Java相当于C＃Linq中的Where子句

Question

I can do this in C# :

int CheetahNumber = 77;
Animal Cheetah = Model.Animals
   .Where(e => e.AnimalNo.Equals(CheetahNumber))
   .FirstOrDefault();

For example in Java I have ArrayList<Animal> Animals

How can I query such an ArrayList? Thanks.

Answer 1

Java 8 introduces the Stream API that allows similar constructs to those in Linq.

Your query for example, could be expressed:

int cheetahNumber = 77;

Animal cheetah = animals.stream()
  .filter((animal) -> animal.getNumber() == cheetahNumber)
  .findFirst()
  .orElse(Animal.DEFAULT);

You'll obviously need to workout if a default exists, which seems odd in this case, but I've shown it because that's what the code in your question does.

Answer 2

You can try it by using streams:

public String getFirstDog(List<Animal> animals) {
    Animal defaultDog = new Dog();
    Animal animal = animalNames.stream(). //get a stream of all animals 
        filter((s) -> s.name.equals("Dog")).findFirst(). //Filter for dogs and find the first one
        orElseGet(() -> defaultDog ); //If no dog available return an default animal.
                                        //You can omit this line.
    return animal;
}

Answer 3

Even though there Java does not provide you with LINQ equal constructs but you can achieve some level what LINQ operations with Java 8 stream constructs.

such as one

List<String> items = new ArrayList<String>();
items.add("one"); 
items.add("two");
items.add("three");

Stream<String> stream = items.stream();  
stream.filter( item ->  item.startsWith("o") );

Take a look at java8 stream