

推荐答案

通常，我们希望将案例转换一次并比较并完成它。但是，代码将案例转换两次，原因在评论中说明了不同的方法 public boolean regionMatches（boolean ignoreCase ，int toffset，String other，int ooffset，int len） ：

Normally, we would expect to convert the case once and compare and be done with it. However, the code convert the case twice, and the reason is stated in the comment on a different method public boolean regionMatches(boolean ignoreCase, int toffset, String other, int ooffset, int len):

不幸的是，转换大写字母对格鲁吉亚字母表不起作用，格式字母表有关于大小写转换的奇怪规则。因此我们需要在退出前进行最后一次检查。

Unfortunately, conversion to uppercase does not work properly for the Georgian alphabet, which has strange rules about case conversion. So we need to make one last check before exiting.

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附录

regionMatches 的代码与 CaseInsenstiveComparator ，但基本上做同样的事情。下面引用该方法的完整代码用于交叉检查：

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Appendix

The code of regionMatches has a few difference from the code in the CaseInsenstiveComparator, but essentially does the same thing. The full code of the method is quoted below for the purpose of cross-checking:

public boolean regionMatches(boolean ignoreCase, int toffset,
                       String other, int ooffset, int len) {
    char ta[] = value;
    int to = offset + toffset;
    char pa[] = other.value;
    int po = other.offset + ooffset;
    // Note: toffset, ooffset, or len might be near -1>>>1.
    if ((ooffset < 0) || (toffset < 0) || (toffset > (long)count - len) ||
            (ooffset > (long)other.count - len)) {
        return false;
    }
    while (len-- > 0) {
        char c1 = ta[to++];
        char c2 = pa[po++];
        if (c1 == c2) {
            continue;
        }
        if (ignoreCase) {
            // If characters don't match but case may be ignored,
            // try converting both characters to uppercase.
            // If the results match, then the comparison scan should
            // continue.
            char u1 = Character.toUpperCase(c1);
            char u2 = Character.toUpperCase(c2);
            if (u1 == u2) {
                continue;
            }
            // Unfortunately, conversion to uppercase does not work properly
            // for the Georgian alphabet, which has strange rules about case
            // conversion.  So we need to make one last check before
            // exiting.
            if (Character.toLowerCase(u1) == Character.toLowerCase(u2)) {
                continue;
            }
        }
        return false;
    }
    return true;
}

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