blocks|key|319446|text|如果对象的运行时类型是您试图将其强制转换为的子类型，则可以进行强制转换。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|319447|编辑：|319448|是的，您试图转换的对象需要实现接口才能成功转换。|319449|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|H|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|I|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|J|8|@]|9|@]|A|$]]|$1|F|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|G|$]]

You can cast if the runtime type of an object is a subtype of what you're trying to cast it into.

EDIT:

Yes, the object that you're trying to cast will need to implement the interface in order for you to cast it successfully.

blocks|key|3062325|text|在Java中，有两种类型的引用变量类型转换：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3062326|3062327|Downcasting：如果您有引用某个子类型对象的引用变量，则可以将其赋给该子类型的引用变量。为此，您必须进行显式强制转换，结果是您可以使用这个新的引用variable.|unordered-list-item|offset|length|style|BOLD|3062328|Upcasting：访问子类型的成员。您可以显式或隐式地将引用变量分配给父类型引用变量。这是一个本质上安全的操作，因为赋值限制了新变量的访问能力。|3062329|3062330|是，您需要直接或间接地实现接口，才能将您的类对象引用分配给接口类型。|3062331|entityMap^0|0|0|0|B|0|0|9|0|0|0|1|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|R|8|@]|9|@]|A|$]]|$1|C|3|D|5|E|7|S|8|@$F|T|G|U|H|I]]|9|@]|A|$]]|$1|J|3|K|5|E|7|V|8|@$F|W|G|X|H|I]]|9|@]|A|$]]|$1|L|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|Z|8|@$F|10|G|11|H|I]]|9|@]|A|$]]|$1|O|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|P|$]]

In Java there are two types of reference variable casting: 

<ul>
<li>Downcasting: If you have a reference
variable that refers to a subtype
object, you can assign it to a
reference variable of the subtype.
You must make an explicit cast to do
this, and the result is that you can
access the subtype's members with
this new reference variable.</li>
<li>Upcasting: You can assign a reference
variable to a supertype reference
variable explicitly or implicitly.
This is an inherently safe operation
because the assignment restricts the
access capabilities of the new
variable.</li>
</ul>

Yes, you need to implement the interface directly or indirectly to enable assigning your class object reference to the interface type.

blocks|key|319464|text|如果：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|319465|interface+MyInterface{}

class+MyClass+implements+MyInterface{}|code-block|syntax|javascript|319466|然后|319467|MyClass+m+=+new+MyClass();
MyInterface+i+=+(MyInterface)m;|319468|是可能的。|319469|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

If:

<pre><code>interface MyInterface{}

class MyClass implements MyInterface{}
</code></pre>

Then

<pre><code>MyClass m = new MyClass();
MyInterface i = (MyInterface)m;
</code></pre>

is possible.

blocks|key|317573|text|有一种直观的方式来思考这一点-你不是用类型转换来改变一个对象，你只是在做一些如果类型已知就会被允许的事情-换句话说，你只能转换成你的对象已经是的类型。因此，只需“向上”查看对象链，看看哪些类型适用于您的对象。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|317574|因此，只有在链中更高的位置定义了接口，才能将其强制转换为接口(例如，如果您的类的父类实现了它，等等)。它必须是显式的-从你的问题听起来你可能在想，如果你实现了方法"void+foo()“，那么你应该能够强制转换为定义方法"void+foo()”的接口-这有时被描述为"duck+typing"+(如果它叫起来像鸭子，它就是鸭子)，但不是java的工作方式。|offset|length|317575|entityMap|0|LINK|mutability|MUTABLE|url|http://en.wikipedia.org/wiki/Duck_typing^0|0|3P|D|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

There's an intuitive way of thinking about this - you're not changing an object with a cast, you're only doing something that would already be permitted if the type was known - inotherwords, you can only cast to a type that your object already is. So just look "up" the object chain to see what kinds apply to your object. 

So you can cast to an interface only if it's defined somewhere higher up in the chain (e.g. if your classes parent implements it, etc. etc). It has to be explicit - from your question it sounds like you may be thinking that if you implement method "void foo()" then you should be able to cast to an interface that defines the method "void foo()" - this is sometimes described as <a href="http://en.wikipedia.org/wiki/Duck_typing" rel="nofollow noreferrer">"duck typing"</a> (if it quacks like a duck, it's a duck) but is not how java works.

blocks|key|316595|text|这将会起作用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|316596|class+Foo+implements+Runnable+{
++++public+void+run()+{}
}

Foo+foo+=+new+Foo();
System.out.println((Runnable)+foo);|code-block|syntax|javascript|316597|但这不会：|316598|class+Bar+{
++++public+void+run()+{}
}

Bar+bar+=+new+Bar();
System.out.println((Runnable)+bar);|316599|因为尽管Bar有一个可以实现Runnable.run()的run()方法，但Bar没有声明为实现Runnable，因此不能强制转换为Runnable。|offset|length|style|CODE|316600|Java要求您按名称声明已实现的接口。与Python和Go等其他语言不同，它没有duck+typing|316601|entityMap|0|LINK|mutability|MUTABLE|url|http://www.python.org/|1|http://golang.org/|2|http://en.wikipedia.org/wiki/Duck_typing^0|0|0|0|0|4|3|E|E|T|5|12|3|1C|8|1U|8|0|K|6|0|R|2|1|14|B|2|0^^$0|@$1|2|3|4|5|6|7|14|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|15|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|16|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|17|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|18|8|@$M|19|N|1A|O|P]|$M|1B|N|1C|O|P]|$M|1D|N|1E|O|P]|$M|1F|N|1G|O|P]|$M|1H|N|1I|O|P]|$M|1J|N|1K|O|P]]|9|@]|A|$]]|$1|Q|3|R|5|6|7|1L|8|@]|9|@$M|1M|N|1N|1|1O]|$M|1P|N|1Q|1|1R]|$M|1S|N|1T|1|1U]]|A|$]]|$1|S|3|-4|5|6|7|1V|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|Z]]|10|$5|V|W|X|A|$Y|11]]|12|$5|V|W|X|A|$Y|13]]]]

This will work:

<pre><code>class Foo implements Runnable {
 public void run() {}
}

Foo foo = new Foo();
System.out.println((Runnable) foo);
</code></pre>

But this will not:

<pre><code>class Bar {
 public void run() {}
}

Bar bar = new Bar();
System.out.println((Runnable) bar);
</code></pre>

Because although <code>Bar</code> has a <code>run()</code> method that could implement <code>Runnable.run()</code>, <code>Bar</code> is not declared to implement <code>Runnable</code> so it cannot be cast to <code>Runnable</code>.

Java requires that you declare implemented interfaces by name. It does not have <a href="http://en.wikipedia.org/wiki/Duck_typing" rel="nofollow noreferrer">duck typing</a>, unlike some other languages such as <a href="http://www.python.org/" rel="nofollow noreferrer">Python</a> and <a href="http://golang.org/" rel="nofollow noreferrer">Go</a>

blocks|key|314550|text|假设我们想将d个对象强制转换为A，|type|unstyled|depth|inlineStyleRanges|entityRanges|data|314551|A+a=+(C)d；|314552|所以在内部，编译器和JVM已经检查了3条规则。JVM编译器在编译时检查前2个规则，将在运行时检查最后一个规则。|offset|length|style|BOLD|314553|规则1(编译时检查)：|314554|'d‘和C的|314555|类型必须有某种关系(子类型到父类型或者父类型对子类型或同时).If如果没有关系，那么我们将得到一个编译错误(不可转换的类型)。|blockquote|314556|314557|314558|规则2(编译时检查)：|314559|314560|+'C‘必须是'A’的相同类型或派生类型(子类)，否则将出现编译错误(类型不兼容)。|314561|314562|314563|规则3(运行时例外)：|314564|“%25d”的|314565|运行时对象类型必须相同或派生为“C”类型，否则将出现运行时异常(ClassCastException异常)。|314566|314567|314568|找到下面的例子来获得更多的想法，|314569|String+s+=+new+String("hello");+StringBuffer+sb+=+(StringBuffer)s;++//+Compile+error+:+Invertible+types+because+there+is+no+relationship+between.

Object+o+=+new+String("hello");+StringBuffer+sb+=+(String)o;+++++++//+Compile+error+:+Incompatible+types+because+String+is+not+child+class+of+StringBuffer.

Object+o+=+new+String("hello");+StringBuffer+sb+=+(StringBuffer)o;+//+Runtime+Exception+:+ClassCastException+because+'o'+is+string+type+and+trying+to+cast+into+StingBuffer+and+there+is+no+relationship+between+String+and+StringBuffer.|code-block|syntax|javascript|314570|entityMap^0|0|0|15|E|0|0|B|0|0|0|0|0|0|B|0|0|0|0|0|0|B|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|1G|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|1H|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|1I|8|@$F|1J|G|1K|H|I]]|9|@]|A|$]]|$1|J|3|K|5|6|7|1L|8|@$F|1M|G|1N|H|I]]|9|@]|A|$]]|$1|L|3|M|5|6|7|1O|8|@]|9|@]|A|$]]|$1|N|3|O|5|P|7|1P|8|@]|9|@]|A|$]]|$1|Q|3|-4|5|6|7|1Q|8|@]|9|@]|A|$]]|$1|R|3|-4|5|6|7|1R|8|@]|9|@]|A|$]]|$1|S|3|T|5|6|7|1S|8|@$F|1T|G|1U|H|I]]|9|@]|A|$]]|$1|U|3|-4|5|6|7|1V|8|@]|9|@]|A|$]]|$1|V|3|W|5|P|7|1W|8|@]|9|@]|A|$]]|$1|X|3|-4|5|6|7|1X|8|@]|9|@]|A|$]]|$1|Y|3|-4|5|6|7|1Y|8|@]|9|@]|A|$]]|$1|Z|3|10|5|6|7|1Z|8|@$F|20|G|21|H|I]]|9|@]|A|$]]|$1|11|3|12|5|6|7|22|8|@]|9|@]|A|$]]|$1|13|3|14|5|P|7|23|8|@]|9|@]|A|$]]|$1|15|3|-4|5|6|7|24|8|@]|9|@]|A|$]]|$1|16|3|-4|5|6|7|25|8|@]|9|@]|A|$]]|$1|17|3|18|5|6|7|26|8|@]|9|@]|A|$]]|$1|19|3|1A|5|1B|7|27|8|@]|9|@]|A|$1C|1D]]|$1|1E|3|-4|5|6|7|28|8|@]|9|@]|A|$]]]|1F|$]]

Suppose we want to cast d object to A,

A a = (C)d;

So internally 3 rules have been checked by Compiler and JVM.
The compiler is checking first 2 rules at Compile time and JVM will check last one rule at Runtime.

Rule 1 (Compile time checking): 

<blockquote>
 Type of 'd' and C must have some relation (child to parent or parent
 to child or same time).If there is no relationship then we will get a
 compile error(inconvertible types).
</blockquote>

Rule 2 (Compile time checking): 

<blockquote>
 'C' must be either same type or derived type(subclass) of 'A'
 otherwise we will get a compile error(incompatible types).
</blockquote>

Rule 3 (Runtime Exception): 

<blockquote>
 Runtime object type of 'd' must be same or derived a type of 'C'
 otherwise we will get a runtime exception (ClassCastException
 Exception).
</blockquote>

Find following examples to get more idea,

<pre><code>String s = new String("hello"); StringBuffer sb = (StringBuffer)s; // Compile error : Invertible types because there is no relationship between.

Object o = new String("hello"); StringBuffer sb = (String)o; // Compile error : Incompatible types because String is not child class of StringBuffer.

Object o = new String("hello"); StringBuffer sb = (StringBuffer)o; // Runtime Exception : ClassCastException because 'o' is string type and trying to cast into StingBuffer and there is no relationship between String and StringBuffer.
</code></pre>

When can a certain object be cast into another object? Does the casted object have to be a subtype of the other object? I'm trying to figure out the rules...

Edit: I realized that I didn't explain my issue at all: basically I am casting an object to an interface type. However, at run-time, I get a <code>java.lang.ClassCastException</code>. What needs to happen with my object so that I can cast it to this interface? Does it have to implement it? 

Thanks

Java rules for casting

编译器

Java

什么时候可以将某个对象转换为另一个对象？强制转换的对象必须是另一个对象的子类型吗？我在试着弄清楚规则...编辑：我意识到我根本没有解释我的问题:基本上我是将一个对象转换为一个接口类型。但是，在运行时，我会得到一个java.lang.ClassCastException。需要对我的对象做些什么才能将它强制转换到这个接口？...

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