blocks|key|1621703|text|你应该看看Java+generics，如果你不指定HashMap的类型，键和值都会是Object类型。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1621704|因此，如果您想要一个包含Integer键和String值的HashMap，例如：|1621705|++++HashMap<Integer,+String>+hashMap=+new+HashMap<Integer,+String>();|code-block|syntax|javascript|1621706|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/tutorial/java/generics/^0|16|6|5|D|0|0|C|7|L|6|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@$9|X|A|Y|1|Z]]|E|$]]|$1|F|3|G|5|6|7|10|8|@$9|11|A|12|B|C]|$9|13|A|14|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|15|8|@]|D|@]|E|$K|L]]|$1|M|3|-4|5|6|7|16|8|@]|D|@]|E|$]]]|N|$O|$5|P|Q|R|E|$S|T]]]]

You should take a look on <a href="http://docs.oracle.com/javase/tutorial/java/generics/" rel="noreferrer">Java generics</a>, if you don't specify the types of the HashMap, both key and value will be <code>Object</code> type.

So, if you want a HashMap with <code>Integer</code> keys and <code>String</code> values for instance:

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

blocks|key|4119800|text|通过指定key和value类型，可以通过启用编译时类型强制来实现更高的类型安全性。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4119801|这使得编写不会意外混淆键和值类型的代码变得更容易，并减少了必须在代码中显式声明的强制转换的数量。|4119802|但是，重要的是要知道，这些类型检查只在编译时进行，即一旦应用程序运行，JVM将允许您对键和值使用任何类型。|4119803|entityMap^0|4|3|8|5|0|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]|$9|O|A|P|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|Q|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|R|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|K|$]]

Specifying the <code>key</code> and the <code>value</code> types allows for greater type-safety by enabling compile-time typing enforcement. 

This makes it easier to write code that doesn't accidentally mix up the key and value types, and reduces the amount of casts that you must explicitly declare in the code. 

However, it is important to be aware the these type-checks are compile-time only, i.e. once the application is running, the JVM will allow you to use any types for the keys and values.

blocks|key|4119844|text|-+Generics可以隐含在类、接口、方法、变量等中。但它使用的最重要原因是使Collection+更具类型安全的。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|CODE|entityRanges|data|4119845|-+Generics确保只有特定类型的对象进入和离开Collections。|4119846|-此外，值得一提的是，有一个称为Erasure的过程，|4119847|->+Erasure是一个过程，其中type+parameters和type+arguments被编译器从泛型类和接口中移除，使其与编写的没有泛型的代码兼容。|4119848|所以,|4119849|HashMap<String,+Integer>+map+=+new+HashMap<String,+Integer>();|4119850|变成了Raw+type，|4119851|HashMap+map+=+new+HashMap();|4119852|entityMap^0|0|1|1F|7|2|8|14|A|0|0|1|2|8|Q|B|0|0|1|G|7|0|3|7|I|F|Y|E|1Z|8|0|0|3|0|0|1Q|0|1Q|0|0|C|3|8|0|0|S|0|S|0^^$0|@$1|2|3|4|5|6|7|W|8|@$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|D]|$9|13|A|14|B|D]]|E|@]|F|$]]|$1|G|3|H|5|6|7|15|8|@$9|16|A|17|B|C]|$9|18|A|19|B|D]|$9|1A|A|1B|B|D]]|E|@]|F|$]]|$1|I|3|J|5|6|7|1C|8|@$9|1D|A|1E|B|C]|$9|1F|A|1G|B|D]]|E|@]|F|$]]|$1|K|3|L|5|6|7|1H|8|@$9|1I|A|1J|B|D]|$9|1K|A|1L|B|D]|$9|1M|A|1N|B|D]|$9|1O|A|1P|B|C]]|E|@]|F|$]]|$1|M|3|N|5|6|7|1Q|8|@$9|1R|A|1S|B|C]]|E|@]|F|$]]|$1|O|3|P|5|6|7|1T|8|@$9|1U|A|1V|B|C]|$9|1W|A|1X|B|D]]|E|@]|F|$]]|$1|Q|3|R|5|6|7|1Y|8|@$9|1Z|A|20|B|C]|$9|21|A|22|B|D]]|E|@]|F|$]]|$1|S|3|T|5|6|7|23|8|@$9|24|A|25|B|C]|$9|26|A|27|B|D]]|E|@]|F|$]]|$1|U|3|-4|5|6|7|28|8|@]|E|@]|F|$]]]|V|$]]

- <code>Generics</code> can be implied to classes, interfaces, methods, variables etc.. but the most important reason for which its used is making the <code>Collection</code> more type safe.

- <code>Generics</code> make sure that only specific type of object enters and comes out of the <code>Collections</code>.

- Moreover its worth mentioning that there is a process known as <code>Erasure</code>,

-> <code>Erasure</code> is a process where the <code>type parameters</code> and <code>type arguments</code> are removed from the generic classes and interfaces by the compiler, making it back compatible with the codes that where written without Generics.

So,

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

becomes of <code>Raw type</code>,

<code>HashMap map = new HashMap();</code>

blocks|key|2009467|text|以下是您拥有的选项：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2009468|J2SE+<5.0样式：|2009469|+Map+map+=+new+HashMap();|code-block|syntax|javascript|2009470|J2SE+5.0%2B样式(使用generics)：|offset|length|2009471|+Map<KeyType,+ValueType>+map+=+new+HashMap<KeyType,+ValueType>();|2009472|Google+Guava样式(更短、更灵活)：|2009473|+Map<KeyType,+ValueType>+map+=+Maps.newHashMap();|2009474|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Generics_in_Java|1|http://code.google.com/p/guava-libraries/^0|0|0|0|E|8|0|0|0|0|C|1|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|13|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|14|8|@]|9|@]|A|$G|H]]|$1|I|3|J|5|6|7|15|8|@]|9|@$K|16|L|17|1|18]]|A|$]]|$1|M|3|N|5|F|7|19|8|@]|9|@]|A|$G|H]]|$1|O|3|P|5|6|7|1A|8|@]|9|@$K|1B|L|1C|1|1D]]|A|$]]|$1|Q|3|R|5|F|7|1E|8|@]|9|@]|A|$G|H]]|$1|S|3|-4|5|6|7|1F|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|Z]]|10|$5|V|W|X|A|$Y|11]]]]

Those are the options you have:

J2SE &lt;5.0 style:

<pre><code> Map map = new HashMap();
</code></pre>

J2SE 5.0+ style (use of <a href="http://en.wikipedia.org/wiki/Generics_in_Java">generics</a>):

<pre><code> Map&lt;KeyType, ValueType&gt; map = new HashMap&lt;KeyType, ValueType&gt;();
</code></pre>

<a href="http://code.google.com/p/guava-libraries/">Google Guava</a> style (shorter and more flexible):

<pre><code> Map&lt;KeyType, ValueType&gt; map = Maps.newHashMap();
</code></pre>

I've been learning about HashMaps recently but I have one question that I can't seem to get a clear answer on. The main difference between -

<pre><code>HashMap hash1 = new HashMap();
</code></pre>

vs 

<pre><code>HashMap&lt;,&gt;hash1 = new HashMap &lt;,&gt; (); //Filled in with whatever Key and Value you want. 
</code></pre>

I thought when you define a HashMap it requires the Key and Value. Any help would be much appreciated. Thank You.

Different Ways of Creating HashMaps

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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我最近一直在学习HashMaps，但我有一个问题似乎无法得到明确的答案。两者的主要区别是-HashMap hash1 = new HashMap();vsHashMap<,>hash1 = new HashMap <,> (); //Filled in with whatever Key and Value you w...

问创建HashMaps的不同方式
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