First, lets look at the Bird.

• Download the starter code.
• The questions skip a key piece: showFrame
• Uncomment the code that shows the bird.
• Questions?

Finishing ArrayList

Last time:

• List vs. Array
  • Interface vs. Built-in thing with special syntax
• ArrayWrapper

    int[] xs = {1, 2, 4};
    List ys = List.of(1, 2, 4);

Problem:

• Arrays have fixed size.
• To add an addional item, we first need to allocate a new array and copy all the existing items over.

ArrayList

The problem with ArrayWrapper#add is that adding one item requires allocating a new array and copying everything over.

New plan: Instead of adding only one space when we allocate a new array, we’ll add several spaces.

On modern computers, we’ve got plenty of memory. And when we’re thinking about things with big-O, we can ignore constant factors. So we’ll double the size of the array whenever it grows.

Some useful methods:

    @SafeVarargs
    public static <T> ArrayList<T> of(T... args) {
      ArrayList<T> ys = new ArrayList<T>();
      for (int ii = args.length - 1; ii >= 0; --ii) {
            ys.append(args[ii]);
        }
        return ys;
    }
    
    @SuppressWarnings("unchecked")
    ArrayList() {
        // This appears to be the standard way to get a new array of T.
        this.data = (T[]) new Object[2];
        this.length = 0;
    }
    
    @SuppressWarnings("unchecked")
    void setCapacity(int nn) {
        T[] data1 = (T[]) new Object[nn];
        int len1 = Math.min(nn, this.length);

        for (int ii = 0; ii < len1; ++ii) {
            data1[ii] = this.data[ii];
        }

        this.data = data1;
        this.length = len1;
    }

Working from existing ArrayWrap:

• Copy to ArrayList
• Revert ArrayWrap to the grow-by-1 logic
• Have add only grow array when it needs to.
• Have the code count array slot writes.

Deep into Iteration #

• Write a method that appends “s” to each item in a list of Strings.
  • Start with for-each loop over array.
  • Run on example with excess capacity - crash.
  • Need C-style for loop.
• Write a method that adds 1 to each item in a list of ints.
  • Try for-each loop again.
  • This one doesn’t crash, but it appends zeros. Why?
• Implement Iterable for our ArrayList.
  • Need to import Iterator, but iterable is available by default.
  • Rewrite with for-each over the object itself.
• Abstract to map method.

  import java.util.function.Function;

  // ArrayList method
  <U> List<U> map(Function<T, U> op) {
    ArrayList ys = new ArrayList<U>();
    
    for (int ii = 0; ii < this.size(); ++ii) {
      ys.add(op.apply(this.get(ii)));
    }
    
    return ys;
  }
 
  // Usage:
  // xs.map((xx) -> xx + 1);