String是最常用的类没有之一，现在开始看一些String的源码。

先看String的hashCode实现。

/**
 * Returns a hash code for this string. The hash code for a
 * {@code String} object is computed as
 * <blockquote><pre>
 * s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
 * </pre></blockquote>
 * using {@code int} arithmetic, where {@code s[i]} is the
 * <i>i</i>th character of the string, {@code n} is the length of
 * the string, and {@code ^} indicates exponentiation.
 * (The hash value of the empty string is zero.)
 *
 * @return  a hash code value for this object.
 */
public int hashCode() {
    int h = hash;
    if (h == 0 && value.length > 0) {
        char val[] = value;

        for (int i = 0; i < value.length; i++) {
            h = 31 * h + val[i];
        }
        hash = h;
    }
    return h;
}

String 的hashCode的实现就是31*hash + char;

public static void main(String[] args) {
    String string = "xueshaoyi";
    System.out.println(string.hashCode());
    int h =0;
    for (int i = 0; i < string.length(); i++) {
        h = 31 * h + string.charAt(i);
    }
    System.out.println(h);
}
结果：
-1560834405
-1560834405