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Making a Collection Read-Only

Making a collection read-only involves wrapping the collection in another object whose mutation methods all throw UnsupportedOperationException.

The Case for Immutable Collections

This is useful if you have a requirement case, where the collection like a history can’t be modified. Further examples: A set of instructions or a processed order list.

List stuff = Arrays.asList(new String[]{"a", "b"});

// Make a list read-only
List list = new ArrayList(stuff);
list = Collections.unmodifiableList(list);

try {
    // Try modifying the list
    list.set(0, "new value");
} catch (UnsupportedOperationException e) {
    // Can't modify
}

Make a Set immutable

// Make a set read-only
Set set = new HashSet(stuff);
set = Collections.unmodifiableSet(set);

Make a Map immutable

// Make a map read-only
Map map = new HashMap();
// Add key/value pairs ...
map = Collections.unmodifiableMap(map);