是否可以制作不可修改地图的可修改副本？

Question

Description:

One of my problem sets is provided with test cases to check if the typed code works correctly. I won't go into much detail, but basically these tests take a map as input. My code worked perfectly for all test cases except for one: when I took a look at the test, I saw that the input map gets changed into an unmodifiable map ( Collections.unmodifiableMap(numbers) ) and this unmodifiable map is passed on to the method that I wrote. This is the only test that does it, so I was wondering if it is possible to make a copy of this unmodifiable map such that it can be modified within my method? The most important modification that I'll have to make is updating the value of a key ( map.put(key, map.get(key)-1 ).

Extension:

An additional question, are there similar ways to do this for other data structures such as Lists or Sets?

Answer 1

The easiest way is creating a new map:

final Map<?, ?> unmodifiable = Collections.unmodifiableMap(new HashMap<>());
final Map<?, ?> modifiable = new HashMap<>(unmodifiable);

Both maps reference the same values.

Extension

For collections like lists and sets, it works similarly. They usually have constructors that you can pass another collection to.

Answer 2

It isn't necessary to create a new List or Map to update the state of their objects. It's the collections that are unmodifiable, not the objects in them.

      List<Foo> mod = IntStream.range(1, 10).mapToObj(Foo::new).collect(
            Collectors.toList());
      List<Foo> unmod = Collections.unmodifiableList(mod);
      // unmod.add(new Foo(20)); <-- can't do this

      List<Foo> subList = unmod.subList(2, 4);
      //  subList.remove(0); <-- or this
      Foo f = unmod.get(3); 
      f.v = 99; // <-- but this is allowed
      System.out.println(unmod);


      class Foo {
          int v;

          Foo(int v) {
              this.v = v;
          }
          public String toString() {
              return v + "";
          }
       }