java 如何从对象的 ArrayList 中获取属性列表
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How to Get attributes list from ArrayList of objects
提问by ninjasense
Suppose I have an object that looks like:
假设我有一个看起来像的对象：
public class Obj {
String foo;
String bar;
}
If I create an arraylist of type Obj and populate it, is there a way I can return a list of all the objects in the array's foo attribute from the ArrayList?
如果我创建一个 Obj 类型的数组列表并填充它，有没有办法可以从 ArrayList 返回数组的 foo 属性中所有对象的列表？
EDIT: I should have been more clear, I did not want to do this via iteration
编辑：我应该更清楚，我不想通过迭代来做到这一点
采纳答案by Brian Agnew
You'll have to iterate through your List<Obj>and collate the foo entries into a new List
您必须遍历您List<Obj>的 foo 条目并将其整理成一个新的List
e.g.
例如
List<String> foos = new ArrayList<String>();
for (Obj obj : objs) {
foos.add(obj.foo)
}
or for Java 8 and beyond, use streams thus:
或者对于 Java 8 及更高版本，请使用流：
objs.stream().map(o -> o.foo).collect(toList());
回答by Surodip
Try this:
试试这个：
Collection<String> names = CollectionUtils.collect(personList, TransformerUtils.invokerTransformer("getName"));
Use apache commons collection api.
使用 apache 公共资源集合 api。
回答by ColinD
Using Guavayou could create a view of the fooproperty of the objects in the Listusing Lists.transformlike this:
使用Guava，您可以foo在Listusing Lists.transform 中创建对象属性的视图，如下所示：
public class Obj {
String foo;
String bar;
public static final Function<Obj, String> FOO = new Function<Obj, String>() {
public String apply(Obj input) {
return input.foo;
}
};
}
public void someMethod(List<Obj> objs) {
List<String> foos = Lists.transform(objs, Obj.FOO);
...
}
Unlike other solutions, this is purely a viewof the List<Obj>and as such it doesn't allocate a whole separate ArrayListor some such in memory and can be created in almost no time regardless of the size of your List<Obj>. Additionally, if you change the original List<Obj>, the fooslist will reflect that change.
与其他解决方案不同，这纯粹是一个视图，List<Obj>因此它不会ArrayList在内存中分配一个完整的单独或一些这样的内容，并且几乎可以在任何时间创建，无论List<Obj>. 此外，如果您更改原始List<Obj>，foos列表将反映该更改。
In Java 8 (due in 2012 sometime), this will become a lot easier with lambda expressions and method references. You'll be able to do something like this:
在 Java 8（将于 2012 年某个时候）中，使用 lambda 表达式和方法引用将变得更加容易。你将能够做这样的事情：
List<Obj> objs = ...
List<String> foos = objs.map(#Obj.getFoo);
回答by jbandi
The answer of @Surodip uses a compact solution based on Apache Commons Collections.
But that solution is not typesafe, since the Transfomer references the property via string expression: TransformerUtils.invokerTransformer("getName")
@Surodip 的答案使用基于Apache Commons Collections的紧凑解决方案。但该解决方案不是类型安全的，因为 Transfomer 通过字符串表达式引用属性：TransformerUtils.invokerTransformer("getName")
Here is a more verbose, but typesafe solution using Apache Commons Collections:
这是一个使用 Apache Commons Collections 的更详细但类型安全的解决方案：
Collection<String> values = CollectionUtils.collect(messages, new Transformer<Obj, String>(){
@Override
public String transform(Obj input) {
return input.getFoo();
}
});
The above solutions uses Apache Commons Collection Version >= 4, which supports generics for type safety.
上述解决方案使用 Apache Commons Collection Version >= 4，它支持泛型以实现类型安全。
Below is the less typesafe version for Apache Collections Version < 4, which does not use generics:
下面是 Apache Collections Version < 4 的类型安全较少的版本，它不使用泛型：
Collection values = CollectionUtils.collect(messages, new Transformer(){
@Override
public Object transform(Object input) {
Obj obj = (Obj) input;
return obj.getFoo();
}
});
回答by John Gardner
something like this?
像这样的东西？
List<String> foos = new ArrayList<String>();
for (Obj obj : objList )
{
foos.addElement(obj.foo);
}
回答by camickr
Iterate through the list and create a Set of all foo properties.
遍历列表并创建一个所有 foo 属性的集合。
