Java 继承 不允许继承的类 不可变的类 final class

public final class Address
{
    private final String detail;
    private final String postCode;

    //在构造方法里初始化两个实例属性
    public Address()
    {
        this.detail = "";                                                                       
        this.postCode = "";

    }
    public Address(String detail , String postCode)
    {
        this.detail = detail;
        this.postCode = postCode;
    }
    //仅为两个实例属性提供getter方法
    public String getDetail()
    {
         return this.detail;
    }

    public String getPostCode()
    {
         return this.postCode;
    }
    //重写equals方法，判断两个对象是否相等。
    public boolean equals(Object obj)
    {
        if (obj instanceof Address)
        {
            Address ad = (Address)obj;
            if (this.getDetail().equals(ad.getDetail()) && this.getPostCode().equals(ad.getPostCode()))
            {
                return true;
            }
        }
        return false;
    }
    public int hashCode()
    {
        return detail.hashCode() + postCode.hashCode();
    }

public static void main(String args[])
{
    Address a = new Address();
    Address b = new Address("abcd","efgh");

    System.out.println("a:"+a.getDetail()+a.getPostCode()+a.hashCode());
     
    System.out.println("b:"+b.getDetail()+b.getPostCode()+b.hashCode());

    System.out.println(a.equals(b));

}
}