hashcode()中 lineNumber是一个变量，怎么可以调用hashCode()方法？

public class PhoneNumber {
private int areaCode;
private String prefix;
private String lineNumber;

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + areaCode;
    result = prime * result
            + ((lineNumber == null) ? 0 : lineNumber.hashCode());
    result = prime * result + ((prefix == null) ? 0 : prefix.hashCode());
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    PhoneNumber other = (PhoneNumber) obj;
    if (areaCode != other.areaCode)
        return false;
    if (lineNumber == null) {
        if (other.lineNumber != null)
            return false;
    } else if (!lineNumber.equals(other.lineNumber))
        return false;
    if (prefix == null) {
        if (other.prefix != null)
            return false;
    } else if (!prefix.equals(other.prefix))
        return false;
    return true;
}

}

hashCode方法在Object类中进行定义的，而Java中所有非Object类都是隐式继承Oject类的，lineNumber是String类型的变量，String继承Object类，所以lineNumber当然可以调用hashCode方法。