是否可以使用 Java Guava 将函数应用于集合？
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Is it possible to apply a function to a collection using Java Guava?
提问by Garrett Hall
I want to apply a function to a collection, map, etc, using Guava.
我想使用 Guava 将函数应用于集合、地图等。
Basically, I need to resize the rows and columns of a Tableseparately so all rows and columns are of equal size, doing something like this:
基本上，我需要分别调整 a 的行和列的Table大小，以便所有行和列的大小相同，执行如下操作：
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 ResizeFunctionshould be. However, I need to applyit, not just return a Collection.
我已经知道ResizeFunction应该是什么了。但是，我需要应用它，而不仅仅是返回一个Collection.
回答by Sean Patrick Floyd
In Guava, you don't convert existing Lists, but instead create a new one using Iterables.transform:
在 Guava 中，您不转换现有列表，而是使用 Iterables.transform 创建一个新列表：
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:
输出：
[racecar, boxcar]
[赛车、棚车]
Or, if you don't need a Listand a Collectionwill do, you can use a transformed live view:
或者，如果您不需要 a Listand a Collectionwill do，则可以使用转换后的实时视图：
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 Collectionis a live view of the underlying one, so changes to listwill be reflected in this Collection.
这Collection是底层视图的实时视图，因此更改list将反映在此 中Collection。
回答by Garrett Hall
What about creating a function like this:
创建这样的函数怎么样：
public static <T> void apply(Iterable<T> iterable, Function<T, Void> function) {
for (T input : iterable)
function.apply(input);
}
回答by Premraj
Sean has already mentioned that Guava don't change the original collection, so you can't really "apply" a function on your existing collection.
Sean 已经提到 Guava 不会更改原始集合，因此您无法真正在现有集合上“应用”功能。
It's not clear what your ResizeFunctionfunction does, if you only changing the value of Cellin Table<Integer, Integer, Cell>then you can use Tables#transformValues()
不清楚你的ResizeFunction函数是做什么的，如果你只改变Cellin的值，Table<Integer, Integer, Cell>那么你可以使用Tables#transformValues()
Guava don't allow you to change the values of Rand Cin Table<R, C, V>(in standard Tablesclass) 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.
Guava 不允许您更改Rand Cin Table<R, C, V>（在标准Tables类中）的值，因为它们在返回行或列映射（Table#rowMap()and Table#columnMap()）时用作键，并且您无法转换它们，因为所有 Guava 的转换和过滤方法都会产生惰性结果意味着函数/谓词仅在需要时应用，因为使用对象。他们不创建副本。正因为如此，转换很容易打破 Set 的要求。
If you still want to do it then you can wrap Tableobject in your own class and provide a required methods.
如果您仍然想这样做，那么您可以将Table对象包装在您自己的类中并提供所需的方法。
回答by John B
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:
我认为最好在填充之前将表格设置为适当的大小/具有适当数量的单元格，即使它们是空的。就像是：
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.
这将确保表格中的每个位置都有一个单元格。只要您仅将单元格添加到 numRows、numColumns 内的行、列位置，您就会保留一个“方形”表格。
