String类对象的比较

1.字符串比较，是按照字符串(String)中每一个字符(char)的字段表顺序进行比较

/**
 * Compares two strings lexicographically(字典序，按照字典顺序).
 * The comparison is based on the Unicode value of each character in
 * the strings. The character sequence represented by this
 * {@code String} object is compared lexicographically to the
 * character sequence represented by the argument string. The result is
 * a negative integer if this {@code String} object
 * lexicographically precedes(先于) the argument string. The result is a
 * positive integer if this {@code String} object lexicographically
 * follows the argument string. The result is zero if the strings
 * are equal; {@code compareTo} returns {@code 0} exactly when
 * the {@link #equals(Object)} method would return {@code true}.
 * <p>
 * This is the definition of lexicographic ordering. If two strings are
 * different, then either they have different characters at some index
 * that is a valid index for both strings, or their lengths are different,
 * or both. If they have different characters at one or more index
 * positions, let <i>k</i> be the smallest such index; then the string
 * whose character at position <i>k</i> has the smaller value, as
 * determined by using the &lt; operator, lexicographically precedes the
 * other string. In this case, {@code compareTo} returns the
 * difference of the two character values at position {@code k} in
 * the two string -- that is, the value:
 * <blockquote><pre>
 * this.charAt(k)-anotherString.charAt(k)
 * </pre></blockquote>
 * If there is no index position at which they differ, then the shorter
 * string lexicographically precedes the longer string. In this case,
 * {@code compareTo} returns the difference of the lengths of the
 * strings -- that is, the value:
 * <blockquote><pre>
 * this.length()-anotherString.length()
 * </pre></blockquote>
 *
 * @param   anotherString   the {@code String} to be compared.
 * @return  the value {@code 0} if the argument string is equal to
 *          this string; a value less than {@code 0} if this string
 *          is lexicographically less than the string argument; and a
 *          value greater than {@code 0} if this string is
 *          lexicographically greater than the string argument.
 */
public int compareTo(String anotherString) {
    int len1 = value.length;
    int len2 = anotherString.value.length;
    int lim = Math.min(len1, len2);
    char v1[] = value;
    char v2[] = anotherString.value;

    int k = 0;
    while (k < lim) {
        char c1 = v1[k];
        char c2 = v2[k];
        if (c1 != c2) {
            return c1 - c2;
        }
        k++;
    }
    return len1 - len2;
}

String str1 = "abd";
String str2 = "aba";
int res1 = str1.compareTo(str2);
//3
System.out.println(res1);

String str1 = "Abd";
String str2 = "aba";
int res1 = str1.compareTo(str2);
//-32
System.out.println(res1);

2.比较时忽略大小写

/**
 * Compares two strings lexicographically, ignoring case
 * differences. This method returns an integer whose sign is that of
 * calling {@code compareTo} with normalized versions of the strings
 * where case differences have been eliminated by calling
 * {@code Character.toLowerCase(Character.toUpperCase(character))} on
 * each character.
 * <p>
 * Note that this method does <em>not</em> take locale into account,
 * and will result in an unsatisfactory ordering for certain locales.
 * The java.text package provides <em>collators</em> to allow
 * locale-sensitive ordering.
 *
 * @param   str   the {@code String} to be compared.
 * @return  a negative integer, zero, or a positive integer as the
 *          specified String is greater than, equal to, or less
 *          than this String, ignoring case considerations.
 * @see     java.text.Collator#compare(String, String)
 * @since   1.2
 */
public int compareToIgnoreCase(String str) {
    return CASE_INSENSITIVE_ORDER.compare(this, str);
}

String str1 = "Abd";
String str2 = "aba";
int res = str1.compareToIgnoreCase(str2);
//3
System.out.println(res);