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public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    private final char value[];
}

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public int compareTo(String anotherString) {
    // 1.取出2字串長度
    int len1 = value.length;
    int len2 = anotherString.value.length;
    
    // 2.取得2個字串最小長度
    int lim = Math.min(len1, len2);

    char v1[] = value;
    char v2[] = anotherString.value;

    // 3.先比較最小長度內，每個字元是否相等
    int k = 0;
    while (k < lim) {
        char c1 = v1[k];
        char c2 = v2[k];
        // 4.字元不相等就傳回字元相減的結果
        if (c1 != c2) {
            return c1 - c2;
        }
        k++;
    }
    // 5.若最小長度內的字元都相等
    // 6.傳回長度相減
    return len1 - len2;
}

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String value = "abcdefg";
String another = "abdd";
int result = value.compareTo(another);

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String value = "abcdefg";
String another = "abcd";
int result = value.compareTo(another);
System.out.println(result);

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String value = "abcd";
String another = "abcdfg";
int result = value.compareTo(another);
System.out.println(result);