Java Tutorial/Collections/Map

A map declared to hold objects of a type T can also hold objects that extend from T

   <source lang="java">

import java.util.HashMap; import java.util.Map; public class Main {

 public static void main(String[] argv) {
   Map<Number, String> numMap = new HashMap<Number, String>();
   numMap.put(.5, "half");
   numMap.put(1, "first");
 }

}</source>

Automatically Removing an Unreferenced Element from a Hash Table

   <source lang="java">

import java.util.Iterator; import java.util.Map; import java.util.WeakHashMap; public class Main {

 public static void main(String[] argv) throws Exception {
   Map weakMap = new WeakHashMap();
   Object keyObject = "";
   Object valueObject = "";
   weakMap.put(keyObject, valueObject);
   Iterator it = weakMap.keySet().iterator();
   while (it.hasNext()) {
     Object key = it.next();
   }
 }

}</source>

A value retrieved from a type-specific collection does not need to be casted

   <source lang="java">

import java.net.MalformedURLException; import java.net.URL; import java.util.HashMap; import java.util.Map; public class Main {

 public static void main(String[] argv) {
   Map<String, URL> urlMap = new HashMap<String, URL>();
   try {
     urlMap.put("java", new URL("http://www.jexp.ru"));
   } catch (MalformedURLException e) {
   }
   String s = urlMap.get("java").getHost();
 }

}</source>

Convert Properties into Map

   <source lang="java">

import java.util.Properties; import java.util.Map; import java.util.HashMap; import java.util.Set; public class Main {

 public static void main(String[] args) {
   Properties properties = new Properties();
   properties.setProperty("name", "Designer");
   properties.setProperty("version", "1.0");
   properties.setProperty("vendor", "Inc");
   Map<String, String> map = new HashMap<String, String>((Map) properties);
   Set propertySet = map.entrySet();
   for (Object o : propertySet) {
     Map.Entry entry = (Map.Entry) o;
     System.out.printf("%s = %s%n", entry.getKey(), entry.getValue());
   }
 }

}</source>

Create an array containing the keys in a map

   <source lang="java">

import java.util.HashMap; import java.util.Map; public class Main {

 public static void main(String[] argv) throws Exception {
   Map map = new HashMap();
   Object[] objectArray = map.keySet().toArray();
   MyClass[] array = (MyClass[]) map.keySet().toArray(new MyClass[map.keySet().size()]);
 }

} class MyClass { }</source>

Create an array containing the values in a map

   <source lang="java">

import java.util.HashMap; import java.util.Map; public class Main {

 public static void main(String[] argv) throws Exception {
   Map map = new HashMap();
   Object[] objectArray = map.values().toArray();
   MyClass[] array = (MyClass[]) map.values().toArray(new MyClass[map.values().size()]);
 }

} class MyClass { }</source>

Create type specific collections

   <source lang="java">

import java.util.Map; import java.util.HashMap; import java.util.List; import java.util.ArrayList; public class Main {

 public static void main(String[] args) {
   Map<Integer, String> grades = new HashMap<Integer, String>();
   grades.put(1, "A");
   grades.put(2, "B");
   grades.put(3, "C");
   grades.put(4, "D");
   grades.put(5, "E");
   String value = grades.get(1);
   List<String> dayNames = new ArrayList<String>();
   dayNames.add("Sunday");
   dayNames.add("Monday");
   dayNames.add("Tuesday");
   dayNames.add("Wednesday");
   String firstDay = dayNames.get(0);
 }

}</source>

Creating a Hash Table

   <source lang="java">

import java.util.HashMap; import java.util.Iterator; import java.util.Map; import java.util.TreeMap; public class Main {

 public static void main(String[] argv) throws Exception {
   Map<String, Integer> map = new HashMap<String, Integer>();
   map = new TreeMap();
   map.put("a", new Integer(1));
   map.put("b", new Integer(2));
   map.put("c", new Integer(3));
   int size = map.size(); // 2
   Object oldValue = map.put("a", new Integer(9)); // 1
   oldValue = map.remove("c"); // 3
   Iterator it = map.keySet().iterator();
   while (it.hasNext()) {
     Object key = it.next();
   }
   it = map.values().iterator();
   while (it.hasNext()) {
     Object value = it.next();
   }
 }

}</source>

Creating a Map That Retains Order-of-Insertion

   <source lang="java">

import java.util.Iterator; import java.util.LinkedHashMap; import java.util.Map; public class Main {

 public static void main(String[] argv) throws Exception {
   Map<String, String> map = new LinkedHashMap<String, String>();
   map.put("1", "value1");
   map.put("2", "value2");
   map.put("3", "value3");
   map.put("2", "value4");
   for (Iterator it = map.keySet().iterator(); it.hasNext();) {
     Object key = it.next();
     Object value = map.get(key);
   }
 }

}</source>

Creating and storing arrays in a map

   <source lang="java">

import java.util.Iterator; import java.util.Map; import java.util.TreeMap; public class Main {

 public static void main(String[] args) {
   Map<String, int[]> map = new TreeMap<String, int[]>();
   int[] array = new int[3];
   array[0] = 0;
   array[1] = 1;
   array[2] = 2;
   map.put("array", array);
   Iterator<String> iter = map.keySet().iterator();
   while (iter.hasNext()) {
     String arrayName = iter.next();
     array = map.get(arrayName);
     System.out.print(arrayName + ":");
     for (int i = 0; i < array.length; i++) {
       System.out.print(array[i]);
     }
   }
 }

} //array:012</source>

Creating a Type-Specific Map [5.0]

   <source lang="java">

import java.util.HashMap; import java.util.Map; public class Main {

 public static void main(String[] argv) throws Exception {
   Map<Integer, String> map = new HashMap<Integer, String>();
   map.put(1, "first");
   map.put(2, "second");
   // map.put(1, 2); <- Syntax error
 }

}</source>

Creating a Type-Specific Map: creates a map whose keys are Integer objects and values are String objects.

   <source lang="java">

import java.util.HashMap; import java.util.Map; public class Main {

 public static void main(String[] argv) {
   Map<Integer, String> map = new HashMap<Integer, String>();
   map.put(1, "first");
   map.put(2, "second");
   // map.put(1, 2); <- Syntax error
 }

}</source>

Get a key from value with an HashMap

   <source lang="java">

import java.util.HashMap; import java.util.Map; public class Main {

 public static void main(String[] argv) {
   Map<String, String> map = new HashMap<String, String>();
   map.put("1","one");
   map.put("2","two");
   map.put("3","three");
   map.put("4","four");
   
   System.out.println(getKeyFromValue(map,"three"));
 }
 public static Object getKeyFromValue(Map hm, Object value) {
   for (Object o : hm.keySet()) {
     if (hm.get(o).equals(value)) {
       return o;
     }
   }
   return null;
 }

}</source>

Map techniques.

   <source lang="java">

import java.util.Collection; import java.util.Map; import java.util.Set; import java.util.TreeMap; public class Main {

 public static void main(String args[]) {
   Map<String, Integer> atomNums = new TreeMap<String, Integer>();
   atomNums.put("A", 1);
   atomNums.put("B", 2);
   atomNums.put("C", 3);
   atomNums.put("D", 4);
   atomNums.put("E", 5);
   atomNums.put("F", 6);
   System.out.println("The map contains these " + atomNums.size() + " entries:");
   Set<Map.Entry<String, Integer>> set = atomNums.entrySet();
   for (Map.Entry<String, Integer> me : set) {
     System.out.print(me.getKey() + ", Atomic Number: ");
     System.out.println(me.getValue());
   }
   TreeMap<String, Integer> atomNums2 = new TreeMap<String, Integer>();
   atomNums2.put("Q", 30);
   atomNums2.put("W", 82);
   atomNums.putAll(atomNums2);
   set = atomNums.entrySet();
   System.out.println("The map now contains these " + atomNums.size() + " entries:");
   for (Map.Entry<String, Integer> me : set) {
     System.out.print(me.getKey() + ", Atomic Number: ");
     System.out.println(me.getValue());
   }
   if (atomNums.containsKey("A"))
     System.out.println("A has an atomic number of " + atomNums.get("A"));
   if (atomNums.containsValue(82))
     System.out.println("The atomic number 82 is in the map.");
   System.out.println();
   if (atomNums.remove("A") != null)
     System.out.println("A has been removed.\n");
   else
     System.out.println("Entry not found.\n");
   Set<String> keys = atomNums.keySet();
   for (String str : keys)
     System.out.println(str + " ");
   Collection<Integer> vals = atomNums.values();
   for (Integer n : vals)
     System.out.println(n + " ");
   atomNums.clear();
   if (atomNums.isEmpty())
     System.out.println("The map is now empty.");
 }

}</source>

Retrieve environment variables (JDK1.5)

   <source lang="java">

import java.util.Iterator; import java.util.Map; public class Main {

 public static void main(String args[]) {
   System.out.println("PATH = " + System.getenv("PATH"));
   Map env = System.getenv();
   for (Iterator it = env.entrySet().iterator(); it.hasNext();) {
     Map.Entry entry = (Map.Entry) it.next();
     System.out.println(entry.getKey() + " = " + entry.getValue());
   }
 }

}</source>

Sort based on the values

   <source lang="java">

import java.util.ArrayList; import java.util.HashMap; import java.util.Iterator; import java.util.LinkedHashMap; import java.util.List; import java.util.Map; import java.util.Set; import java.util.TreeSet; public class Main {

 public static void main(String[] a) {
   Map<String, String> yourMap = new HashMap<String, String>();
   yourMap.put("1", "one");
   yourMap.put("2", "two");
   yourMap.put("3", "three");
   Map<String, Object> map = new LinkedHashMap<String, Object>();
   List<String> keyList = new ArrayList<String>(yourMap.keySet());
   List<String> valueList = new ArrayList<String>(yourMap.values());
   Set<String> sortedSet = new TreeSet<String>(valueList);
   
   Object[] sortedArray = sortedSet.toArray();
   int size = sortedArray.length;
   for (int i = 0; i < size; i++) {
     map.put(keyList.get(valueList.indexOf(sortedArray[i])), sortedArray[i]);
   }
   Set ref = map.keySet();
   Iterator it = ref.iterator();
   while (it.hasNext()) {
     String i = (String) it.next();
     System.out.println(i);
   }
 }

}</source>