Is it possible to apply a function to a collection using Java Guava?

问题

I want to apply a function to a collection, map, etc, using Guava.

Basically, I need to resize the rows and columns of a Table separately so all rows and columns are of equal size, doing something like this:

    Table<Integer, Integer, Cell> table = HashBasedTable.create();
    Maps.transformValues(table.columnMap(), new ResizeFunction(BlockDimension.WIDTH));
    Maps.transformValues(table.rowMap(), new ResizeFunction(BlockDimension.HEIGHT));

public interface Cell {
    int getSize(BlockDimension dimension);
    void setSize(BlockDimension dimension);
}

I already have an idea of what the ResizeFunction should be. However, I need to apply it, not just return a Collection.

回答1:

In Guava, you don't convert existing Lists, but instead create a new one using Iterables.transform:

final List<String> list = Arrays.asList("race", "box");
final List<String> transformed =
    Lists.newArrayList(Iterables.transform(list, new Function<String, String>() {

        @Override
        public String apply(final String input) {
            return new StringBuilder().append(input).append("car").toString();
        }
    }));
System.out.println(transformed);

Output:

[racecar, boxcar]

Or, if you don't need a List and a Collection will do, you can use a transformed live view:

final Collection<String> transformed =
    Collections2.transform(list, new Function<String, String>() {

        @Override
        public String apply(final String input) {
            return new StringBuilder().append(input).append("car").toString();
        }
    });

This Collection is a live view of the underlying one, so changes to list will be reflected in this Collection.

回答2:

What about creating a function like this:

public static <T> void apply(Iterable<T> iterable, Function<T, Void> function) {
    for (T input : iterable)
        function.apply(input);
}

回答3:

Sean has already mentioned that Guava don't change the original collection, so you can't really "apply" a function on your existing collection.

It's not clear what your ResizeFunction function does, if you only changing the value of Cell in Table<Integer, Integer, Cell> then you can use Tables#transformValues()

Guava don't allow you to change the values of R and C in Table<R, C, V> (in standard Tables class) because those are used as keys while returning row or column map (Table#rowMap() and Table#columnMap()) and you cannot transform those because all of Guava's methods for transforming and filtering produce lazy results means the function/predicate is only applied when needed as the object is used. They don't create copies. Because of that, though, a transformation can easily break the requirements of a Set.

If you still want to do it then you can wrap Table object in your own class and provide a required methods.

回答4:

I think would would be better off setting up the table before population to be the appropriate size / have the appropriate number of cells even if they are empty. Something like:

for (int i=0; i<numRows; i++)
   for (int j=0; j<numColumns; j++)
       table.put(i,j,null);

This would ensure there is a cell for each position in your table. So long as you only add cells into row, column positions that are within numRows, numColumns you will keep a "square" table.

来源：https://stackoverflow.com/questions/7447002/is-it-possible-to-apply-a-function-to-a-collection-using-java-guava

标签

java

collections

Guava