如果我们讨论的是JDK不可修改*和番石榴不可修改*，实际上它们的区别还在于性能。如果不可变集合不是常规集合的包装器(JDK实现就是包装器)，那么它们可以更快、更节省内存。 以番石榴团队为例:

JDK提供了集合。不可修改的xxx方法，但在我们看来，这些可以

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低效:数据结构仍然有可变集合的所有开销，包括并发修改检查，哈希表中的额外空间等。

Java™教程介绍了以下内容:

Unlike synchronization wrappers, which add functionality to the wrapped collection, the unmodifiable wrappers take functionality away. In particular, they take away the ability to modify the collection by intercepting all the operations that would modify the collection and throwing an UnsupportedOperationException. Unmodifiable wrappers have two main uses, as follows: To make a collection immutable once it has been built. In this case, it's good practice not to maintain a reference to the backing collection. This absolutely guarantees immutability. To allow certain clients read-only access to your data structures. You keep a reference to the backing collection but hand out a reference to the wrapper. In this way, clients can look but not modify, while you maintain full access.

我认为这是一个很好的解释，足以理解其中的区别。

基本上不可修改集合是一个视图，所以它仍然可以间接地从其他一些可修改的引用中“修改”。也因为它只是另一个集合的只读视图，当源集合发生变化时，不可修改集合将始终显示最新的值。

然而，不可变集合可以被视为另一个集合的只读副本，不能被修改。在这种情况下，当源集合发生变化时，不可变集合不反映这些变化

下面是一个测试用例来可视化这种差异。

@Test
public void testList() {

    List<String> modifiableList = new ArrayList<String>();
    modifiableList.add("a");

    System.out.println("modifiableList:"+modifiableList);
    System.out.println("--");


    //unModifiableList

    assertEquals(1, modifiableList.size());

    List<String> unModifiableList=Collections.unmodifiableList(
                                        modifiableList);

    modifiableList.add("b");

    boolean exceptionThrown=false;
    try {
        unModifiableList.add("b");
        fail("add supported for unModifiableList!!");
    } catch (UnsupportedOperationException e) {
        exceptionThrown=true;
        System.out.println("unModifiableList.add() not supported");
    }
    assertTrue(exceptionThrown);

    System.out.println("modifiableList:"+modifiableList);
    System.out.println("unModifiableList:"+unModifiableList);

    assertEquals(2, modifiableList.size());
    assertEquals(2, unModifiableList.size());
            System.out.println("--");



            //immutableList


    List<String> immutableList=Collections.unmodifiableList(
                            new ArrayList<String>(modifiableList));

    modifiableList.add("c");

    exceptionThrown=false;
    try {
        immutableList.add("c");
        fail("add supported for immutableList!!");
    } catch (UnsupportedOperationException e) {
        exceptionThrown=true;
        System.out.println("immutableList.add() not supported");
    }
    assertTrue(exceptionThrown);


    System.out.println("modifiableList:"+modifiableList);
    System.out.println("unModifiableList:"+unModifiableList);
    System.out.println("immutableList:"+immutableList);
    System.out.println("--");

    assertEquals(3, modifiableList.size());
    assertEquals(3, unModifiableList.size());
    assertEquals(2, immutableList.size());

}

输出

modifiableList:[a]
--
unModifiableList.add() not supported
modifiableList:[a, b]
unModifiableList:[a, b]
--
immutableList.add() not supported
modifiableList:[a, b, c]
unModifiableList:[a, b, c]
immutableList:[a, b]
--

如上所述，不可修改的不像不可变的，因为不可修改的集合可以被改变，例如，如果一个不可修改的集合有一个被其他对象引用的底层委托集合，而该对象改变了它。

Regarding immutable, it's not even well defined. However, generally it means that the object "will not change", but that would need to be defined recursively. For example, I can define immutable on classes whose instance variables are all primitives and whose methods all contain no arguments and return primitives. The methods then recursively allow the instance variables to be immutable and all methods to contain arguments that are immutable and that return immutable values. The methods should be guaranteed to return the same value over time.

Assuming that we can do that, there is also the concept thread safe. And you might be led to believe that immutable (or not changeble over time) also implies thread safe. However that is not the case and that is the main point I am making here that has not yet been noted in other answers. I can construct an immutable object that always returns the same results yet is not thread safe. To see this suppose that I construct an immutable collection by maintaining additions and deletions over time. Now the immutable collection returns its elements by looking at the internal collection (which may be changing over time) and then (internally) adding and deleting the elements that were added or deleted after creation of the collection. Clearly, although the collection would always return the same elements, it is not thread safe merely because it will never change value.

现在我们可以将不可变定义为线程安全且永远不会改变的对象。有一些创建不可变类的指导原则，这些类通常会导致这样的类，然而，请记住，可能有一些创建不可变类的方法，需要注意线程安全，例如，如上面的“快照”集合示例所述。

    // normal list
    List list1 = new ArrayList();
    list1.add(1);

    // unmodifiable list
    List list2 = Collections.unmodifiableList(list1);

    // immutable list
    List list3 = Collections.unmodifiableList(new ArrayList<>(list1));

    list1.add(2);
    list1.add(3);

    System.out.println(list1);
    System.out.println(list2);
    System.out.println(list3);

输出:

[1, 2, 3]
[1, 2, 3]
[1]

[不可更改和不可更改]

不可修改的集合(对象)仍然可以通过改变原始对象来改变。使用参考是可能的。

Java提供了几种方法来创建一个不可修改的映射:

Collections.unmodifiableMap () Java 9 Map.of()， Map.ofEntries()