blocks|key|586386|text|你不能。你可以在switch中使用原语(int，char，short，byte)和字符串(仅在java7中)。基元不能为空。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|586387|在切换之前，在单独的条件下检查i。|offset|length|style|CODE|586388|entityMap^0|0|F|1|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@$D|L|E|M|F|G]]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

You can't. You can use primitives (int, char, short, byte) and String (Strings in java 7 only) in switch. primitives can't be null. 
Check <code>i</code> in separate condition before switch.

blocks|key|370810|text|如果I为null，switch(i)将抛出NullPointerException，因为它将尝试将Integer拆箱为int。因此，碰巧是非法的case+null，无论如何都不会被联系到。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|370811|您需要在switch语句之前检查i是否不为null。|370812|entityMap^0|4|4|9|9|1D|7|1N|3|20|9|0|4|6|0^^$0|@$1|2|3|4|5|6|7|J|8|@$9|K|A|L|B|C]|$9|M|A|N|B|C]|$9|O|A|P|B|C]|$9|Q|A|R|B|C]|$9|S|A|T|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|U|8|@$9|V|A|W|B|C]]|D|@]|E|$]]|$1|H|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|I|$]]

<code>switch(i)</code> will throw a NullPointerException if i is <code>null</code>, because it will try to unbox the <code>Integer</code> into an <code>int</code>. So <code>case null</code>, which happens to be illegal, would never have been reached anyway.

You need to check that i is not null before the <code>switch</code> statement.

blocks|key|370833|text|你必须做一个|type|unstyled|depth|inlineStyleRanges|entityRanges|data|370834|if+(i+==+null)+{
+++doSomething0();
}+else+{
+++switch+(i)+{
+++}
}|code-block|syntax|javascript|370835|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|$]]

You have to make a 

<pre><code>if (i == null) {
 doSomething0();
} else {
 switch (i) {
 }
}
</code></pre>

blocks|key|586506|text|Java文档清楚地指出：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|586507|禁止使用null作为开关标签可以防止编写永远无法执行的代码。如果开关表达式是引用类型，如装箱的基元类型或枚举，则如果表达式在运行时计算为null，则会发生运行时错误。|586508|在执行Swithch语句之前，必须验证是否为null。|586509|if+(i+==+null)|code-block|syntax|javascript|586510|请参阅The+Switch+Statement|offset|length|586511|case+null:+//+will+never+be+executed,+therefore+disallowed.|586512|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/tutorial/java/nutsandbolts/switch.html^0|0|0|0|0|3|K|0|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|Z|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|10|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|11|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|12|8|@]|9|@$M|13|N|14|1|15]]|A|$]]|$1|O|3|P|5|H|7|16|8|@]|9|@]|A|$I|J]]|$1|Q|3|-4|5|6|7|17|8|@]|9|@]|A|$]]]|R|$S|$5|T|U|V|A|$W|X]]]]

Java docs clearly stated that:

The prohibition against using null as a switch label prevents one from writing code that can never be executed. If the switch expression is of a reference type, such as a boxed primitive type or an enum, a run-time error will occur if the expression evaluates to null at run-time.

You must have to verify for null before Swithch statement execution. 

<pre><code>if (i == null)
</code></pre>

See <a href="http://docs.oracle.com/javase/tutorial/java/nutsandbolts/switch.html" rel="noreferrer">The Switch Statement</a>

<pre><code>case null: // will never be executed, therefore disallowed.
</code></pre>

blocks|key|586518|text|switch+((i+!=+null)+?+i+:+DEFAULT_VALUE)+{
++++++++//...
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|586519|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>switch ((i != null) ? i : DEFAULT_VALUE) {
 //...
}
</code></pre>

blocks|key|586581|text|给定：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|586582|public+enum+PersonType+{
++++COOL_GUY(1),
++++JERK(2);

++++private+final+int+typeId;
++++private+PersonType(int+typeId)+{
++++++++this.typeId+=+typeId;
++++}

++++public+final+int+getTypeId()+{
++++++++return+typeId;
++++}

++++public+static+PersonType+findByTypeId(int+typeId)+{
++++++++for+(PersonType+type+:+values())+{
++++++++++++if+(type.typeId+==+typeId)+{
++++++++++++++++return+type;
++++++++++++}
++++++++}
++++++++return+null;
++++}
}|code-block|syntax|javascript|586583|对我来说，这通常与数据库中的查找表一致(仅适用于很少更新的表)。|586584|但是，当我尝试在switch语句中使用findByTypeId时(很可能来自用户输入)……|offset|length|style|CODE|586585|int+userInput+=+3;
PersonType+personType+=+PersonType.findByTypeId(userInput);
switch(personType)+{
case+COOL_GUY:
++++//+Do+things+only+a+cool+guy+would+do.
++++break;
case+JERK:
++++//+Push+back.+Don't+enable+him.
++++break;
default:
++++//+I+don't+know+or+care+what+to+do+with+this+mess.
}|586586|其他人已经声明，这将导致NPE+@+switch(personType)+{。我开始实现的一个变通方法(即“解决方案”)是添加一个UNKNOWN(-1)类型。|586587|public+enum+PersonType+{
++++UNKNOWN(-1),
++++COOL_GUY(1),
++++JERK(2);
++++...
++++public+static+PersonType+findByTypeId(int+id)+{
++++++++...
++++++++return+UNKNOWN;
++++}
}|586588|现在，您不必在它计数的地方进行null检查，您可以选择处理或不处理UNKNOWN类型。(注意:在业务场景中，-1是一个不太可能的标识符，但显然要选择对您的用例有意义的内容)。|586589|entityMap^0|0|0|0|J|C|0|0|I|K|1T|B|0|0|X|7|1I|2|0^^$0|@$1|2|3|4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|10|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|11|8|@$K|12|L|13|M|N]]|9|@]|A|$]]|$1|O|3|P|5|D|7|14|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|15|8|@$K|16|L|17|M|N]|$K|18|L|19|M|N]]|9|@]|A|$]]|$1|S|3|T|5|D|7|1A|8|@]|9|@]|A|$E|F]]|$1|U|3|V|5|6|7|1B|8|@$K|1C|L|1D|M|N]|$K|1E|L|1F|M|N]]|9|@]|A|$]]|$1|W|3|-4|5|6|7|1G|8|@]|9|@]|A|$]]]|X|$]]

Given:

<pre><code>public enum PersonType {
 COOL_GUY(1),
 JERK(2);

 private final int typeId;
 private PersonType(int typeId) {
 this.typeId = typeId;
 }

 public final int getTypeId() {
 return typeId;
 }

 public static PersonType findByTypeId(int typeId) {
 for (PersonType type : values()) {
 if (type.typeId == typeId) {
 return type;
 }
 }
 return null;
 }
}
</code></pre>

For me, this typically aligns with a look-up table in a database (for rarely-updated tables only).

However, when I try to use <code>findByTypeId</code> in a switch statement (from, most likely, user input)...

<pre><code>int userInput = 3;
PersonType personType = PersonType.findByTypeId(userInput);
switch(personType) {
case COOL_GUY:
 // Do things only a cool guy would do.
 break;
case JERK:
 // Push back. Don't enable him.
 break;
default:
 // I don't know or care what to do with this mess.
}
</code></pre>

...as others have stated, this results in an NPE @ <code>switch(personType) {</code>. One work-around (i.e., "solution") I started implementing was to add an <code>UNKNOWN(-1)</code> type.

<pre><code>public enum PersonType {
 UNKNOWN(-1),
 COOL_GUY(1),
 JERK(2);
 ...
 public static PersonType findByTypeId(int id) {
 ...
 return UNKNOWN;
 }
}
</code></pre>

Now, you don't have to do null-checking where it counts and you can choose to, or not to, handle <code>UNKNOWN</code> types. (NOTE: <code>-1</code> is an unlikely identifier in a business scenario, but obviously choose something that makes sense for your use-case).

blocks|key|371022|text|一些库试图提供内置java+switch语句的替代方案。Vavr就是其中之一，他们将其推广到模式匹配。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|371023|下面是来自their+documentation的一个示例|371024|String+s+=+Match(i).of(
++++Case($(1),+"one"),
++++Case($(2),+"two"),
++++Case($(),+"?")
);|code-block|syntax|javascript|371025|您可以使用任何谓词，但他们提供了许多开箱即用的谓词，而且$(null)是完全合法的。我发现这是一个比替代方案更优雅的解决方案，但这需要java8和对vavr库的依赖……|371026|entityMap|0|LINK|mutability|MUTABLE|url|http://www.vavr.io/|1|http://www.vavr.io/vavr-docs/#_pattern_matching^0|E|6|S|4|0|0|5|J|1|0|0|S|7|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]]|D|@$9|11|A|12|1|13]]|E|$]]|$1|F|3|G|5|6|7|14|8|@]|D|@$9|15|A|16|1|17]]|E|$]]|$1|H|3|I|5|J|7|18|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|19|8|@$9|1A|A|1B|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|1C|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]|W|$5|R|S|T|E|$U|X]]]]

Some libraries attempt to offer alternatives to the builtin java <code>switch</code> statement. <a href="http://www.vavr.io" rel="nofollow noreferrer">Vavr</a> is one of them, they generalize it to pattern matching.

Here is an example from <a href="http://www.vavr.io/vavr-docs/#_pattern_matching" rel="nofollow noreferrer">their documentation</a>:

<pre><code>String s = Match(i).of(
 Case($(1), "one"),
 Case($(2), "two"),
 Case($(), "?")
);
</code></pre>

You can use any predicate, but they offer many of them out of the box, and <code>$(null)</code> is perfectly legal. I find this a more elegant solution than the alternatives, but this requires java8 and a dependency on the vavr library...

blocks|key|586640|text|switch+(String.valueOf(value)){
++++case+"null":
++++default:+
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|586641|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>switch (String.valueOf(value)){
 case "null":
 default: 
}
</code></pre>

blocks|key|586676|text|基于@tetsuo答案，使用java+8：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|586677|Integer+i+=+...

switch+(Optional.ofNullable(i).orElse(DEFAULT_VALUE))+{
++++case+DEFAULT_VALUE:
++++++++doDefault();
++++++++break;++++
}|code-block|syntax|javascript|586678|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|$]]

Based on @tetsuo answer, with java 8 : 

<pre><code>Integer i = ...

switch (Optional.ofNullable(i).orElse(DEFAULT_VALUE)) {
 case DEFAULT_VALUE:
 doDefault();
 break; 
}
</code></pre>

blocks|key|586741|text|我今天才学到，你不需要再加一层缩进/大括号，因为if需要检查null。您可以执行以下任一操作：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|586742|if+(i+==+null)+{
++++//+...
}+else+switch+(i)+{
++++case+1:
++++++++//+...
++++++++break;
++++default:
++++++++//+...
}|code-block|syntax|javascript|586743|或|586744|if+(i+!=+null)+switch+(i)+{
++++case+1:
++++++++//+...
++++++++break;
++++default:
++++++++//+...
}+else+{
++++//+...
}|586745|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|$]]

I just learned today that you dont have to put another layer of indentation/curly brackets just because of the if that is checking for null. You can do either of these:
<pre><code>if (i == null) {
 // ...
} else switch (i) {
 case 1:
 // ...
 break;
 default:
 // ...
}
</code></pre>
or
<pre><code>if (i != null) switch (i) {
 case 1:
 // ...
 break;
 default:
 // ...
} else {
 // ...
}
</code></pre>

<pre><code>Integer i = ...

switch (i){
 case null:
 doSomething0();
 break; 
 }
</code></pre>

In the code above I cant use null in switch case statement. How can I do this differently?
I can't use <code>default</code> because then I want to do something else.

How to use null in switch

数据库

Java

Integer i = ...switch (i){    case null:        doSomething0();        break;        }在上面的代码中，我不能在switch case语句中使用null。我怎样才能以不同的方式做到这一点呢？我不能使用default，因为我想做其他的事情...

问如何在开关中使用null
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

问如何在开关中使用nullEN

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

问如何在开关中使用null
EN