Java 集合/映射应用方法等效吗？
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Java collection/map apply method equivalent?
提问by joshjdevl
I would like to apply a function to a Java collection, in this particular case a map. Is there a nice way to do this? I have a map and would like to just run trim() on all the values in the map and have the map reflect the updates.
我想将一个函数应用于 Java 集合，在这种特殊情况下是一个地图。有没有很好的方法来做到这一点？我有一张地图，只想对地图中的所有值运行 trim() 并让地图反映更新。
采纳答案by erickson
With Java 8's lambdas, this is a one liner:
使用 Java 8 的 lambdas，这是一个单行代码：
map.replaceAll((k, v) -> v.trim());
For the sake of history, here's a version without lambdas:
为了历史起见，这里有一个没有 lambdas 的版本：
public void trimValues(Map<?, String> map) {
for (Map.Entry<?, String> e : map.entrySet()) {
String val = e.getValue();
if (val != null)
e.setValue(val.trim());
}
}
Or, more generally:
或者，更一般地说：
interface Function<T> {
T operate(T val);
}
public static <T> void replaceValues(Map<?, T> map, Function<T> f)
{
for (Map.Entry<?, T> e : map.entrySet())
e.setValue(f.operate(e.getValue()));
}
Util.replaceValues(myMap, new Function<String>() {
public String operate(String val)
{
return (val == null) ? null : val.trim();
}
});
回答by Alnitak
Whether you can modify your collection in-place or not depends on the class of the objects in the collection.
是否可以就地修改集合取决于集合中对象的类。
If those objects are immutable (which Strings are) then you can't just take the items from the collection and modify them - instead you'll need to iterate over the collection, call the relevant function, and then put the resulting value back.
如果这些对象是不可变的（字符串是），那么您不能只从集合中获取项目并修改它们 - 相反，您需要遍历集合，调用相关函数，然后将结果值放回去。
回答by Steve Kuo
You'll have to iterate over all the entries and trim each String value. Since String is immutable you'll have to re-put it in the map. A better approach might be to trim the values as they're placed in the map.
您必须遍历所有条目并修剪每个 String 值。由于 String 是不可变的，因此您必须将其重新放入地图中。更好的方法可能是在值放置在地图中时对其进行修剪。
回答by CoverosGene
Might be overkill for something like this, but there are a number of really good utilities for these types of problems in the Apache Commons Collectionslibrary.
对于这样的事情可能有点矫枉过正，但是Apache Commons Collections库中有许多非常好的实用程序可以解决这些类型的问题。
Map<String, String> map = new HashMap<String, String>();
map.put("key1", "a ");
map.put("key2", " b ");
map.put("key3", " c");
TransformedMap.decorateTransform(map,
TransformerUtils.nopTransformer(),
TransformerUtils.invokerTransformer("trim"));
I highly recommend the Jakarta Commons Cookbookfrom O'Reilly.
我强烈推荐O'Reilly的Jakarta Commons Cookbook。
回答by Fortyrunner
You could also take a look at Google Collections
你也可以看看谷歌收藏
回答by Jean Hominal
I don't know a way to do that with the JDK libraries other than your accepted response, however Google Collections lets you do the following thing, with the classes com.google.collect.Mapsand com.google.common.base.Function:
除了您接受的响应之外，我不知道使用 JDK 库的方法，但是 Google Collections 允许您使用类com.google.collect.Maps和执行以下操作com.google.common.base.Function：
Map<?,String> trimmedMap = Maps.transformValues(untrimmedMap, new Function<String, String>() {
public String apply(String from) {
if (from != null)
return from.trim();
return null;
}
}
The biggest difference of that method with the proposed one is that it provides a view to your original map, which means that, while it is always in sync with your original map, the applymethod could be invoked many times if you are manipulating said map heavily.
该方法与建议方法的最大区别在于它提供了原始地图的视图，这意味着虽然它始终与原始地图同步，但apply如果您大量操作所述地图，则可以多次调用该方法.
A similar Collections2.transform(Collection<F>,Function<F,T>)method exists for collections.
Collections2.transform(Collection<F>,Function<F,T>)集合存在类似的方法。
回答by Raphael Jolivet
I have come up with a "Mapper" class
我想出了一个“映射器”类
public static abstract class Mapper<FromClass, ToClass> {
private Collection<FromClass> source;
// Mapping methods
public abstract ToClass map(FromClass source);
// Constructors
public Mapper(Collection<FromClass> source) {
this.source = source;
}
public Mapper(FromClass ... source) {
this.source = Arrays.asList(source);
}
// Apply map on every item
public Collection<ToClass> apply() {
ArrayList<ToClass> result = new ArrayList<ToClass>();
for (FromClass item : this.source) {
result.add(this.map(item));
}
return result;
}
}
That I use like that :
我这样使用：
Collection<Loader> loaders = new Mapper<File, Loader>(files) {
@Override public Loader map(File source) {
return new Loader(source);
}
}.apply();
回答by nmr
I ended up using a mutation of @erickson's answer, mutated to:
我最终使用了@erickson 答案的变异，变异为：
- return a new
Collection, not modify in place - return
Collections with elements of type equal to the return type of theFunction - support mapping over either the values of a map or the elements of a list
- 返回一个新的
Collection，而不是就地修改 - 返回
Collection元素类型等于返回类型的 sFunction - 支持映射映射的值或列表的元素
Code:
代码：
public static interface Function<L, R> {
L operate(R val);
}
public static <K, L, R> Map<K, L> map(Map<K, R> map, Function<L, R> f) {
Map<K, L> retMap = new HashMap<K, L>();
for (Map.Entry<K, R> e : map.entrySet()) retMap.put(e.getKey(), f.operate(e.getValue()));
return retMap;
}
public static <L, R> List<L> map(List<R> list, Function<L, R> f) {
List<L> retList = new ArrayList<L>();
for (R e : list) retList.add(f.operate(e));
return retList;
}
