blocks|key|400833|text|不同之处在于，例如，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|400834|List<HashMap<String,String>>|code-block|syntax|javascript|400835|是一个|400836|List<?+extends+Map<String,String>>|400837|但不是|400838|List<Map<String,String>>|400839|所以：|400840|void+withWilds(+List<?+extends+Map<String,String>>+foo+){}
void+noWilds(+List<Map<String,String>>+foo+){}

void+main(+String[]+args+){
++++List<HashMap<String,String>>+myMap;

++++withWilds(+myMap+);+//+Works
++++noWilds(+myMap+);+//+Compiler+error
}|400841|您可能认为a+List+of+HashMaps应该是a+List+of+Maps，但有一个很好的理由证明它不是：|offset|length|style|CODE|400842|假设你可以这样做：|400843|List<HashMap<String,String>>+hashMaps+=+new+ArrayList<HashMap<String,String>>();

List<Map<String,String>>+maps+=+hashMaps;+//+Won't+compile,
++++++++++++++++++++++++++++++++++++++++++//+but+imagine+that+it+could

Map<String,String>+aMap+=+Collections.singletonMap("foo","bar");+//+Not+a+HashMap

maps.add(+aMap+);+//+Perfectly+legal+(adding+a+Map+to+a+List+of+Maps)

//+But+maps+and+hashMaps+are+the+same+object,+so+this+should+be+the+same+as

hashMaps.add(+aMap+);+//+Should+be+illegal+(aMap+is+not+a+HashMap)|400844|这就是为什么a+List+of+HashMaps不应该是List+of+Maps的原因。|400845|entityMap^0|0|0|0|0|0|0|0|0|7|4|F|7|S|4|10|3|0|0|0|8|4|G|7|S|4|10|3|0^^$0|@$1|2|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|18|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|19|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|1A|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|1B|8|@]|9|@]|A|$E|F]]|$1|O|3|P|5|6|7|1C|8|@]|9|@]|A|$]]|$1|Q|3|R|5|D|7|1D|8|@]|9|@]|A|$E|F]]|$1|S|3|T|5|6|7|1E|8|@$U|1F|V|1G|W|X]|$U|1H|V|1I|W|X]|$U|1J|V|1K|W|X]|$U|1L|V|1M|W|X]]|9|@]|A|$]]|$1|Y|3|Z|5|6|7|1N|8|@]|9|@]|A|$]]|$1|10|3|11|5|D|7|1O|8|@]|9|@]|A|$E|F]]|$1|12|3|13|5|6|7|1P|8|@$U|1Q|V|1R|W|X]|$U|1S|V|1T|W|X]|$U|1U|V|1V|W|X]|$U|1W|V|1X|W|X]]|9|@]|A|$]]|$1|14|3|-4|5|6|7|1Y|8|@]|9|@]|A|$]]]|15|$]]

The difference is that, for example, a

<pre><code>List&lt;HashMap&lt;String,String&gt;&gt;
</code></pre>

is a

<pre><code>List&lt;? extends Map&lt;String,String&gt;&gt;
</code></pre>

but not a

<pre><code>List&lt;Map&lt;String,String&gt;&gt;
</code></pre>

So:

<pre><code>void withWilds( List&lt;? extends Map&lt;String,String&gt;&gt; foo ){}
void noWilds( List&lt;Map&lt;String,String&gt;&gt; foo ){}

void main( String[] args ){
 List&lt;HashMap&lt;String,String&gt;&gt; myMap;

 withWilds( myMap ); // Works
 noWilds( myMap ); // Compiler error
}
</code></pre>

You would think a <code>List</code> of <code>HashMap</code>s should be a <code>List</code> of <code>Map</code>s, but there's a good reason why it isn't:

Suppose you could do:

<pre><code>List&lt;HashMap&lt;String,String&gt;&gt; hashMaps = new ArrayList&lt;HashMap&lt;String,String&gt;&gt;();

List&lt;Map&lt;String,String&gt;&gt; maps = hashMaps; // Won't compile,
 // but imagine that it could

Map&lt;String,String&gt; aMap = Collections.singletonMap("foo","bar"); // Not a HashMap

maps.add( aMap ); // Perfectly legal (adding a Map to a List of Maps)

// But maps and hashMaps are the same object, so this should be the same as

hashMaps.add( aMap ); // Should be illegal (aMap is not a HashMap)
</code></pre>

So this is why a <code>List</code> of <code>HashMap</code>s shouldn't be a <code>List</code> of <code>Map</code>s.

blocks|key|400939|text|今天，我已经使用了这个功能，所以这是我非常新鲜的现实生活中的例子。(我已经将类和方法名称更改为泛型名称，这样它们就不会分散对实际问题的注意力。)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|400940|我有一个方法，用于接受我最初使用此签名编写的A对象的Set：|offset|length|style|CODE|400941|void+myMethod(Set<A>+set)|code-block|syntax|javascript|400942|但它实际上想用A的子类的Set来调用它。但这是不允许的！(这样做的原因是，myMethod可以向set添加类型为A的对象，但不是set的对象在调用方站点声明的子类型。因此，如果可能的话，这可能会破坏类型系统。)|400943|现在来看一下泛型，因为如果我使用这个方法签名，它就能正常工作：|400944|<T+extends+A>+void+myMethod(Set<T>+set)|400945|或者更短一些，如果你不需要在方法体中使用实际的类型：|400946|void+myMethod(Set<?+extends+A>+set)|400947|这样，set的类型就变成了A的实际子类型的对象集合，因此可以在子类中使用它，而不会危及类型系统。|400948|entityMap^0|0|M|1|Q|3|0|0|7|1|C|3|11|8|1C|3|1K|1|1S|3|0|0|0|0|0|3|3|D|1|0^^$0|@$1|2|3|4|5|6|7|10|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|11|8|@$D|12|E|13|F|G]|$D|14|E|15|F|G]]|9|@]|A|$]]|$1|H|3|I|5|J|7|16|8|@]|9|@]|A|$K|L]]|$1|M|3|N|5|6|7|17|8|@$D|18|E|19|F|G]|$D|1A|E|1B|F|G]|$D|1C|E|1D|F|G]|$D|1E|E|1F|F|G]|$D|1G|E|1H|F|G]|$D|1I|E|1J|F|G]]|9|@]|A|$]]|$1|O|3|P|5|6|7|1K|8|@]|9|@]|A|$]]|$1|Q|3|R|5|J|7|1L|8|@]|9|@]|A|$K|L]]|$1|S|3|T|5|6|7|1M|8|@]|9|@]|A|$]]|$1|U|3|V|5|J|7|1N|8|@]|9|@]|A|$K|L]]|$1|W|3|X|5|6|7|1O|8|@$D|1P|E|1Q|F|G]|$D|1R|E|1S|F|G]]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|1T|8|@]|9|@]|A|$]]]|Z|$]]

Today, I have used this feature, so here's my very fresh real-life example. (I have changed class and method names to generic ones so they won't distract from the actual point.)

I have a method that's meant to accept a <code>Set</code> of <code>A</code> objects that I originally wrote with this signature:

<pre><code>void myMethod(Set&lt;A&gt; set)
</code></pre>

But it want to actually call it with <code>Set</code>s of subclasses of <code>A</code>. But this is not allowed! (The reason for that is, <code>myMethod</code> could add objects to <code>set</code> that are of type <code>A</code>, but not of the subtype that <code>set</code>'s objects are declared to be at the caller's site. So this could break the type system if it were possible.)

Now here come generics to the rescue, because it works as intended if I use this method signature instead:

<pre><code>&lt;T extends A&gt; void myMethod(Set&lt;T&gt; set)
</code></pre>

or shorter, if you don't need to use the actual type in the method body:

<pre><code>void myMethod(Set&lt;? extends A&gt; set)
</code></pre>

This way, <code>set</code>'s type becomes a collection of objects of the actual subtype of <code>A</code>, so it becomes possible to use this with subclasses without endangering the type system.

Is there any difference between

<pre><code>List&lt;Map&lt;String, String&gt;&gt;
</code></pre>

and

<pre><code>List&lt;? extends Map&lt;String, String&gt;&gt;
</code></pre>

?

If there is no difference, what is the benefit of using <code>? extends</code>?

List<Map<String, String>> vs List<? extends Map<String, String>>

两者之间有什么区别吗？List<Map<String, String>>和List<? extends Map<String, String>>如果没有区别，使用? extends的好处是什么？

问List<Map<String，String>> vs List<？扩展Map<String、String>>
EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问List<Map<String，String>> vs List<？扩展Map<String、String>>EN

社区

活动

资源

关于

腾讯云开发者

热门产品

热门推荐

更多推荐

问List<Map<String，String>> vs List<？扩展Map<String、String>>
EN