blocks|key|80124|text|main是一个有效的java标识符，老师错了。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|80125|相关文档在Java+Language+Specification中，就在这里：|80126|第三章。“词汇结构”，3.8节。“标识符”：|80127|https://docs.oracle.com/javase/specs/jls/se10/html/jls-3.html#jls-3.8|80128|上面写着：|80129|Java标识符是由|80130|字母和数字组成的无长度限制的序列，其中第一个必须是Java字母...标识符不能与关键字(§3.9)、布尔文字(§3.10.3)或空文字(§3.10.7)具有相同的拼写(Unicode字符序列)，否则会发生编译时错误。|blockquote|80131|80132|80133|这意味着您可以通过以下方式证明它是有效的标识符：|80134|80135|在java关键字列表中查找它(提示:您不会在那里找到它！)或者简单地通过|unordered-list-item|80136|使用它作为标识符，并观察到没有发生编译时错误。|80137|80138|entityMap|0|LINK|mutability|MUTABLE|url^0|0|4|0|0|0|0|1X|0|0|0|0|0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1A|8|@$9|1B|A|1C|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1D|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|1E|8|@]|D|@]|E|$]]|$1|J|3|K|5|6|7|1F|8|@]|D|@$9|1G|A|1H|1|1I]]|E|$]]|$1|L|3|M|5|6|7|1J|8|@]|D|@]|E|$]]|$1|N|3|O|5|6|7|1K|8|@]|D|@]|E|$]]|$1|P|3|Q|5|R|7|1L|8|@]|D|@]|E|$]]|$1|S|3|-4|5|6|7|1M|8|@]|D|@]|E|$]]|$1|T|3|-4|5|6|7|1N|8|@]|D|@]|E|$]]|$1|U|3|V|5|6|7|1O|8|@]|D|@]|E|$]]|$1|W|3|-4|5|6|7|1P|8|@]|D|@]|E|$]]|$1|X|3|Y|5|Z|7|1Q|8|@]|D|@]|E|$]]|$1|10|3|11|5|Z|7|1R|8|@]|D|@]|E|$]]|$1|12|3|-4|5|6|7|1S|8|@]|D|@]|E|$]]|$1|13|3|-4|5|6|7|1T|8|@]|D|@]|E|$]]]|14|$15|$5|16|17|18|E|$19|K]]]]

<code>main</code> is a valid java identifier, and the teacher is wrong. 

The relevant documentation is in the Java Language Specification, right here:

Chapter 3. "Lexical Structure", section 3.8. "Identifiers":

<a href="https://docs.oracle.com/javase/specs/jls/se10/html/jls-3.html#jls-3.8" rel="nofollow noreferrer">https://docs.oracle.com/javase/specs/jls/se10/html/jls-3.html#jls-3.8</a>

It says:

<blockquote>
 An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter... An identifier cannot have the same spelling (Unicode character sequence) as a keyword (§3.9), boolean literal (§3.10.3), or the null literal (§3.10.7), or a compile-time error occurs.
</blockquote>

Which means that you can prove that it is a valid identifier either by:

<ul>
<li>looking for it in the list of java keywords (hint: you won't find it there!) or simply by </li>
<li>using it as an identifier and observing that no compile-time error occurs.</li>
</ul>

blocks|key|80312|text|main是完全有效的，因为它，from+the+docs|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|80313|Java+|80314|是“一系列Java字母和Java数字，其中第一个是Java字母”，|unordered-list-item|80315|
|80316|不是keyword|80317|80318|Is不是null+literal|80319|
，即"true“或”false“|80320|+boolean+literal不是null+literal|80321|80322|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/specs/jls/se8/html/jls-3.html#jls-Identifier|1|https://docs.oracle.com/javase/specs/jls/se10/html/jls-3.html#jls-3.8|2|https://docs.oracle.com/javase/specs/jls/se8/html/jls-3.html#jls-3.10.7|3|https://docs.oracle.com/javase/specs/jls/se8/html/jls-3.html#jls-3.10.3|4^0|0|4|F|D|0|0|0|0|0|2|7|1|0|0|4|C|2|0|0|1|F|3|I|C|4|0|0^^$0|@$1|2|3|4|5|6|7|1B|8|@$9|1C|A|1D|B|C]]|D|@$9|1E|A|1F|1|1G]]|E|$]]|$1|F|3|G|5|6|7|1H|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1I|8|@]|D|@]|E|$]]|$1|K|3|L|5|6|7|1J|8|@]|D|@]|E|$]]|$1|M|3|N|5|J|7|1K|8|@]|D|@$9|1L|A|1M|1|1N]]|E|$]]|$1|O|3|L|5|6|7|1O|8|@]|D|@]|E|$]]|$1|P|3|Q|5|J|7|1P|8|@]|D|@$9|1Q|A|1R|1|1S]]|E|$]]|$1|R|3|S|5|6|7|1T|8|@]|D|@]|E|$]]|$1|T|3|U|5|J|7|1U|8|@]|D|@$9|1V|A|1W|1|1X]|$9|1Y|A|1Z|1|20]]|E|$]]|$1|V|3|-4|5|6|7|21|8|@]|D|@]|E|$]]|$1|W|3|-4|5|6|7|22|8|@]|D|@]|E|$]]]|X|$Y|$5|Z|10|11|E|$12|13]]|14|$5|Z|10|11|E|$12|15]]|16|$5|Z|10|11|E|$12|17]]|18|$5|Z|10|11|E|$12|19]]|1A|$5|Z|10|11|E|$12|17]]]]

<code>main</code> is perfectly valid because it, <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-3.html#jls-Identifier" rel="noreferrer">from the docs</a>:

<ol>
<li>Is a "sequence of Java letters and Java digits, the first of which is a Java letter"</li>
<li>Is not a <a href="https://docs.oracle.com/javase/specs/jls/se10/html/jls-3.html#jls-3.8" rel="noreferrer">keyword</a></li>
<li>Is not a <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-3.html#jls-3.10.3" rel="noreferrer">boolean literal</a> i.e. "true" or "false"</li>
<li>Is not <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-3.html#jls-3.10.7" rel="noreferrer">null literal</a></li>
</ol>

blocks|key|80140|text|public+class+J+{
++++public+static+void+main(String[]+args)
++++{
++++++++String+main+=+"The+character+sequence+\"main\"+is+an+identifier,+not+a+keyword+or+reserved+word.";
++++++++System.out.println(main);
++++}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|80141|这段代码将进行编译，并在执行时发出以下输出：|unstyled|80142|The+character+sequence+"main"+is+an+identifier,+not+a+keyword+or+reserved+word.|80143|字符序列main是标识符，而不是关键字或保留字。|offset|length|style|CODE|80144|The+relevant+section+of+the+JLS+is+3.8|80145|80146|标识符是由Java字母和Java数字组成的无长度限制的序列，其中第一个必须是Java字母。|blockquote|80147|80148|标识符：|80149|返回IdentifierChars，但不是关键字、BooleanLiteral或NullLiteral|80150|IdentifierChars：|80151|推送JavaLetter+{JavaLetterOrDigit}|80152|JavaLetter：|80153|不包括任何为"Java字母“的Unicode字符|80154|JavaLetterOrDigit：|80155|不允许任何"Java字母或数字“的Unicode字符|80156|80157|字符序列main符合上述描述，并且不在keyword+list+in+Section+3.9中。|80158|(出于同样的原因，字符序列java1234也是一个标识符。)|80159|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/specs/jls/se10/html/jls-3.html#jls-3.8|1|https://docs.oracle.com/javase/specs/jls/se10/html/jls-3.html#jls-3.9^0|0|0|0|4|4|0|4|Y|0|0|0|0|0|0|0|0|0|0|0|0|0|0|4|4|J|R|1|0|D|8|0^^$0|@$1|2|3|4|5|6|7|1Q|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|1R|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|1S|8|@]|9|@]|A|$B|C]]|$1|I|3|J|5|F|7|1T|8|@$K|1U|L|1V|M|N]]|9|@]|A|$]]|$1|O|3|P|5|F|7|1W|8|@]|9|@$K|1X|L|1Y|1|1Z]]|A|$]]|$1|Q|3|-4|5|F|7|20|8|@]|9|@]|A|$]]|$1|R|3|S|5|T|7|21|8|@]|9|@]|A|$]]|$1|U|3|-4|5|F|7|22|8|@]|9|@]|A|$]]|$1|V|3|W|5|F|7|23|8|@]|9|@]|A|$]]|$1|X|3|Y|5|F|7|24|8|@]|9|@]|A|$]]|$1|Z|3|10|5|F|7|25|8|@]|9|@]|A|$]]|$1|11|3|12|5|F|7|26|8|@]|9|@]|A|$]]|$1|13|3|14|5|F|7|27|8|@]|9|@]|A|$]]|$1|15|3|16|5|F|7|28|8|@]|9|@]|A|$]]|$1|17|3|18|5|F|7|29|8|@]|9|@]|A|$]]|$1|19|3|1A|5|F|7|2A|8|@]|9|@]|A|$]]|$1|1B|3|-4|5|F|7|2B|8|@]|9|@]|A|$]]|$1|1C|3|1D|5|F|7|2C|8|@$K|2D|L|2E|M|N]]|9|@$K|2F|L|2G|1|2H]]|A|$]]|$1|1E|3|1F|5|F|7|2I|8|@$K|2J|L|2K|M|N]]|9|@]|A|$]]|$1|1G|3|-4|5|F|7|2L|8|@]|9|@]|A|$]]]|1H|$1I|$5|1J|1K|1L|A|$1M|1N]]|1O|$5|1J|1K|1L|A|$1M|1P]]]]

<pre><code>public class J {
 public static void main(String[] args)
 {
 String main = &quot;The character sequence \&quot;main\&quot; is an identifier, not a keyword or reserved word.&quot;;
 System.out.println(main);
 }
}
</code></pre>
This compiles, and when executed, emits this output:
<pre><code>The character sequence &quot;main&quot; is an identifier, not a keyword or reserved word.
</code></pre>
The character sequence <code>main</code> is an identifier, not a keyword or reserved word.
The <a href="https://docs.oracle.com/javase/specs/jls/se10/html/jls-3.html#jls-3.8" rel="noreferrer">relevant section of the JLS is 3.8</a>:
<blockquote>
An identifier is an unlimited-length sequence of Java letters and Java digits, the first of which must be a Java letter.
Identifier:
    IdentifierChars but not a Keyword or BooleanLiteral or NullLiteral
IdentifierChars:
    JavaLetter {JavaLetterOrDigit}
JavaLetter:
    any Unicode character that is a &quot;Java letter&quot;
JavaLetterOrDigit:
    any Unicode character that is a &quot;Java letter-or-digit&quot;
</blockquote>
The character sequence <code>main</code> fits the above description and is not in the <a href="https://docs.oracle.com/javase/specs/jls/se10/html/jls-3.html#jls-3.9" rel="noreferrer">keyword list in Section 3.9</a>.
(The character sequence <code>java1234</code> is also an identifier, for the same reasons.)

blocks|key|82072|text|当main被用作声明"main“方法的标识符时，它怎么不能用作标识符呢？|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|82073|对于这样一个经典的成语：|82074|public+class+Foo{
+++public+static+void+main(String[]+args){
+++}
}|code-block|syntax|javascript|82075|main不是一个关键字，由于明显的追溯兼容性原因，它在Java语言中可能永远不会成为关键字。|82076|82077|关于这个问题，main是一个好的标识符吗？|82078|首先:对编译器有效并不意味着一定是好的。|82079|例如，提出的java1234选项也是一个有效的标识符，但这确实应该避免。|82080|main有一个非常特殊和重要的含义:它用作java命令行执行的类和jars的入口点方法。|82081|如果方法名不符合java命令行使用的标准，则使用main作为变量名或类名可能更有意义，这会产生误导。|82082|例如，将表示应用程序入口点的类定义为应用程序的Main类是可以接受的，因此也可以将其用作变量名，例如：|82083|public+class+Main+{

++public+static+void+main(String+args[]){
+++++Main+main+=+new+Main();
+++++//+...
++}++++++

}|82084|一般来说，在Java中，多个字符或“单词”被认为是编译器的有效标识符，但强烈建议不要在客户端代码中使用它们(但生成的代码可能会这样做:例如:嵌套类)，因为它们不可读和/或确实具有误导性。|82085|例如，这可能对编译器有效：|82086|public+class+Object+{+//+1
++++public+void+foo()+{
+++++++...
++++}
}

public+class+BadChosenIdentifier+{

++++public+static+void+main()+{+//+2
++++++++new+BadChosenIdentifier().toString(new+Object());++
++++}

++++public+void+toString(Object+java1234)+{+//+3,+4
++++++++String+_result$+=+java1234+%2B+"+->+to+avoid";+//+4
++++++++System.out.println(_result$);
++++}++++
}|82087|但我们不想：|82088|82089|将Object命名为java.lang(1)中定义的类。如果不满足java命令行使用的标准，则使用|unordered-list-item|82090|将方法命名为main()+(2)。|82091|重载Object.toString()方法(3)。|82092|使用_、$或任何违反共享命名约定的令人惊讶的/无意义的字符来命名变量(4)。|82093|82094|entityMap^0|1|4|0|0|0|0|4|0|0|7|4|0|0|6|8|0|0|4|L|4|0|8|4|O|4|0|N|4|0|0|0|0|0|0|0|1|6|A|9|X|4|0|6|6|0|2|H|0|2|1|4|1|0|0^^$0|@$1|2|3|4|5|6|7|1O|8|@$9|1P|A|1Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|1R|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|1S|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1T|8|@$9|1U|A|1V|B|C]]|D|@]|E|$]]|$1|O|3|-4|5|6|7|1W|8|@]|D|@]|E|$]]|$1|P|3|Q|5|6|7|1X|8|@$9|1Y|A|1Z|B|C]]|D|@]|E|$]]|$1|R|3|S|5|6|7|20|8|@]|D|@]|E|$]]|$1|T|3|U|5|6|7|21|8|@$9|22|A|23|B|C]]|D|@]|E|$]]|$1|V|3|W|5|6|7|24|8|@$9|25|A|26|B|C]|$9|27|A|28|B|C]]|D|@]|E|$]]|$1|X|3|Y|5|6|7|29|8|@$9|2A|A|2B|B|C]|$9|2C|A|2D|B|C]]|D|@]|E|$]]|$1|Z|3|10|5|6|7|2E|8|@$9|2F|A|2G|B|C]]|D|@]|E|$]]|$1|11|3|12|5|J|7|2H|8|@]|D|@]|E|$K|L]]|$1|13|3|14|5|6|7|2I|8|@]|D|@]|E|$]]|$1|15|3|16|5|6|7|2J|8|@]|D|@]|E|$]]|$1|17|3|18|5|J|7|2K|8|@]|D|@]|E|$K|L]]|$1|19|3|1A|5|6|7|2L|8|@]|D|@]|E|$]]|$1|1B|3|-4|5|6|7|2M|8|@]|D|@]|E|$]]|$1|1C|3|1D|5|1E|7|2N|8|@$9|2O|A|2P|B|C]|$9|2Q|A|2R|B|C]|$9|2S|A|2T|B|C]]|D|@]|E|$]]|$1|1F|3|1G|5|1E|7|2U|8|@$9|2V|A|2W|B|C]]|D|@]|E|$]]|$1|1H|3|1I|5|1E|7|2X|8|@$9|2Y|A|2Z|B|C]]|D|@]|E|$]]|$1|1J|3|1K|5|1E|7|30|8|@$9|31|A|32|B|C]|$9|33|A|34|B|C]]|D|@]|E|$]]|$1|1L|3|-4|5|6|7|35|8|@]|D|@]|E|$]]|$1|1M|3|-4|5|6|7|36|8|@]|D|@]|E|$]]]|1N|$]]

How <code>main</code> could not be used as an identifier while it is used as identifier to declare the "main" method ? 

For such a classic idiom : 

<pre><code>public class Foo{
 public static void main(String[] args){
 }
}
</code></pre>

<code>main</code> is not a keyword and it would probably never be a keyword in Java for obvious retro compatibility reasons. 

<hr>

About the question, is <code>main</code> a good identifier ? 

First : valid for a compiler doesn't mean necessarily good. 
For example the <code>java1234</code> option that is proposed is also a valid identifier but that should really be avoided. 

<code>main</code> has a very particularly and important meaning : it is used as the entry point method of classes and jars executed by the <code>java</code> command line. 
Using <code>main</code> for a method name that doesn't fill the criteria to be used by the <code>java</code> command line would be just misleading while using it as variable name or a class name could make sense. 
For example defining the class representing the entry point of an application as the <code>Main</code> class of the application is acceptable and so using it as variable name too such as : 

<pre><code>public class Main {

 public static void main(String args[]){
 Main main = new Main();
 // ...
 } 

}
</code></pre>

In a general way, in Java, multiple characters or "words" are considered valid identifiers for the compiler but are strongly discouraged to be used in the client code (but generated code may do that : nested classes for example) as not readable and/or really misleading. 

For example this could be valid for the compiler :

<pre><code>public class Object { // 1
 public void foo() {
 ...
 }
}

public class BadChosenIdentifier {

 public static void main() { // 2
 new BadChosenIdentifier().toString(new Object()); 
 }

 public void toString(Object java1234) { // 3, 4
 String _result$ = java1234 + " -&gt; to avoid"; // 4
 System.out.println(_result$);
 } 
}
</code></pre>

But we don't want :

<ul>
<li>to name <code>Object</code> our class as this is defined in <code>java.lang</code>(1). </li>
<li>to name a method <code>main()</code> if doesn't fill the criteria to be used by the <code>java</code> command line (2). </li>
<li>to overload the <code>Object.toString()</code> method (3). </li>
<li>to name our variables with <code>_</code>, <code>$</code> or any surprising/unmeaningful characters that go against the shared naming conventions (4). </li>
</ul>

blocks|key|82680|text|public+class+Main+{
++++private+static+String+main;
++++public+static+void+main(String[]+main)+{
++++++++Main.main+=+main[0];
++++++++new+Main().main(Main.main);
++++}
++++private+void+main(String+main)+{
++++++++System.out.println(main);
++++}
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|82681|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 class="lang-java prettyprint-override"><code>public class Main {
 private static String main;
 public static void main(String[] main) {
 Main.main = main[0];
 new Main().main(Main.main);
 }
 private void main(String main) {
 System.out.println(main);
 }
}
</code></pre>

blocks|key|2595055|text|这在Java+1.8上编译得很好...|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2595056|public+class+main+{

++++public+String+main+=+"main";+

++++public+void+main(String+main)+{
++++++++System.out.println("This+object+is+an+instance+of+the+class+"+%2B+this.getClass().getCanonicalName());
++++++++System.out.println("The+value+of+the+argument+\"main\"+for+this+call+to+the+method+\"main(String+main)\"+is+"+%2B+main);
++++++++System.out.println("The+value+of+the+field+\"main\"+is+"+%2B+this.main);
++++}

++++public+static+void+main(String[]+args)+{
++++++++main+main+=+new+main();
++++++++main.main(main.main+%2B+main.main);
++++}
}|code-block|syntax|javascript|2595057|2595058|This+object+is+an+instance+of+the+class+main
The+value+of+the+argument+"main"+for+this+call+to+the+method+"main(String+main)"+is+mainmain
The+value+of+the+field+"main"+is+main|2595059|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|M|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|N|8|@]|9|@]|A|$]]|$1|H|3|I|5|D|7|O|8|@]|9|@]|A|$E|F]]|$1|J|3|-4|5|6|7|P|8|@]|9|@]|A|$]]]|K|$]]

This compiles fine on Java 1.8...

<pre class="lang-java prettyprint-override"><code>public class main {

 public String main = "main"; 

 public void main(String main) {
 System.out.println("This object is an instance of the class " + this.getClass().getCanonicalName());
 System.out.println("The value of the argument \"main\" for this call to the method \"main(String main)\" is " + main);
 System.out.println("The value of the field \"main\" is " + this.main);
 }

 public static void main(String[] args) {
 main main = new main();
 main.main(main.main + main.main);
 }
}
</code></pre>

...and when executed produces the output:

<pre><code>This object is an instance of the class main
The value of the argument "main" for this call to the method "main(String main)" is mainmain
The value of the field "main" is main
</code></pre>

blocks|key|82714|text|它是有效的标识符吗？是。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|82715|它是一个好的标识符吗？如果您将其用于除JVM启动时启动的方法之外的任何其他用途，则不会。|82716|是否列出了另一个有效标识符？是。|82717|测试说明是否说要选择最好的答案？|82718|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|J|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|K|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|L|8|@]|9|@]|A|$]]|$1|F|3|G|5|6|7|M|8|@]|9|@]|A|$]]|$1|H|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|I|$]]

Is it a valid identifier? Yes.

Is it a good identifier? Not if you're using it for anything other than the method that starts at JVM launch.

Is another valid identifier listed? Yes.

Did the test instructions say to choose the best answer?

blocks|key|82161|text|我会说main是一个有效的标识符。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|82162|package+main;

public+class+main+{

++++static+main+main;
++++String+Main;

++++main(String+main)+{
++++++++Main+=+main;
++++}

++++main(main+main)+{
++++++++System.out.println(main.Main);
++++}

++++main+main(main+main)+{
++++++++return+new+main(main);
++++}

++++public+static+void+main(main...Main)+{
++++++++main:
++++++++for+(main+main+:+Main)+{
++++++++++++main+=+(main+instanceof+Main)+?+new+main(main):+main.main(main);
++++++++++++break+main;
++++++++}
++++}

++++public+static+void+main(String[]+args)+{
++++++++main+=+new+main("main");
++++++++main.main(main,+main);
++++++++main+=+main.new+Main(main)+{
++++++++++++main+main(main+main)+{
++++++++++++++++return+((Main)main).main();
++++++++++++}
++++++++};
++++++++main.main(main);
++++++++main.main(main,main);
++++}

++++abstract+class+Main+extends+main+{
++++++++Main(main+main)+{
++++++++++++super("main");
++++++++}

++++++++main+main()+{
++++++++++++main.Main+=+"Main";
++++++++++++return+main;
++++++++}
++++}
}|code-block|syntax|javascript|82163|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|$]]

I threw everything I could at it, and it appears to work. I'd say main is a valid identifier.

<pre><code>package main;

public class main {

 static main main;
 String Main;

 main(String main) {
 Main = main;
 }

 main(main main) {
 System.out.println(main.Main);
 }

 main main(main main) {
 return new main(main);
 }

 public static void main(main...Main) {
 main:
 for (main main : Main) {
 main = (main instanceof Main) ? new main(main): main.main(main);
 break main;
 }
 }

 public static void main(String[] args) {
 main = new main("main");
 main.main(main, main);
 main = main.new Main(main) {
 main main(main main) {
 return ((Main)main).main();
 }
 };
 main.main(main);
 main.main(main,main);
 }

 abstract class Main extends main {
 Main(main main) {
 super("main");
 }

 main main() {
 main.Main = "Main";
 return main;
 }
 }
}
</code></pre>

blocks|key|80485|text|main和java123都是有效的主标识符，不是保留关键字，所以使用它是完全可以接受的，就测试而言，你至少应该得到一分或半分。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|BOLD|entityRanges|data|80486|entityMap^0|0|4|5|7|E|8|0^^$0|@$1|2|3|4|5|6|7|I|8|@$9|J|A|K|B|C]|$9|L|A|M|B|C]|$9|N|A|O|B|D]]|E|@]|F|$]]|$1|G|3|-4|5|6|7|P|8|@]|E|@]|F|$]]]|H|$]]

Both <code>main</code> and <code>java123</code> are valid identifiers, main isn’t a reserved keyword so it’s perfectly acceptable to use, as far as the test goes you should’ve gotten a point or half a point at least.

blocks|key|80494|text|那个老师犯了一个小错误，要么假设main不是一个有效的标识符，要么只是把问题写错了。他可能是想说“一个好的识别者”。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|80495|但是，忽视您儿子的论点，从而阻碍他检查相关文献(Java规范)和执行实验(编写示例程序)的科学方法，是与教师应该做的事情完全相反的。|80496|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

That teacher made a minor mistake in either assuming main is not a valid identifier or simply phrasing the question wrong. He possibly meant to say "a good identifier". 
But ignoring your sons arguments and thereby discouraging his scientific approach of checking relevant literature (Java specification) and performing an experiment (writing a sample program) is the exact opposite of what a teacher is supposed to do.

blocks|key|2595212|text|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2595213|应为单个单词。也就是说，不允许使用空格。|ordered-list-item|2595214|2595215|例如:芒果是有效的，但芒果的价格不是valid.|2595216|Should，以字母(字母)或下划线或$开头。|unordered-list-item|offset|length|style|BOLD|2595217|2595218|例如:+identifiers.|2595219|Should，_price和$price是有效的，而不是的关键字，因为关键字对编译器有特殊的意义。|2595220|2595221|示例:+class+or+void+etc.|2595222|Should不是以数字开头，而是数字可以在中间或结尾。|2595223|2595224|例如:5mangoes是无效的，mango5cost和mangocost5是一个标识符的valid.|2595225|Length在Java中可以是65,535个字符，并且都是有意义的。标识符区分大小写。也就是说，芒果和芒果都被区别对待。可以包含所有大写字母、小写字母或混合字母。|2595226|2595227|标识符：它们是类名、方法名、变量名……|2595228|由于main不是保留字，并且根据上面用于定义标识符的解释，main是有效的标识符，因此和java1234+also.Remaining选项由于上述解释而无效。|2595229|entityMap^0|0|0|0|0|K|3|0|0|0|G|0|S|L|0|0|0|M|0|0|R|0|0|0|1E|0|0|29|0|0|0|J|0|0|27|0^^$0|@$1|2|3|-4|4|5|6|19|7|@]|8|@]|9|$]]|$1|A|3|B|4|C|6|1A|7|@]|8|@]|9|$]]|$1|D|3|-4|4|5|6|1B|7|@]|8|@]|9|$]]|$1|E|3|F|4|5|6|1C|7|@]|8|@]|9|$]]|$1|G|3|H|4|I|6|1D|7|@$J|1E|K|1F|L|M]]|8|@]|9|$]]|$1|N|3|-4|4|5|6|1G|7|@]|8|@]|9|$]]|$1|O|3|P|4|5|6|1H|7|@$J|1I|K|1J|L|M]]|8|@]|9|$]]|$1|Q|3|R|4|I|6|1K|7|@$J|1L|K|1M|L|M]]|8|@]|9|$]]|$1|S|3|-4|4|5|6|1N|7|@]|8|@]|9|$]]|$1|T|3|U|4|5|6|1O|7|@$J|1P|K|1Q|L|M]]|8|@]|9|$]]|$1|V|3|W|4|I|6|1R|7|@$J|1S|K|1T|L|M]]|8|@]|9|$]]|$1|X|3|-4|4|5|6|1U|7|@]|8|@]|9|$]]|$1|Y|3|Z|4|5|6|1V|7|@$J|1W|K|1X|L|M]]|8|@]|9|$]]|$1|10|3|11|4|I|6|1Y|7|@$J|1Z|K|20|L|M]]|8|@]|9|$]]|$1|12|3|-4|4|5|6|21|7|@]|8|@]|9|$]]|$1|13|3|14|4|5|6|22|7|@$J|23|K|24|L|M]]|8|@]|9|$]]|$1|15|3|16|4|5|6|25|7|@$J|26|K|27|L|M]]|8|@]|9|$]]|$1|17|3|-4|4|5|6|28|7|@]|8|@]|9|$]]]|18|$]]

<ol>
<li>Should be single word. That is spaces are not allowed.

Example: mangoprice is valid but mango price is not valid.</li>
<li>Should start with a letter (alphabet) or underscore or $ symbol.

Example: price, _price and $price are valid identifiers.</li>
<li>Should not be a keyword of Java as keyword carries special meaning to the compiler.

Example: class or void etc.</li>
<li>Should not start with a digit but digit can be in the middle or at the end.

Example: 5mangoescost is not valid and mango5cost and mangocost5 are valid.</li>
<li>Length of an identifier in Java can be of 65,535 characters and all are significant.
Identifiers are case-sensitive. That is both mango and Mango are treated differently.
Can contain all uppercase letters or lowercase letters or a mixture.</li>
</ol>

IDENTIFIER: they are class names, method names, variable names ...

As main is not a reserved word and according to the explanation above for defining an identifier main is a valid identifier and java1234 also.Remaining options are not valid because of the above explanation.

One of my kids is taking Java in high school and had this on one of his tests:
<blockquote>
Which of the following is a valid identifier in Java?
a. <code>123java</code> 
b. <code>main</code> 
c. <code>java1234</code> 
d. <code>{abce</code> 
e. <code>)whoot</code>
</blockquote>
He answered b and got it wrong.
I looked at the question and argued that <code>main</code> is a valid identifier and that it should have been right.
We took a look at the Java <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-3.html" rel="noreferrer">spec</a> for identifiers and it reinforced that point. We also wrote a sample program that had a variable called <code>main</code>, as well as a method. He created a written rebuttal that included the Java documentation reference, the test program and the teacher ignored it and says the answer is still incorrect.
Is <code>main</code> a valid identifier?

Is main a valid Java identifier?

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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我的一个孩子在高中学习Java，并在他的一次测试中做了以下测试：以下哪一项是Java中的有效标识符？a. 123javab. mainc. java1234d. {abcee. )whoot他回答了b，但答错了。我看了看这个问题，认为main是一个有效的标识符，它应该是正确的。我们研究了用于标识符的Java ，它强化了...

问main是有效的Java标识符吗？
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