1. Introduction
This cookbook article is organized into small and focused recipes and code snippets for using Guava style collections.
The format is that of a growing list of code examples with no additional explanation necessary – it is meant to keep common usages of the API easy to access during development.
2. The Recipes
downcast a List<Parent> to a List<Child>
– note: this is a workaround for non-covariant generified collections in Java
class CastFunction<F, T extends F> implements Function<F, T> {
@Override
public final T apply(final F from) {
return (T) from;
}
}
List<TypeParent> originalList = Lists.newArrayList();
List<TypeChild> theList = Lists.transform(originalList,
new CastFunction<TypeParent, TypeChild>());
simpler alternative without Guava – involving 2 cast operations
List<Number> originalList = Lists.newArrayList();
List<Integer> theList = (List<Integer>) (List<? extends Number>) originalList;
adding an iterable to a collection
Iterable<String> iter = Lists.newArrayList();
Collection<String> collector = Lists.newArrayList();
Iterables.addAll(collector, iter);
check if collection contains element(s) according to a custom matching rule
Iterable<String> theCollection = Lists.newArrayList("a", "bc", "def");
boolean contains = Iterables.any(theCollection, new Predicate<String>() {
@Override
public boolean apply(final String input) {
return input.length() == 1;
}
});
assertTrue(contains);
alternative solution using search
Iterable<String> theCollection = Sets.newHashSet("a", "bc", "def");
boolean contains = Iterables.find(theCollection, new Predicate<String>() {
@Override
public boolean apply(final String input) {
return input.length() == 1;
}
}) != null;
assertTrue(contains);
alternative solution only applicable to Sets
Set<String> theCollection = Sets.newHashSet("a", "bc", "def");
boolean contains = !Sets.filter(theCollection, new Predicate<String>() {
@Override
public boolean apply(final String input) {
return input.length() == 1;
}
}).isEmpty();
assertTrue(contains);
NoSuchElementException on Iterables.find when nothing is found
Iterable<String> theCollection = Sets.newHashSet("abcd", "efgh", "ijkl");
Predicate<String> inputOfLengthOne = new Predicate<String>() {
@Override
public boolean apply(final String input) {
return input.length() == 1;
}
};
String found = Iterables.find(theCollection, inputOfLengthOne);
– this will throw the NoSuchElementException exception:
java.util.NoSuchElementException
at com.google.common.collect.AbstractIterator.next(AbstractIterator.java:154)
at com.google.common.collect.Iterators.find(Iterators.java:712)
at com.google.common.collect.Iterables.find(Iterables.java:643)
– solution: there is an overloaded find method that takes the default return value as an argument and can be called with null for the desired behavior:
String found = Iterables.find(theCollection, inputOfLengthOne, null);
remove all null values from a collection
List<String> values = Lists.newArrayList("a", null, "b", "c");
Iterable<String> withoutNulls = Iterables.filter(values, Predicates.notNull());
create immutable List/Set/Map directly
ImmutableList<String> immutableList = ImmutableList.of("a", "b", "c");
ImmutableSet<String> immutableSet = ImmutableSet.of("a", "b", "c");
ImmutableMap<String, String> imuttableMap =
ImmutableMap.of("k1", "v1", "k2", "v2", "k3", "v3");
create immutable List/Set/Map from a standard collection
List<String> muttableList = Lists.newArrayList();
ImmutableList<String> immutableList = ImmutableList.copyOf(muttableList);
Set<String> muttableSet = Sets.newHashSet();
ImmutableSet<String> immutableSet = ImmutableSet.copyOf(muttableSet);
Map<String, String> muttableMap = Maps.newHashMap();
ImmutableMap<String, String> imuttableMap = ImmutableMap.copyOf(muttableMap);
alternative solution using builders
List<String> muttableList = Lists.newArrayList();
ImmutableList<String> immutableList =
ImmutableList.<String> builder().addAll(muttableList).build();
Set<String> muttableSet = Sets.newHashSet();
ImmutableSet<String> immutableSet =
ImmutableSet.<String> builder().addAll(muttableSet).build();
Map<String, String> muttableMap = Maps.newHashMap();
ImmutableMap<String, String> imuttableMap =
ImmutableMap.<String, String> builder().putAll(muttableMap).build();
3. More Guava Cookbooks
Guava is a comprehensive and fantastically useful library – here’s a few more APIs covered in cookbook form:
Enjoy.
4. Going Forward
As I mentioned in the beginning, I am experimenting with this different format – the cookbook – to try to gather simple common tasks of using Guava Collections in a single place. The focus of this format is simplicity and speed, so most recipes have no additional explanation other than the code example itself.
Finally – I am looking at this as a living document – I’m going to keep adding recipes and examples as I run into them. Feel free to provide more in the comments and I’ll look to incorporate them into the cookbook.
The implementation of all these examples and code snippets can be found over on GitHub – this is a Maven-based project, so it should be easy to import and run as it is.
