blocks|key|4522238|text|实际答案是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4522239|Map<Object,Object>+valueMap+=+...;
@SuppressWarnings("unchecked")
Map<String,String>+targetMap+=+(Map)valueMap;|code-block|syntax|javascript|4522240|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

The actual answer is:

<pre><code>Map&lt;Object,Object&gt; valueMap = ...;
@SuppressWarnings("unchecked")
Map&lt;String,String&gt; targetMap = (Map)valueMap;
</code></pre>

blocks|key|4522264|text|在Chris+J's+answer的基础上，下面是他的“黑客”代码，展示了它的工作原理：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|4522265|public+static+void+main(String[]+args)+{
++++Map<Object,+Object>+valueMap+=+new+HashMap<Object,+Object>();
++++valueMap.put("foo",+"bar");
++++@SuppressWarnings("unchecked")
++++Map<String,+String>+targetMap+=+(Map<String,+String>)+((Object)+valueMap);+//+hack+alert!
++++for+(Map.Entry<String,+String>+entry+:+targetMap.entrySet())+{
++++++++String+key+=+entry.getKey();
++++++++String+value+=+entry.getValue();
++++++++System.out.println(key+%2B+"="+%2B+value);+//+prints+foo=bar+:)
++++}
}|code-block|syntax|javascript|4522266|哇!无运行时异常:)|4522267|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/7788177/java-cast-a-map/7788191#7788191^0|1|G|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@$A|T|B|U|1|V]]|C|$]]|$1|D|3|E|5|F|7|W|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|X|8|@]|9|@]|C|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|C|$]]]|L|$M|$5|N|O|P|C|$Q|R]]]]

Building on <a href="https://stackoverflow.com/questions/7788177/java-cast-a-map/7788191#7788191">Chris J's answer</a>, here's his "hack" code in action, showing it works:

<pre><code>public static void main(String[] args) {
 Map&lt;Object, Object&gt; valueMap = new HashMap&lt;Object, Object&gt;();
 valueMap.put("foo", "bar");
 @SuppressWarnings("unchecked")
 Map&lt;String, String&gt; targetMap = (Map&lt;String, String&gt;) ((Object) valueMap); // hack alert!
 for (Map.Entry&lt;String, String&gt; entry : targetMap.entrySet()) {
 String key = entry.getKey();
 String value = entry.getValue();
 System.out.println(key + "=" + value); // prints foo=bar :)
 }
}
</code></pre>

Wow! No runtime exceptions :)

blocks|key|4522317|text|我认为这是一个好主意，解释为什么简单的解决方案不起作用，为什么你永远不应该使用它。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4522318|假设您可以将List<Object>转换为List<String>+(这同样适用于Map，只是一个更简单的接口)。您希望下面的代码会发生什么情况：|offset|length|style|CODE|4522319|List<Object>+m+=+Something;
m.add("Looks+good.");
m.add(42);
List<String>+s+=+(List<String>)m;+//+uhuh,+no+we+don't+want+that.
String+myString+=+s.get(1);+//+huh+exception+here.|code-block|syntax|javascript|4522320|现在您确实可以使用波西米亚人/Chris解决方案来破解它，但您基本上破坏了Java的类型系统。你不想让一个List<String>包含一个整数！后面的调试会很有趣-循环所有变量的额外代码将避免许多令人头疼的问题，而且几乎不会成为性能问题。|4522321|如果有理由将Map声明为接受对象而不是字符串，有人可能会向其添加任何对象-通常您应该能够通过更好的泛型来避免这种情况。|4522322|entityMap^0|0|6|C|L|C|0|0|1H|C|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@$D|U|E|V|F|G]|$D|W|E|X|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|Y|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|Z|8|@$D|10|E|11|F|G]]|9|@]|A|$]]|$1|O|3|P|5|6|7|12|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|13|8|@]|9|@]|A|$]]]|R|$]]

I think it's a good idea to explain why the simple solution doesn't work and why you never, ever should use this.

Assume you could cast <code>List&lt;Object&gt;</code> to <code>List&lt;String&gt;</code> (the same applies to Map, just a simpler interface). What would you expect to happen from the following code:

<pre><code>List&lt;Object&gt; m = Something;
m.add("Looks good.");
m.add(42);
List&lt;String&gt; s = (List&lt;String&gt;)m; // uhuh, no we don't want that.
String myString = s.get(1); // huh exception here.
</code></pre>

Now you CAN hack it indeed using Bohemians/Chris solution, but you basically destroy Java's type system. DON'T DO THAT. You don't want a <code>List&lt;String&gt;</code> to contain an Integer! Have fun debugging that later on - the additional code of looping through all variables will avoid lots of headaches and hardly is a performance problem.

If there's a reason to declare the Map as taking an Object instead of a String someone may add any object to it - usually you should be able to avoid this with a better generic.

blocks|key|683164|text|如果你想要干净的东西，你必须转换成“深度”！键和值。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|683165|使用Java+8流，这只需要几行代码：|683166|static+public+Map<String,+String>+toStringString(Map<?+extends+Object,+?+extends+Object>+map)+{
++++++++return+map
++++++++++++++++.entrySet()
++++++++++++++++.stream()
++++++++++++++++.collect(Collectors.toMap(
++++++++++++++++++++e+->+e.getKey().toString(),
++++++++++++++++++++e+->+e.getValue().toString()
++++++++++++++++));
++++}|code-block|syntax|javascript|683167|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

If you want something clean, you must convert "deeply"! keys and values.

Using Java 8 stream, this take only a few lines:

<pre><code>static public Map&lt;String, String&gt; toStringString(Map&lt;? extends Object, ? extends Object&gt; map) {
 return map
 .entrySet()
 .stream()
 .collect(Collectors.toMap(
 e -&gt; e.getKey().toString(),
 e -&gt; e.getValue().toString()
 ));
 }
</code></pre>

blocks|key|4804314|text|+public++Map<String,+String>+castOnlyStringValues(final+Map<String,+Object>+map)+{
++++return+map.entrySet().stream()
++++++++++++++.filter(x+->+String.class.isAssignableFrom(x.getValue().getClass()))
++++++++++++++.map(x+->+(Entry<?,+?>)+x).map(x+->+(Entry<String,+String>)+x)
++++++++++++++.collect(toMap(Entry::getKey,+Entry::getValue));
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|4804315|unstyled|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|G|8|@]|9|@]|A|$B|C]]|$1|D|3|-4|5|E|7|H|8|@]|9|@]|A|$]]]|F|$]]

<pre><code> public Map&lt;String, String&gt; castOnlyStringValues(final Map&lt;String, Object&gt; map) {
 return map.entrySet().stream()
 .filter(x -&gt; String.class.isAssignableFrom(x.getValue().getClass()))
 .map(x -&gt; (Entry&lt;?, ?&gt;) x).map(x -&gt; (Entry&lt;String, String&gt;) x)
 .collect(toMap(Entry::getKey, Entry::getValue));
}
</code></pre>

How can I cast a <code>Map&lt;Object,Object&gt;</code> to <code>Map&lt;String,String&gt;</code> in the cleanest way?

Is there a way to do that without iterating over the map?

Thanks

Java - Cast a Map

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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如何以最简洁的方式将Map<Object,Object>转换为Map<String,String>？有没有一种方法可以在不遍历map的情况下做到这一点？谢谢

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