Java基礎 String

final class String {

1 空串的檢查

public boolean isEmpty() {
        return value.length == 0;
    }

2  相等

public boolean equals(Object anObject) {
        if (this == anObject) { //Identifical reference
            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;
    }

3 比大小

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; //第一個相異的字符大小表明串的大小。
            }
            k++;
        }
        return len1 - len2; //沒有相異的， 長串比短串大
    }

4 Hash

public int hashCode() {
        int h = hash; //初始值。
        if (h == 0 && value.length > 0) { //h == 0用於狀態。避免重複計算。
            char val[] = value;

            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i]; // 經典算法 y = k * x + b, 線性映射。 k = 31 爲素數。
            }
            hash = h;
        }
        return h;
    }

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