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cs2381 Notes: 10 Higher Order Methods

··1 min

Introducing Map

Setup:

  • A method that takes a List<Pet> and ages each pet by one year.
  • A method that doubles each number in a list.
  // Import
  import java.util.function.Function;

  // List
  <U> List<U> map(Function<T, U> op);

  // Empty
  @Override
  public <U> List<U> map(Function<T, U> op) {
    return new Empty<U>();
  }

  // Map
  @Override
  public <U> List<U> map(Function<T, U> op) {
    return new Cell<U>(op.apply(this.first()), this.rest.map(op));
  }
  
  // Add one to each number.
  void example() {
    var xs = List.list(1,2,3,4);
    var ys = xs.map((xx) -> xx + 1);
  }

Now rewrite our two example methods to use map.

Notes:

  • The thing with “->” is called an anonymous function or a lambda function.
  • It constructs an object with no name, of a type with no name, that implements the interface Function[T, U].

Introducing Filter

Setup:

  • A method that takes a List<Pet> and selects only dogs.
  • A method that takes a list of int and takes only even numbers.

Punchline:

  • Write List#filter, which abstracts filtering with a function arg.
  • Rewrite the two things using filter.