blocks|key|4067360|text|java编译器不知道throwIAE总是会抛出异常，所以它假定您最终会到达run方法的末尾，当这种情况发生时，需要一个返回值。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4067361|entityMap^0|A|8|11|3|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]|$9|K|A|L|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|M|8|@]|D|@]|E|$]]]|G|$]]

The java compiler does not know that the
<code>throwIAE</code>
will always throw an exception so it
assumes that you will eventually reach the end of the 
<code>run</code>
method and,
when that happens,
a return value is required.

blocks|key|4066136|text|你的方法有一个返回类型，所以它应该返回一个布尔结果。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4066137|+++public+boolean+run()+{
+++++throwIAE();
+++++//+Missing+return+statement
+++++return+false;
+++}|code-block|syntax|javascript|4066138|或者你应该直接在方法中抛出你的异常：|4066139|+++public+boolean+run()+{
+++++throw+new+IllegalStateException("error");+//+it+will+compile
+++}|4066140|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|$]]

Your method has a return type so it should return a boolean result.

<pre><code> public boolean run() {
 throwIAE();
 // Missing return statement
 return false;
 }
</code></pre>

Or you should throw your exception directly in method :

<pre><code> public boolean run() {
 throw new IllegalStateException("error"); // it will compile
 }
</code></pre>

blocks|key|4066303|text|即使现在的编译器很聪明，他们也不能“看到”你的throwIAE()方法总是返回异常，因为这会在运行时发生，而不是在编译时。如果你处理这个异常呢？那么你最终必须添加一个返回值。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4066304|entityMap^0|N|A|0^^$0|@$1|2|3|4|5|6|7|H|8|@$9|I|A|J|B|C]]|D|@]|E|$]]|$1|F|3|-4|5|6|7|K|8|@]|D|@]|E|$]]]|G|$]]

Even if compilers are smart nowadays, they cannot 'see' that your <code>throwIAE()</code> method always returns an Exception because this will happen during runtime and not compile time. What if you handle the exception? Then you must add a return value eventually.

blocks|key|4067613|text|我真的不希望这样做，但有趣的是，您可以这样做，这会使代码更具可读性：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4067614|++++public+static+<T>+T+unsupported()
++++{
++++++++throw+new+UnsupportedOperationException();
++++}

+++++...
++++
++++return+unsupported();|code-block|syntax|javascript|4067615|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 didn't really expect this to work, but interestingly you CAN do this, which makes the code a little more readable:
<pre><code> public static &lt;T&gt; T unsupported()
 {
 throw new UnsupportedOperationException();
 }

 ...
 
 return unsupported();
</code></pre>

blocks|key|4066240|text|抛出的异常将终止throwIAE方法，但不会终止调用此方法的方法。将其理解为返回输出的方法(从void开始为空return;+)。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4066241|public+void+throwIAE()+{
+++++return;++++++++++++++++//+is+not+responsible+for+terminating+run()
}|code-block|syntax|javascript|4066242|你看到了吗？return终止throwIAE方法，程序继续运行。return和throw都与方法本身的作用域相关，而与调用方法无关。您需要：|4066243|public+boolean+run()+{
+++++throwIAE();++++++++++++//+throws+exception+and+terminates+throwIAR()
+++++return+false;++++++++++//+terminates+run()
}|4066244|entityMap^0|8|8|1B|4|1J|7|0|0|6|6|E|8|W|6|13|5|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]|$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|17|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|18|8|@]|D|@]|E|$]]]|P|$]]

The exception thrown terminates the <code>throwIAE</code> method, but not the method which calls this one. Understand this as a method returning an output (empty <code>return;</code> since <code>void</code>). 

<pre><code>public void throwIAE() {
 return; // is not responsible for terminating run()
}
</code></pre>

Do you see? The <code>return</code> terminates the <code>throwIAE</code> method and the program continues. Both the <code>return</code> and <code>throw</code> is related to the scope of the method itself, not the calling method. You need:

<pre><code>public boolean run() {
 throwIAE(); // throws exception and terminates throwIAR()
 return false; // terminates run()
}
</code></pre>

blocks|key|4067541|text|到目前为止，答案还不错。我想知道为什么没有人提到AOP(面向方面的编程)。有了面向方面编程，你可以很容易地在运行时拦截你的方法throwIAE()并捕获异常。调用方法run()不能知道这一点。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4067542|下面是一些例子：|4067543|package+yourPackage;

public+class+YourClass+{

++++public+boolean+run()+{
++++++++throwIAE();
++++++++System.out.println("Hello+world!");//+prints+Hello+World!+even+if+method+throws+exception
++++++++return+false;//+does+not+compile+without+return+statement
++++}

++++public+void+throwIAE()+{
++++++++throw+new+IllegalStateException("error");
++++}

}|code-block|syntax|javascript|4067544|以及在运行时拦截throwIAE()方法的另一个方面类：|4067545|package+yourPackage;

@Aspect
public+class+ExceptionHandlingAspect+{
++++@Around("execution(*+yourPackage.YourClass.throwIAE())")
++++public+Object+handleException(ProceedingJoinPoint+pjp)+throws+Throwable+{
++++++++try+{
++++++++++++return+pjp.proceed();
++++++++}+catch+(Exception+e)+{
++++++++++++System.out.println("exception+caught");
++++++++++++return+null;
++++++++}
++++}
}++|4067546|entityMap^0|1R|A|2B|5|0|0|0|8|A|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|X|8|@]|D|@]|E|$]]|$1|H|3|I|5|J|7|Y|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|Z|8|@$9|10|A|11|B|C]]|D|@]|E|$]]|$1|O|3|P|5|J|7|12|8|@]|D|@]|E|$K|L]]|$1|Q|3|-4|5|6|7|13|8|@]|D|@]|E|$]]]|R|$]]

Good answers so far. I wonder how nobody mentioned AOP(Aspect Oriented Programming). With AOP you can easily intercept your method <code>throwIAE()</code> during RUNTIME and catch the exception. The calling method <code>run()</code> cannot know this. 

Some example:

<pre><code>package yourPackage;

public class YourClass {

 public boolean run() {
 throwIAE();
 System.out.println("Hello world!");// prints Hello World! even if method throws exception
 return false;// does not compile without return statement
 }

 public void throwIAE() {
 throw new IllegalStateException("error");
 }

}
</code></pre>

And another Aspect class which intercepts <code>throwIAE()</code> method during runtime: 

<pre><code>package yourPackage;

@Aspect
public class ExceptionHandlingAspect {
 @Around("execution(* yourPackage.YourClass.throwIAE())")
 public Object handleException(ProceedingJoinPoint pjp) throws Throwable {
 try {
 return pjp.proceed();
 } catch (Exception e) {
 System.out.println("exception caught");
 return null;
 }
 }
} 
</code></pre>

blocks|key|4067573|text|想象一下：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4067574|+public+class+NotReallyACompilerIssue+extends+CompilerIssue+{

+++@Override++++
+++public+void+throwIAE()+{
+++++//+doesn't+really+throw+the+IAE!
+++}
+}|code-block|syntax|javascript|4067575|如果您的原始类在run()中没有使用return语句进行编译，那么NotReallyACompilerIssue中的run()方法应该已经成功完成并且没有return。那就不好了，不是吗？|offset|length|style|CODE|4067576|如果类为final，或者方法throwIAE为final，则可以检测到这种情况。但事实并非如此，编译器(至少是Oracle+J8_131+)不进行这样的检查。我认为这是一个非常具体的案例，不值得你费力去做。|4067577|entityMap^0|0|0|8|5|X|N|1M|5|0|4|5|N|5|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@$I|T|J|U|K|L]|$I|V|J|W|K|L]|$I|X|J|Y|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|Z|8|@$I|10|J|11|K|L]|$I|12|J|13|K|L]]|9|@]|A|$]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$]]

Imagine this:

<pre><code> public class NotReallyACompilerIssue extends CompilerIssue {

 @Override 
 public void throwIAE() {
 // doesn't really throw the IAE!
 }
 }
</code></pre>

If your original class compiled without the return statement in <code>run()</code>, the <code>run()</code> method in <code>NotReallyACompilerIssue</code> whould've completed successfully and had no return. Which wouldn't be good, would it?

It is be possible to detect such situation if the class is<code>final</code>, or method throwIAE is<code>final</code>. It isn't the case though, the compiler (Oracle J8_131 at least) does not make such checks. I imagine this is a very specific case and it wasn't worth the effort.

Why does this not compile (tried with java 8 and java 10)? It yields a missing return statement error. 

<pre><code> public class CompilerIssue {
 public boolean run() {
 throwIAE();
 // Missing return statement
 }

 public void throwIAE() {
 throw new IllegalStateException("error");
 }
 }
</code></pre>

Java expects return value when RuntimeException is thrown

为什么这不能编译(用java8和java10尝试过)？它会产生一个缺少返回语句的错误。 public class CompilerIssue {   public boolean run() {     throwIAE();     // Missing return statement   }   public void throwIAE() {     throw new IllegalSt

问当抛出RuntimeException时，Java需要返回值
EN

回答 7

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

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

问当抛出RuntimeException时，Java需要返回值EN

回答 7

Stack Overflow用户

Stack Overflow用户

Stack Overflow用户

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

问当抛出RuntimeException时，Java需要返回值
EN