Java instanceof - 不兼容的条件操作数类型
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instanceof - incompatible conditional operand types
提问by java_geek
The following compiles fine:
以下编译正常：
Object o = new Object();
System.out.println(o instanceof Cloneable);
But this doesn't:
但这不会：
String s = new String();
System.out.println(s instanceof Cloneable);
A compiler error is thrown.
抛出编译器错误。
What is the problem?
问题是什么？
采纳答案by polygenelubricants
A more blatant incarnation of your problem is the following:
您的问题更明显的化身如下：
if ("foo" instanceof Number)
// "Incompatible conditional operand types String and Number"
This is specified in JLS 15.20.2 Type comparison operator instanceof:
这在JLS 15.20.2 类型比较运算符中instanceof指定：
RelationalExpression: RelationalExpression instanceof ReferenceTypeIf a cast of the RelationalExpressionto the ReferenceTypewould be rejected as a compile-time error, then the
instanceofrelational expression likewise produces a compile-time error. In such a situation, the result of theinstanceofexpression could never be true.
RelationalExpression: RelationalExpression instanceof ReferenceType如果将RelationalExpression转换为ReferenceType将被拒绝作为编译时错误，那么
instanceof关系表达式同样会产生编译时错误。在这种情况下，instanceof表达式的结果永远不可能为真。
That is, since this cast expression generates a compile time error:
也就是说，由于此强制转换表达式会生成编译时错误：
(Number) "foo"
so must this expression:
这个表达式也必须：
("foo" instanceof Number)
Your case is a bit more subtle, but the principle is the same:
你的情况有点微妙，但原理是一样的：
Stringis a final classStringdoes not implementCloneable- Therefore you can't do
(Cloneable) aString - Therefore also you can't do
aString instanceof Cloneable
String是最后一堂课String不执行Cloneable- 因此你不能做
(Cloneable) aString - 因此你也不能做
aString instanceof Cloneable
回答by Jon Skeet
The compiler knows that Stringis a final class and doesn't implement Cloneable. So no instance of String can everbe an instance of Cloneable. It's stopping you from thinking you've got a meaningful test when actually it will always print "false".
编译器知道这String是一个 final 类并且没有实现Cloneable. 因此，任何 String的实例都不能成为Cloneable. 它阻止你认为你有一个有意义的测试，而实际上它总是打印“false”。
回答by Some Guy
A related issue that I have come across recently (and which led me to this page, before I figured out what was going on) is that the Eclipse environment can report "Incompatible conditional operand types" in an 'instanceof' expression erroneously due to a missing 'import' statement for the type on the right of the 'instanceof'. I spent a while trying to figure out how the types in question could possibly be incompatible before figuring out that a missing import was causing the whole problem. Hopefully this information saves somebody some time.
我最近遇到的一个相关问题（在我弄清楚发生了什么之前将我带到了这个页面）是 Eclipse 环境可以错误地报告“instanceof”表达式中的“不兼容的条件操作数类型”，因为“instanceof”右侧的类型缺少“import”语句。我花了一段时间试图弄清楚有问题的类型如何可能不兼容，然后才发现丢失的导入导致了整个问题。希望这些信息可以为某人节省一些时间。
