java - 泛型双重标准

Question

public class Boxcar<S extends Things> {
public ArrayList<S> contents = new ArrayList<S>(); //an arraylist of things

public static void main(String [] args){
    Boxcar test = new Boxcar();
    test.addContents(new Person("239-235-2535", "Ronald", 36)); //works 100%
}

public Boxcar(Things type, int maxElements){
    this.type = type;
    boxcarId = boxcarIdCount;
    boxcarIdCount++;
    this.maxElements = maxElements;
}

public void addContents(S thing) {
    contents.add(thing);
  }
...

}//end boxcar class

public class Person implements Things {
int age;
String govtId, name;

public Person(String govtId, String name, int age){
    this.govtId = govtId;
    this.name = name;
    this.age = age;
}//end Consrtructor

public void load(ArrayList<Boxcar<?>> train){
    Person dude = new Person("239-235-235", "Ronald", 36);
    train.get(i).addContents(dude); // won't compile
}
...
}//end Person class

public interface Things {

public void load(ArrayList<Boxcar<?>> train, String [] params);

}//end interface Things

public class Train {
ArrayList<Boxcar<?>> train = new ArrayList<Boxcar<?>>(); 

    public void load(Things thing, String [] params){
    thing.load(train, params);
}
...
}

在上面的代码中，方法 addContents 在 Boxcar 类中执行时似乎工作正常。然而，当从 Person 类以完全相同的方式调用时，它的行为会有所不同。

造成这种情况的原因是什么以及如何解决？

Answer 1

Java 编译器不允许访问未绑定(bind)参数化类型的引用上的方法，Boxcar<?>在您的情况下，因为类型未知。

您应该定义通配符的界限并按如下方式使用它:

public void load(ArrayList<Boxcar<? super Things>> train)
{
    Person dude = new Person("239-235-235", "Ronald", 36);
    train.get(0).addContents(dude); 
}