Scala允许调用java.util.HashMap获取带有错误参数数量的方法

Question

在交互式scala控制台中运行以下代码

val map = new java.util.HashMap[String, Integer]();

map.put("key1", 5)

"Test " + map.get("key1") + " " + map.get() + " " + map.get("key1", "key2") + " " + map.get("key1", "key2", "key3")

返回以下内容

Test 5 null null null

我希望这个代码在第一次调用之外的所有调用中都会导致调用get方法的错误参数数量的编译器错误.为什么这个成功编译并返回null？

Answer 1

Java映射不是类型安全的,特别是该get方法具有以下签名:

public V get(Object key);

所以你可以用任何东西作为钥匙.在Scala中,您会看到所谓的自动翻译,这在Scala 2.11中已被弃用,所以如果使用编译项目-deprecation,您会看到:

[warn] ... Adaptation of argument list by inserting () has been deprecated: leaky (Object-receiving) target makes this especially dangerous.
[warn]         signature: HashMap.get(x$1: Any): V
[warn]   given arguments: <none>
[warn]  after adaptation: HashMap.get((): Unit)
[warn]   "Test " + map.get("key1") + " " + map.get() + " " + map.get("key1", "key2") + " " + map.get("key1", "key2", "key3")
[warn]                                            ^

您可以使用-Xfuture编译器标志将其转换为错误:

[error] ... Adaptation of argument list by inserting () has been removed.
[error]         signature: HashMap.get(x$1: Any): V
[error]   given arguments: <none>
[error]   "Test " + map.get("key1") + " " + map.get() + " " + map.get("key1", "key2") + " " + map.get("key1", "key2", "key3")
[error]                                            ^

自动几倍手段map.get()将被视为map.get(())和map.get("key1", "key2")将被视为map.get(("key1", "key2")).

我建议使用Scala自己的集合类型,除非您有非特别的理由不这样做.