Bloch的Effective Java第2版中常量字段的定义

Question

Quote:

If a static final field has a mutable reference type, it can still be a constant field if the referenced object is immutable.

I'm not sure what this mean; can someone give an example of that?

Answer 1

An example that Josh is talking about would be List , which is a mutable type ( add() , remove() , etc), but you can assign an immutable instance to it:

public static final List<String> NAMES = Collections.unmodifiableList( Arrays.asList("foo", "bar")); // immutable

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By the way, a great example of something that looks like a constant, but isn't, is a Date constant:

public static final Date EPOCH = new Date(0);

but then some code can do this:

EPOCH.setTime(123456789); // oops!

Date is mutable ! Everyone will see such a change.

In constrast with this is something like String , which is immutable:

public static final String NAME = "Agent Smith"; // immutable

Answer 2

You can have a mutable type with an immutable subtype:

class Mutable {}  // Not immutable, because it can be extended.

final class Immutable extends Mutable {}

// Reference type is mutable, but referenced object is immutable.
static final Mutable CONSTANT = new Immutable();