使用Scala中的Java泛型方法

Question

我在这个例子中遇到了解Scalac错误消息的问题:

Bean.java

public class Bean {
  static public class Attribute<T> {
    public final String name;
    public Attribute(String name) {this.name = name;}
    // equals and hashcode omitted for simplicity
  }

  public <T> void set(Attribute<T> attribute, T value) {}

  public static Attribute<Long> AGE = new Attribute<Long>("age");
}

Test.scala

object Test {
  def test() {
    val bean = new Bean();
    bean.set(Bean.AGE, 2L);
  }
}

编译yeilds this(尝试用scalac 2.9.2):

Test.scala:4: error: type mismatch;
 found   : Bean.Attribute[java.lang.Long]
 required: Bean.Attribute[Any]
Note: java.lang.Long <: Any, but Java-defined class Attribute is invariant in type T.
You may wish to investigate a wildcard type such as `_ <: Any`. (SLS 3.2.10)
bean.set(Bean.AGE, 2L);
              ^
one error found

为什么需要属性[Any]？在Java中做同样的工作正常

谢谢

Answer 1

该错误是由于java.lang.Longscala和scala 之间的不匹配造成的Long.

Bean.AGE是类型的Bean.Attribute[java.lang.Long].因此,scala编译器期望a java.lang.Long作为另一个参数.但你传球的是2L哪个scala.Long而不是java.lang.Long.因此它显示错误.

这样做会按预期工作:

 b.set(Bean.AGE,new java.lang.Long(23))

感谢@senia,以下是更好的选择:

bean.set[java.lang.Long](Bean.AGE, 23)
bean.set(Bean.AGE, 23:java.lang.Long)