Java继承: hiding fields

Question

我有一个泛型类，它声明了一些字段和一个与它们一起使用的构造函数:

public abstract class GenericClass extends JFrame
{
    protected static String FIELD_1;
    protected static String FIELD_2;
    protected static String FIELD_3;

    public GenericClass()
    {
        super(FIELD_1+" "+FIELD_2+FIELD_3);
    }
}

以及应该隐藏字段并使用父类(super class)构造函数的子类:

public class ChildClass1
{
    protected static String FIELD_1 = "hello";
    protected static String FIELD_2 = "world";
    protected static String FIELD_3 = "!";

    public ChildClass
    {
        super();
    }
}
public class ChildClass2
{
    protected static String FIELD_1 = "another";
    protected static String FIELD_2 = "class";
    protected static String FIELD_3 = "!";

    public ChildClass
    {
        super();
    }
}

我不明白为什么创建的 JFrame 具有标题 null nullnull 。我做错了什么？

更新

这些类的使用非常简单:

public class Main
{

    public static void main(final String[] args)
    {
        new ChildClass1();
        new ChildClass2();
    }
}

Answer 1

因为 GenericClass 对于那些 FIELD_1、FIELD_2、FIELD_3 具有 null 值

public abstract class GenericClass extends JFrame
{
    protected static String FIELD_1;
    protected static String FIELD_2;
    protected static String FIELD_3;

    public GenericClass()
    {   // null +" " +null+null = `null nullnull`
        super(FIELD_1+" "+FIELD_2+FIELD_3);
    }
}

This was the idea: GenericClass should just DECLARE fields and subclasses should initialize them. – Danylo Esterman 41 secs ago

字段不能这样，您需要重载抽象类的构造函数并强制子类在构造函数期间传递参数(通过隐藏默认构造函数)，然后将这些传递的参数用于 JFrame的构造函数