使用享元自定义Collecction和Map

• 享元模式：共享对象，用于减少应用程序创建的对象数量，以减少内存占用和提高性能。

• 创建一个只读数组

  public class CountingIntegerList
  extends AbstractList <Integer> {
    private int size;
    public CountingIntegerList() { size = 0; }
    public CountingIntegerList(int size) {
      this.size = size < 0 ? 0 : size;
    }
      //重写 get 及 set
      //并没有实际存储 list，取值时再计算，可以创建无限大的数据集
    @Override public Integer get(int index) {
      return index;
    }
    @Override public int size() { return size; }
    public static void main(String [] args) {
      List <Integer> cil =
        new CountingIntegerList(30);
      System.out.println(cil);
      System.out.println(cil.get(500));
    }
  }
  

• 实现Map享元

  public class CountMap
  extends AbstractMap <Integer,String> {
    private int size;
    private static char [] chars =
      "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
      //自动计算 value
    private static String value(int key) {
      return
        chars [key % chars.length] +
        Integer.toString(key / chars.length);
    }
    public CountMap(int size) {
      this.size = size < 0 ? 0 : size;
    }
    @Override public String get(Object key) {
      return value((Integer)key);
    }
    private static class Entry
    implements Map.Entry <Integer,String> {
      int index;
      Entry(int index) { this.index = index; }
      @Override   public boolean equals(Object o) {
        return o instanceof Entry &&
          Objects.equals(index, ((Entry)o).index);
      }
      @Override public Integer getKey() { return index; }
      @Override public String getValue() {
        return value(index);
      }
      @Override public String setValue(String value) {
        throw new UnsupportedOperationException();
      }
      @Override public int hashCode() {
        return Objects.hashCode(index);
      }
    }
    @Override
    public Set <Map.Entry<Integer,String> > entrySet() {
      // LinkedHashSet retains initialization order:
      return IntStream.range(0, size)
        .mapToObj(Entry:: new)
        .collect(Collectors
          .toCollection(LinkedHashSet:: new));
    }