JDK之String的equals和equalsIgnoreCase的实现

克虏伯

发布于 2020-07-15 10:24:50

4150

发布于 2020-07-15 10:24:50

文章被收录于专栏：软件开发-青出于蓝

JDK8

这俩个方法经常用，今天突然好奇怎么实现的，之前也看过，不过今天再来看下，记录下来

equalsIgnoreCase

List-1

public boolean equalsIgnoreCase(String anotherString) {
    return (this == anotherString) ? true
            : (anotherString != null)
            && (anotherString.value.length == value.length)
            && regionMatches(true, 0, anotherString, 0, value.length);
}

...
public boolean regionMatches(boolean ignoreCase, int toffset,
                             String other, int ooffset, int len) {
    char ta[] = value;
    int to = toffset;
    char pa[] = other.value;
    int po = ooffset;
    // Note: toffset, ooffset, or len might be near -1>>>1.
    if ((ooffset < 0) || (toffset < 0)
            || (toffset > (long)value.length - len)
            || (ooffset > (long)other.value.length - 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;
}

如List-1所示:

判断是否是本身
判断不为空，判断长度是否相等
在regionMatches方法中，俩个char[]从左边开始往右边逐个对比，如果直接比较俩个字符，不相等的话，将俩个字符先都转换为大写进行比较，如果大写不相等，那么再转换为小写——注释上写着格鲁吉亚的字符有问题

equals

List-2

public boolean equals(Object anObject) {
    if (this == anObject) {
        return true;
    }
    if (anObject instanceof String) {
        String anotherString = (String)anObject;
        int n = value.length;
        if (n == anotherString.value.length) {
            char v1[] = value;
            char v2[] = anotherString.value;
            int i = 0;
            while (n-- != 0) {
                if (v1[i] != v2[i])
                    return false;
                i++;
            }
            return true;
        }
    }
    return false;
}