迭代java中的泛型hashmap-Iteratingoveragenerichashmapinjava

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For the "putting" part in init - the problem is that you're saying it's a map of some type of value where that type extends Drawn, but you're not saying what it is. That means you can't legitimately put any entries in. You really will need to change it to a Map - and that should be absolutely fine with subclasses, and the iteration code below will still be fine.

对于init中的“put”部分 - 问题是你说它是某种类型值的映射，其中该类型扩展为Drawn，但你不是说它是什么。这意味着你不能合法地输入任何条目。你真的需要将它改为Map - 这对于子类应该是绝对正确的，下面的迭代代码仍然可以。

The reason you can't do what you want is that the type system is trying to prevent this sort of problem:

你不能做你想做的事情的原因是类型系统试图阻止这种问题：

Map stringMap = new HashMap();
Map objectMap = stringMap; // This is fine
objectMap.put(10, new Object()); // This *mustn't* be valid...
String value = stringMap.get(10); // Or this would be dangerous

When you fetch a value from the map, you'll only know that it's a Drawn reference - if you want to use any subclass-specific methods, you'll need to cast to the right type. That's unavoidable if you want to have a map that can have multiple different kinds of value.

当你从地图中获取一个值时，你只会知道它是一个绘制引用 - 如果你想使用任何子类特定的方法，你需要转换为正确的类型。如果你想拥有一个可以有多种不同价值的地图，这是不可避免的。

When you're iterating just over values then you can just use:

当您只是迭代值时，您可以使用：

for (Drawn drawn : Drawn.drawns.values()) {
    drawn.draw();
}

Or if you need the key as well:

或者如果您还需要密钥：

// Or Map.Entry after the earlier change
for (Map.Entry entry : Drawn.drawns.entrySet()) {
    // Use entry.getKey() and entry.getValue() here
}