Immutable vs Unmodifiable collection

Basically unModifiable Collection is a view, So indirectly it could still be 'modified' from some other reference that is modifiable. Also as its just a readonly view of annother collection , When the source collection changes unModifiable Collection will always present with latest values.

However immutable Collection can be treated as a readonly copy of another collection and can not be modified. In this case when the source collection changes , immutable Collection do not reflect the changes

Here is a testcase to visualise this difference.

@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());

}

Output

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]
--

An unmodifiable collection is often a wrapper around a modifiable collection which other code may still have access to. So while you can't make any changes to it if you only have a reference to the unmodifiable collection, you can't rely on the contents not changing.

An immutable collection guarantees that nothing can change the collection any more. If it wraps a modifiable collection, it makes sure that no other code has access to that modifiable collection. Note that although no code can change which objects the collection contains references to, the objects themselves may still be mutable - creating an immutable collection of StringBuilder doesn't somehow "freeze" those objects.

Basically, the difference is about whether other code may be able to change the collection behind your back.