Suppose I have an object that looks like:
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?
EDIT: I should have been more clear, I did not want to do this via iteration
Try this:
Collection<String> names = CollectionUtils.collect(personList, TransformerUtils.invokerTransformer("getName"));
Use apache commons collection api.
You'll have to iterate through your List<Obj> and collate the foo entries into a new List
e.g.
List<String> foos = new ArrayList<String>();
for (Obj obj : objs) {
foos.add(obj.foo)
}
or for Java 8 and beyond, use streams thus:
objs.stream().map(o -> o.foo).collect(toList());
var foos = objs.Select( x=>x.foo); until i realized it was a java question. Damn you LINQ goodness!
map in plain Java using something like Guava. There's (a bit) more work involved obviously, but it has advantages over allocating memory for a copy of all the foo values in the original list.
Using Guava you could create a view of the foo property of the objects in the List using Lists.transform like this:
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 view of the List<Obj> and as such it doesn't allocate a whole separate ArrayList or 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 foos list will reflect that change.
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:
List<Obj> objs = ...
List<String> foos = objs.map(#Obj.getFoo);
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")
Here is a more verbose, but typesafe solution using Apache Commons Collections:
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.
Below is the less typesafe version for Apache Collections Version < 4, which does not use generics:
Collection values = CollectionUtils.collect(messages, new Transformer(){
@Override
public Object transform(Object input) {
Obj obj = (Obj) input;
return obj.getFoo();
}
});
Iterate through the list and create a Set of all foo properties.
something like this?
List<String> foos = new ArrayList<String>();
for (Obj obj : objList )
{
foos.addElement(obj.foo);
}
