The following is the specification of useful members of a Java (generic) map/dictionary/hashtable class HashMap useful in developing translators. A closely equivalent C# (generic) version appears lower on this page.
The earlier versions of Java and C# did not have generic versions of classes like these, but worked with elements of the Object class, which necessitated casting operations.
import java.util.*;
class HashMap<K,V>
// Class for constructing a dictionary (map) of elements with keys of type and values of type V in C#
public HashMap<K,V>()
// Empty dictionary constructor
public HashMap<K,V>(int capacity)
// Empty dictionary with specified initial capacity
public void clear()
// Clears all elements from dictionary
public int size()
// Returns number of elements in dictionary
public V put(K key, V value)
// Adds (K, V) to dictionary (replacing any existing entry with that key)
public V get(K key)
// Retrieves an element with specified key from dictionary, or null if no such element exists
public boolean isEmpty()
// Returns true if dictionary is empty
public boolean containsKey(K key)
// Returns true if element with specified key is in the dictionary
public boolean containsValue(V value)
// Returns true if element with specified value is in the dictionary
public remove(K key)
// Removes the element with specified key from dictionary (no effect if no such element exists)
public Set<K> keySet()
// Returns a set of all the keys in the dictionary (useful for iterations)
public Collection<V> values()
// Returns a collection of all the values in the dictionary (useful for iterations)
public Set<MapEntry<K, V>> entrySet()
// Returns a set of all the key/value pairs in the dictionary (useful for iterations)
} // HashMap
Here is a sample program showing the use of this library
import library.*;
import java.util.*;
class ShowGenericDictionary {
// Demonstrate simple application of the generic HashMap dictionary class in Java
static class Entry {
public String name;
public int age; // other fields could be added
public Entry(String name, int age) { // constructor
this.name = name;
this.age = age;
}
public String toString() {
return name + " " + age;
} // toString
} // Entry
static HashMap<String, Entry> dict = new HashMap<String, Entry>(); // global for simplicity here!
public static void main (String[] args) {
boolean found = false;
Entry lookEntry = null;
// Build a dictionary of students' IDs, names and ages
IO.writeLine("Supply a list of student numbers, first names and ages. CTRL-Z to terminate");
do {
String studentNumber = IO.readWord();
if (IO.eof()) break;
String name = IO.readWord();
int age = IO.readInt();
IO.readLn();
dict.put(studentNumber, new Entry(name, age)); // add to dictionary (hash table)
} while (!IO.eof());
// Now demonstrate accesses to this dictionary
Entry patEntry = new Entry("Pat", 61); // that fellow again!
dict.put("63T0844", patEntry); // add him to the dictionary anyway
IO.writeLine(dict.size() + " items stored"); // report size of dictionary
IO.writeLine(dict.containsKey("Pat")); // false
IO.writeLine(dict.containsKey("63T0844")); // true
IO.writeLine(dict.containsValue(patEntry)); // true
dict.put("66A1234", new Entry("Peter", 69)); // not there so add("66A1234", "Peter", 69);
IO.writeLine(dict.get("66A1234")); // just to make sure, use indexer to retrieve entry value
patEntry.age = 69; // Modify Pat's record (values are public, so you can)
dict.put("66A1234", new Entry("Richard", 63)); // Replace the old entry completely
IO.writeLine(dict.get("63T0844")); // Just to convince you
IO.writeLine(dict.get("66A1234")); // Just to convince you
found = dict.get("66A1234") != null; // get should succeed, so return true
IO.writeLine("Peter " + found); // Peter true
found = dict.get("00F3434") != null; // get should fail gracefully, so return null
IO.writeLine("Fred " + found); // Fred false true
IO.writeLine();
Set<String> keys = dict.keySet();
for (String s : keys) IO.writeLine(s); // iterate over keys
IO.writeLine();
for (String s : dict.keySet()) IO.writeLine(s); // iterate over keys directly
IO.writeLine();
Collection<Entry> values = dict.values();
for (Entry e : values) IO.writeLine(e); // iterate over values
IO.writeLine();
for (Entry e : dict.values()) IO.writeLine(e); // iterate over values directly
IO.writeLine();
dict.remove("63T0844"); // Pat has had his turn - get rid of him
for (Map.Entry<String, Entry> me : dict.entrySet()) // retrieve both key and value components
IO.writeLine(me.getKey() + " : " + me.getValue());
IO.writeLine();
dict.remove("63T0844"); // Pat has had his turn - and we already get rid of him
dict.remove("60Z5674"); // Zed was never there - ignore gracefully
} // main
} // ShowGenericDictionary
The following is the specification of useful members of the C# generic map/dictionary/hashtable Dictionary class useful in developing translators. A closely equivalent Java version appears earlier on this page.
The earlier versions of Java and C# did not have generic versions of such classes, but worked with elements of the Object class, which necessitated casting operations.
using System.Collections.Generic;
class Dictionary<K,V>
// Class for constructing a dictionary (map) of elements with keys of type and values of type V in C#
public Dictionary<K,V>()
// Empty dictionary constructor
public Dictionary<K,V>(int capacity)
// Empty dictionary with specified initial capacity
public void Clear()
// Clears all elements from dictionary
public int Count ( get; }
// Returns number of elements in dictionary
public void Add(K key, V value)
// Adds (K, V) to dictionary (replacing any existing entry with that key)
V this [K key] { set; }
// indexer - adds new element, or replaces existing element, using specified key
V this [K key] { get; }
// indexer - retrieves element using specified key, or raises exception if not present
// (Protect yourself by using ContainsKey or TryGetValue!)
// public bool IsEmpty() - not a member
public bool ContainsKey(K key)
// Returns true if element with specified key is in the dictionary
public bool ContainsValue(V value)
// Returns true if element with specified value is in the dictionary
public bool TryGetValue(K key, out V value)
// Returns true if element with specified key is in the dictionary, returning that value
// Returns false if element with specified key is notin the dictionary, returning null
public Remove(K key)
// Removes the element with specified key from dictionary (no effect if no such element exists)
public ICollection<K> Keys { get; }
// Returns a set of all the keys in the dictionary (useful for iterations)
public ICollection<V> Values ( get; }
// Returns a collection of all the values in the dictionary (useful for iterations)
// Iterating drectly over a generic dictionay interface returns a sequence of KeyValuePair
// constructs
//
// public struct KeyValuePair <K, V> {
// public K Key { get ; }
// public V Value { get ; }
// } // KeyValuePair
} // Dictionary
Here is a sample program like the one given earlier showing the use of this library
using Library;
using System.Collections.Generic;
using System.Text;
class ShowGenericDictionary {
// Demonstrate simple application of the generic Dictionary class in C#
class Entry {
public string name;
public int age; // other fields could be added
public Entry(string name, int age) { // constructor
this.name = name;
this.age = age;
}
public override string ToString() {
return name + " " + age;
} // ToString
} // Entry
static Dictionary<string, Entry> dict = new Dictionary<string, Entry>(); // global for simplicity here!
public static void Main (string[] args) {
bool found = false;
Entry lookEntry = null;
// Build a dictionary of students' IDs, names and ages
IO.WriteLine("Supply a list of student numbers, first names and ages. CTRL-Z to terminate");
do {
string studentNumber = IO.ReadWord();
if (IO.EOF()) break;
string name = IO.ReadWord();
int age = IO.ReadInt();
IO.ReadLn();
dict.Add(studentNumber, new Entry(name, age)); // add to end of list
} while (!IO.EOF());
// Now demonstrate accesses to this dictionary
Entry patEntry = new Entry("Pat", 61); // that fellow again!
dict.Add("63T0844", patEntry); // add him to the dictionary anyway
IO.WriteLine(dict.Count + " items stored"); // report size of dictionary
IO.WriteLine(dict.ContainsKey("Pat")); // false
IO.WriteLine(dict.ContainsKey("63T0844")); // true
IO.WriteLine(dict.ContainsValue(patEntry)); // true
dict["66A1234"] = new Entry("Peter", 69); // not there so add("66A1234", "Peter", 69);
IO.WriteLine(dict["66A1234"]); // just to make sure, use indexer to retrieve entry value
patEntry.age = 69; // Modify Pat's record (values are public, so you can)
dict["66A1234"] = new Entry("Richard", 63); // Replace the old entry completely
IO.WriteLine(dict["63T0844"]); // Just to convince you
IO.WriteLine(dict["66A1234"]); // Just to convince you
found = dict.TryGetValue("66A1234", out lookEntry); // should succeed returning a valid entry
IO.WriteLine("Peter " + found + " | " + (lookEntry == null)); // Peter true false
found = dict.TryGetValue("00F3434", out lookEntry); // should fail gracefully, returning null
IO.WriteLine("Fred " + found + " | " + (lookEntry == null)); // Fred false true
IO.WriteLine();
ICollection<string> keys = dict.Keys;
foreach (string s in keys) IO.WriteLine(s); // iterate over keys
IO.WriteLine();
foreach (string s in dict.Keys) IO.WriteLine(s); // iterate over keys directly
IO.WriteLine();
ICollection<Entry> values = dict.Values;
foreach (Entry e in values) IO.WriteLine(e); // iterate over values
IO.WriteLine();
foreach (Entry e in dict.Values) IO.WriteLine(e); // iterate over values directly
IO.WriteLine();
dict.Remove("63T0844"); // Pat has had his turn - get rid of him
foreach (KeyValuePair<string, Entry> kv in dict) // retrieve both key and value components
IO.WriteLine(kv.Key + " : " + kv.Value);
IO.WriteLine();
dict.Remove("63T0844"); // Pat has had his turn - and we already get rid of him
dict.Remove("60Z5674"); // Zed was never there - ignore gracefully
// lookEntry = dict["Not there"]; // throws exception; rather protect with TryGetValue or ContainsKey
} // Main
} // ShowGenericDictionary
Revised: 2014/08/09