Question

How do I replace elements in a list with another?

For example I want all two to become one?

Answer 1

You can use:

Collections.replaceAll(list, "two", "one");

From the documentation:

Replaces all occurrences of one specified value in a list with another. More formally, replaces with newVal each element e in list such that (oldVal==null ? e==null : oldVal.equals(e)). (This method has no effect on the size of the list.)

The method also return a boolean to indicate whether or not any replacement was actually made.

java.util.Collections has many more static utility methods that you can use on List (e.g. sort, binarySearch, shuffle, etc).

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Snippet

The following shows how Collections.replaceAll works; it also shows that you can replace to/from null as well:

    List<String> list = Arrays.asList(
        "one", "two", "three", null, "two", null, "five"
    );
    System.out.println(list);
    // [one, two, three, null, two, null, five]

    Collections.replaceAll(list, "two", "one");
    System.out.println(list);
    // [one, one, three, null, one, null, five]

    Collections.replaceAll(list, "five", null);
    System.out.println(list);
    // [one, one, three, null, one, null, null]

    Collections.replaceAll(list, null, "none");
    System.out.println(list);
    // [one, one, three, none, one, none, none]