OnJava8-Examples/onjava/CountMap.java

// onjava/CountMap.java
// (c)2021 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://OnJava8.com for more book information.
// Unlimited-length Map containing sample data
// {java onjava.CountMap}
package onjava;
import java.util.*;
import java.util.stream.*;

public class CountMap
extends AbstractMap<Integer,String> {
  private int size;
  private static char[] chars =
    "ABCDEFGHIJKLMNOPQRSTUVWXYZ".toCharArray();
  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));
  }
  public static void main(String[] args) {
    final int size = 6;
    CountMap cm = new CountMap(60);
    System.out.println(cm);
    System.out.println(cm.get(500));
    cm.values().stream()
      .limit(size)
      .forEach(System.out::println);
    System.out.println();
    new Random(47).ints(size, 0, 1000)
      .mapToObj(cm::get)
      .forEach(System.out::println);
  }
}
/* Output:
{0=A0, 1=B0, 2=C0, 3=D0, 4=E0, 5=F0, 6=G0, 7=H0, 8=I0,
9=J0, 10=K0, 11=L0, 12=M0, 13=N0, 14=O0, 15=P0, 16=Q0,
17=R0, 18=S0, 19=T0, 20=U0, 21=V0, 22=W0, 23=X0, 24=Y0,
25=Z0, 26=A1, 27=B1, 28=C1, 29=D1, 30=E1, 31=F1, 32=G1,
33=H1, 34=I1, 35=J1, 36=K1, 37=L1, 38=M1, 39=N1, 40=O1,
41=P1, 42=Q1, 43=R1, 44=S1, 45=T1, 46=U1, 47=V1, 48=W1,
49=X1, 50=Y1, 51=Z1, 52=A2, 53=B2, 54=C2, 55=D2, 56=E2,
57=F2, 58=G2, 59=H2}
G19
A0
B0
C0
D0
E0
F0

Y9
J21
R26
D33
Z36
N16
*/