blocks|key|980288|text|static+final+int+i;|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|980289|需要在静态初始化器中初始化，因为它是最终的。|unstyled|980290|static{
++++++++if(3<2){
++++++++i+=+0;
++++++++}
++++}|980291|由于3<2文字或常量，编译器能够检测到代码的无效部分，并且不会初始化i。|980292|添加一个else用例并在其中执行一些i初始化。|offset|length|style|CODE|980293|entityMap^0|0|0|0|0|4|4|I|1|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|T|8|@]|9|@]|A|$]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$B|C]]|$1|I|3|J|5|F|7|V|8|@]|9|@]|A|$]]|$1|K|3|L|5|F|7|W|8|@$M|X|N|Y|O|P]|$M|Z|N|10|O|P]]|9|@]|A|$]]|$1|Q|3|-4|5|F|7|11|8|@]|9|@]|A|$]]]|R|$]]

<pre><code>static final int i;
</code></pre>

need to be initialized at static initializer, because it is final.

<pre><code>static{
 if(3&lt;2){
 i = 0;
 }
 }
</code></pre>

because of the 3&lt;2 literal or constant, the compiler was able to detect your dead part of the code and do not initialize the i.

add an <code>else</code> case and do some <code>i</code> initialization there.

blocks|key|4541234|text|它与编译器如何确定语句是否将被执行有关。它是在JLS+#16中定义的|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|4541235|4541236|每个局部变量和每个空的最终字段在其值发生任何访问时都必须有一个明确分配的值。|blockquote|4541237|4541238|4541239|在您的示例中，编译器可以确定在编译时是否明确分配了i，因为if中的表达式是常量表达式。|style|CODE|4541240|JLS+#15.28定义常量表达式|4541241|4541242|编译时常量表达式是表示基元类型的值或字符串的表达式，该表达式不会突然完成，并且仅使用以下内容组成：|4541243|4541244|4541245|4541246|原语类型的文字...|unordered-list-item|4541247|关系运算符<、<=、>和>=+...|4541248|4541249|有趣的是，尽管Test2可以编译，但这个修改后的Test2版本不能编译：|4541250|static+class+Test3+{
++++static+final+int+i;
++++static+{
++++++++int+j+=+3;
++++++++if+(j+>+2)+{
++++++++++++i+=+0;
++++++++}
++++}
}|code-block|syntax|javascript|4541251|原因是j+>+2不再是一个常量表达式。将j设为final会使这个类再次编译。|4541252|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/specs/jls/se7/html/jls-16.html|1|http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.28^0|N|7|0|0|0|0|0|0|P|1|T|2|0|0|A|1|0|0|0|0|0|0|0|0|0|0|0|3|5|N|5|0^^$0|@$1|2|3|4|5|6|7|1K|8|@]|9|@$A|1L|B|1M|1|1N]]|C|$]]|$1|D|3|-4|5|6|7|1O|8|@]|9|@]|C|$]]|$1|E|3|F|5|G|7|1P|8|@]|9|@]|C|$]]|$1|H|3|-4|5|6|7|1Q|8|@]|9|@]|C|$]]|$1|I|3|-4|5|6|7|1R|8|@]|9|@]|C|$]]|$1|J|3|K|5|6|7|1S|8|@$A|1T|B|1U|L|M]|$A|1V|B|1W|L|M]]|9|@]|C|$]]|$1|N|3|O|5|6|7|1X|8|@]|9|@$A|1Y|B|1Z|1|20]]|C|$]]|$1|P|3|-4|5|6|7|21|8|@]|9|@]|C|$]]|$1|Q|3|R|5|G|7|22|8|@]|9|@]|C|$]]|$1|S|3|-4|5|6|7|23|8|@]|9|@]|C|$]]|$1|T|3|-4|5|6|7|24|8|@]|9|@]|C|$]]|$1|U|3|-4|5|6|7|25|8|@]|9|@]|C|$]]|$1|V|3|W|5|X|7|26|8|@]|9|@]|C|$]]|$1|Y|3|Z|5|X|7|27|8|@]|9|@]|C|$]]|$1|10|3|-4|5|6|7|28|8|@]|9|@]|C|$]]|$1|11|3|12|5|6|7|29|8|@]|9|@]|C|$]]|$1|13|3|14|5|15|7|2A|8|@]|9|@]|C|$16|17]]|$1|18|3|19|5|6|7|2B|8|@$A|2C|B|2D|L|M]|$A|2E|B|2F|L|M]]|9|@]|C|$]]|$1|1A|3|-4|5|6|7|2G|8|@]|9|@]|C|$]]]|1B|$1C|$5|1D|1E|1F|C|$1G|1H]]|1I|$5|1D|1E|1F|C|$1G|1J]]]]

It has to do with how the compiler determines if a statement will be executed or not. It is defined in the <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-16.html" rel="nofollow">JLS #16</a>:

<blockquote>
 Each local variable and every blank final field must have a definitely assigned value when any access of its value occurs.
</blockquote>

In your example, the compiler can determine that <code>i</code> is (or is not) definitely assigned at compile time because the expression in the <code>if</code> is a constant expression.

<a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.28" rel="nofollow">JLS #15.28</a> defines constant expressions:

<blockquote>
 A compile-time constant expression is an expression denoting a value of primitive type or a String that does not complete abruptly and is composed using only the following:
 
 <ul>
 <li>Literals of primitive type [...]</li>
 <li>The relational operators &lt;, &lt;=, >, and >= [...]</li>
 </ul>
</blockquote>

Interestingly, this modified version of Test2 does not compile although Test2 does compile:

<pre><code>static class Test3 {
 static final int i;
 static {
 int j = 3;
 if (j &gt; 2) {
 i = 0;
 }
 }
}
</code></pre>

The reason is that <code>j &gt; 2</code> is not a constant expression any longer. Making j <code>final</code> would make the class compile again.

blocks|key|4541285|text|根据Java+Language+Specification的说法，if语句的条件部分中的表达式是编译时表达式。在Test1编译器知道我永远不会被初始化，这就是为什么它会报错。在第二种情况下，编译器知道我将被初始化。如果将编译时表达式替换为更动态的表达式，两个类都将无法编译，因为编译器不能保证我已被初始化。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|4541286|entityMap|0|LINK|mutability|MUTABLE|url|http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.28^0|2|R|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]]]]

According to <a href="http://docs.oracle.com/javase/specs/jls/se7/html/jls-15.html#jls-15.28" rel="nofollow">Java Language Specification</a> the expression in the condition parts of your if statements are compile-time expressions. In Test1 compiler knows that i will never be initialized, that's why it complains. In the second case the compiler knows that i will be initialized. If you replace your compile-time expressions with something more dynamic, both classes will fail to compile because compiler will not be able to guarantee that i is initialized.

blocks|key|980352|text|可以将if+(3<2)优化为if+(false)，因此Java编译会检测到不可达语句(错误)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|980353|可以将if+(3>2)优化为if+(true)，然后始终执行包含的语句。这没问题。|980354|编译器可以很容易地检测到这些是常量标量，因此可以在编译时计算(即优化)表达式。我们对一个像样的编译器也有同样的期望。|980355|entityMap^0|3|8|E|A|0|3|8|E|9|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|@$9|R|A|S|B|C]|$9|T|A|U|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|V|8|@]|D|@]|E|$]]|$1|J|3|-4|5|6|7|W|8|@]|D|@]|E|$]]]|K|$]]

<code>if (3&lt;2)</code> can be optimised to <code>if (false)</code> hence Java compilation detects an unreachable statement (error).

<code>if (3&gt;2)</code> can be optimised to <code>if (true)</code> and the enclosed statement is then always executed. No problem with that.

The compiler can easily detect that these are constant scalars and hence can evaluate (i.e. optimise) the expression at compile time. We would expect nothing less from a decent compiler.

<pre><code>public class Test1 {
 static final int i;
 static{
 if(3&lt;2){
 i = 0;
 }
 } 
}
</code></pre>



<pre><code>public class Test2 {
 static final int i;
 static{
 if(3&gt;2){
 i = 0;
 }
 } 
}
</code></pre>

Class <code>Test1</code> failed to compile, 
Class <code>Test2</code> compiled successfully.

Can any body tell me how compiler is able to evaluate expression in if condition?

Can an expression be evaluated at compile time?

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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public class Test1 { static final int i; static{ if(3<2){ i = 0; } } }public class Test2 { static final int i; static{ ...

问表达式可以在编译时求值吗？
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问表达式可以在编译时求值吗？EN