blocks|key|1184512|text|在Java中，不能将原始类型用作泛型参数。请改用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1184513|Map<String,+Integer>+myMap+=+new+HashMap<String,+Integer>();|code-block|syntax|javascript|1184514|使用auto-boxing/unboxing，代码几乎没有什么不同。自动装箱意味着你可以写：|offset|length|1184515|myMap.put("foo",+3);|1184516|而不是：|1184517|myMap.put("foo",+new+Integer(3));|1184518|自动装箱意味着第一个版本被隐式转换为第二个版本。自动拆箱意味着你可以写：|1184519|int+i+=+myMap.get("foo");|1184520|1184521|int+i+=+myMap.get("foo").intValue();|1184522|对intValue()的隐式调用意味着如果找不到密钥，它将生成一个NullPointerException，例如：|style|CODE|1184523|int+i+=+myMap.get("bar");+//+NullPointerException|1184524|原因是type+erasure。与C#不同的是，泛型类型在运行时不会被保留。它们只是显式强制转换的“语法糖”，可以省去你这样做的麻烦：|1184525|Integer+i+=+(Integer)myMap.get("foo");|1184526|举个例子，这段代码是完全合法的：|1184527|Map<String,+Integer>+myMap+=+new+HashMap<String,+Integer>();
Map<Integer,+String>+map2+=+(Map<Integer,+String>)myMap;
map2.put(3,+"foo");|1184528|entityMap|0|LINK|mutability|MUTABLE|url|http://java.sun.com/j2se/1.5.0/docs/guide/language/autoboxing.html|1|http://java.sun.com/docs/books/tutorial/java/generics/erasure.html^0|0|0|2|K|0|0|0|0|0|0|0|0|0|1|A|X|K|0|0|3|C|1|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1L|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|1M|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|1N|8|@]|9|@$I|1O|J|1P|1|1Q]]|A|$]]|$1|K|3|L|5|D|7|1R|8|@]|9|@]|A|$E|F]]|$1|M|3|N|5|6|7|1S|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|1T|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|1U|8|@]|9|@]|A|$]]|$1|S|3|T|5|D|7|1V|8|@]|9|@]|A|$E|F]]|$1|U|3|N|5|6|7|1W|8|@]|9|@]|A|$]]|$1|V|3|W|5|D|7|1X|8|@]|9|@]|A|$E|F]]|$1|X|3|Y|5|6|7|1Y|8|@$I|1Z|J|20|Z|10]|$I|21|J|22|Z|10]]|9|@]|A|$]]|$1|11|3|12|5|D|7|23|8|@]|9|@]|A|$E|F]]|$1|13|3|14|5|6|7|24|8|@]|9|@$I|25|J|26|1|27]]|A|$]]|$1|15|3|16|5|D|7|28|8|@]|9|@]|A|$E|F]]|$1|17|3|18|5|6|7|29|8|@]|9|@]|A|$]]|$1|19|3|1A|5|D|7|2A|8|@]|9|@]|A|$E|F]]|$1|1B|3|-4|5|6|7|2B|8|@]|9|@]|A|$]]]|1C|$1D|$5|1E|1F|1G|A|$1H|1I]]|1J|$5|1E|1F|1G|A|$1H|1K]]]]

You can't use primitive types as generic arguments in Java. Use instead:

<pre><code>Map&lt;String, Integer&gt; myMap = new HashMap&lt;String, Integer&gt;();
</code></pre>

With <a href="http://java.sun.com/j2se/1.5.0/docs/guide/language/autoboxing.html" rel="noreferrer">auto-boxing/unboxing</a> there is little difference in the code. Auto-boxing means you can write:

<pre><code>myMap.put("foo", 3);
</code></pre>

instead of:

<pre><code>myMap.put("foo", new Integer(3));
</code></pre>

Auto-boxing means the first version is implicitly converted to the second. Auto-unboxing means you can write:

<pre><code>int i = myMap.get("foo");
</code></pre>

instead of:

<pre><code>int i = myMap.get("foo").intValue();
</code></pre>

The implicit call to <code>intValue()</code> means if the key isn't found it will generate a <code>NullPointerException</code>, for example:

<pre><code>int i = myMap.get("bar"); // NullPointerException
</code></pre>

The reason is <a href="http://java.sun.com/docs/books/tutorial/java/generics/erasure.html" rel="noreferrer">type erasure</a>. Unlike, say, in C# generic types aren't retained at runtime. They are just "syntactic sugar" for explicit casting to save you doing this:

<pre><code>Integer i = (Integer)myMap.get("foo");
</code></pre>

To give you an example, this code is perfectly legal:

<pre><code>Map&lt;String, Integer&gt; myMap = new HashMap&lt;String, Integer&gt;();
Map&lt;Integer, String&gt; map2 = (Map&lt;Integer, String&gt;)myMap;
map2.put(3, "foo");
</code></pre>

blocks|key|1184556|text|GNU+Trove支持这一点，但不使用泛型。http://trove4j.sourceforge.net/javadocs/gnu/trove/TObjectIntHashMap.html|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1184557|entityMap|0|LINK|mutability|MUTABLE|url|http://trove4j.sourceforge.net/javadocs/gnu/trove/TObjectIntHashMap.html^0|M|20|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

GNU Trove support this but not using generics. <a href="http://trove4j.sourceforge.net/javadocs/gnu/trove/TObjectIntHashMap.html" rel="noreferrer">http://trove4j.sourceforge.net/javadocs/gnu/trove/TObjectIntHashMap.html</a>

blocks|key|1400099|text|您可以在泛型参数中使用引用类型，而不是基元类型。所以在这里你应该使用|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1400100|Map<String,+Integer>+myMap+=+new+HashMap<String,+Integer>();|code-block|syntax|javascript|1400101|并将值存储为|1400102|myMap.put("abc",+5);|1400103|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

You can use reference type in generic arguments, not primitive type.
So here you should use

<pre><code>Map&lt;String, Integer&gt; myMap = new HashMap&lt;String, Integer&gt;();
</code></pre>

and store value as

<pre><code>myMap.put("abc", 5);
</code></pre>

blocks|key|1184627|text|不能在HashMap中使用基元类型。int或double不起作用。你必须使用它的封闭类型。举个例子|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1184628|Map<String,Integer>+m+=+new+HashMap<String,Integer>();|code-block|syntax|javascript|1184629|现在两者都是对象，所以这将会起作用。|1184630|entityMap^0|3|7|I|3|M|6|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]|$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|V|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|W|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

You cannot use primitive types in <code>HashMap</code>. <code>int</code>, or <code>double</code> don't work. You have to use its enclosing type. for an example

<pre><code>Map&lt;String,Integer&gt; m = new HashMap&lt;String,Integer&gt;();
</code></pre>

Now both are objects, so this will work.

blocks|key|1400204|text|int是一种基本类型，你可以理解java+here中基本类型的含义，而Map是一个将对象作为输入的接口：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1400205|public+interface+Map<K+extends+Object,+V+extends+Object>|code-block|syntax|javascript|1400206|object意味着一个类，它还意味着您可以创建从它扩展的其他类，但不能创建从int扩展的类。因此您不能将int变量用作对象。对于你的问题，我有两个解决方案：|1400207|Map<String,+Integer>+map+=+new+HashMap<>();|1400208|或|1400209|Map<String,+int[]>+map+=+new+HashMap<>();
int+x+=+1;

//put+x+in+map
int[]+x_+=+new+int[]{x};
map.put("x",+x_);

//get+the+value+of+x
int+y+=+map.get("x")[0];|1400210|entityMap|0|LINK|mutability|MUTABLE|url|https://www.w3schools.com/java/java_data_types.asp^0|L|4|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@$A|Z|B|10|1|11]]|C|$]]|$1|D|3|E|5|F|7|12|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|13|8|@]|9|@]|C|$]]|$1|K|3|L|5|F|7|14|8|@]|9|@]|C|$G|H]]|$1|M|3|N|5|6|7|15|8|@]|9|@]|C|$]]|$1|O|3|P|5|F|7|16|8|@]|9|@]|C|$G|H]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|C|$]]]|R|$S|$5|T|U|V|C|$W|X]]]]

int is a primitive type, you can read what does mean a primitive type in java <a href="https://www.w3schools.com/java/java_data_types.asp" rel="nofollow noreferrer">here</a>, and a Map is an interface that has to objects as input:

<pre class="lang-java prettyprint-override"><code>public interface Map&lt;K extends Object, V extends Object&gt;
</code></pre>

object means a class, and it means also that you can create an other class that exends from it, but you can not create a class that exends from int.
So you can not use int variable as an object. I have tow solutions for your problem:

<pre class="lang-java prettyprint-override"><code>Map&lt;String, Integer&gt; map = new HashMap&lt;&gt;();
</code></pre>

or

<pre class="lang-java prettyprint-override"><code>Map&lt;String, int[]&gt; map = new HashMap&lt;&gt;();
int x = 1;

//put x in map
int[] x_ = new int[]{x};
map.put("x", x_);

//get the value of x
int y = map.get("x")[0];
</code></pre>

<code>HashMap&lt;String, int&gt;</code> doesn't seem to work but <code>HashMap&lt;String, Integer&gt;</code> does work.
Any ideas why?

java: HashMap<String, int> not working

Java

HashMap<String, int>似乎不起作用，但HashMap<String, Integer>确实起作用。你知道为什么吗？

问java: HashMap<String，int>不工作
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