|
Java - HashMap
How to create and print a Map ?
|
HashMap01.java
|
|
import java.io.*;
import java.util.*;
public class HashMap01 {
public static void main(String[] args) {
Map<Integer,String> hMap=new HashMap<Integer,String>();
hMap.put(1001,"Item1");
hMap.put(1002,"Item2");
hMap.put(1003,"Item3");
System.out.println(hMap.toString());
for(Map.Entry m:hMap.entrySet()){
System.out.println(m.getKey()+","+m.getValue());
}
}
}
|
How to pass a Map into a Method ?
|
HashMap02.java
|
|
import java.util.*;
public class HashMap02 {
public static void main(String[] args) {
Map<Integer,String> hMap=new HashMap<Integer,String>();
hMap.put(1001,"Item1");
hMap.put(1002,"Item2");
hMap.put(1003,"Item3");
printMap(hMap);
}
public static void printMap(Map<Integer,String> hMap)
{
System.out.println("\n----------------------------------");
for(Map.Entry m:hMap.entrySet()){
System.out.println(m.getKey()+","+m.getValue());
}
}
}
|
How to retrieve an Entry from a Map ?
|
HashMap03.java
|
|
import java.util.*;
public class HashMap03 {
public static void main(String[] args) {
Map<Integer,String> hMap=new HashMap<Integer,String>();
hMap.put(1001,"Item1");
hMap.put(1002,"Item2");
hMap.put(1003,"Item3");
String hme = hMap.get(1002);
System.out.println(hme);
}
}
|
How to remove an Entry from a Map ?
|
HashMap04.java
|
|
import java.util.*;
public class HashMap04 {
public static void main(String[] args) {
Map<Integer,String> hMap=new HashMap<Integer,String>();
hMap.put(1001,"Item1");
hMap.put(1002,"Item2");
hMap.put(1003,"Item3");
System.out.print("Number of Entry = " + hMap.size());
printMap(hMap);
hMap.remove(1002);
System.out.print("Number of Entry = " + hMap.size());
printMap(hMap);
hMap.clear();
System.out.print("Number of Entry = " + hMap.size());
printMap(hMap);
}
public static void printMap(Map<Integer,String> hMap)
{
System.out.println("\n----------------------------------");
if(hMap.size() > 0) {
for(Map.Entry m:hMap.entrySet()){
System.out.println(m.getKey()+","+m.getValue());
}
System.out.println("");
} else {
System.out.println("No Entry");
};
}
}
|
Output :
Number of Entry = 3
----------------------------------
1001,Item1
1002,Item2
1003,Item3
Number of Entry = 2
----------------------------------
1001,Item1
1003,Item3
Number of Entry = 0
----------------------------------
No Entry
How to replace an Entry in a Map ?
|
HashMap05.java
|
|
iimport java.util.*;
public class HashMap05 {
public static void main(String[] args) {
Map<Integer,String> hMap=new HashMap<Integer,String>();
hMap.put(1001,"Item1");
hMap.put(1002,"Item2");
hMap.put(1003,"Item3");
System.out.print("Number of Entry = " + hMap.size());
printMap(hMap);
hMap.replace(1002,"Updated Item2");
System.out.print("Number of Entry = " + hMap.size());
printMap(hMap);
}
public static void printMap(Map<Integer,String> hMap)
{
System.out.println("\n----------------------------------");
if(hMap.size() > 0) {
for(Map.Entry m:hMap.entrySet()){
System.out.println(m.getKey()+","+m.getValue());
}
System.out.println("");
} else {
System.out.println("No Entry");
};
}
}
|
How to check the existence of an Entry in a Map ?
|
HashMap06.java
|
|
import java.util.*;
public class HashMap06 {
public static void main(String[] args) {
Map<Integer,String> hMap=new HashMap<Integer,String>();
hMap.put(1001,"Item1");
hMap.put(1002,"Item2");
hMap.put(1003,"Item3");
System.out.println(hMap.containsValue(1002));
System.out.println(hMap.containsValue("Item2"));
System.out.println(hMap.containsValue(1004));
System.out.println(hMap.containsValue("Item4"));
}
}
|
Output :
false <-- System.out.println(hMap.containsValue(1002));
true <-- System.out.println(hMap.containsValue("Item2"));
false <-- System.out.println(hMap.containsValue(1004));
false <-- System.out.println(hMap.containsValue("Item4"));
How to put an Entry only when the same Entry is not in the Map ?
|
HashMap07.java
|
|
import java.util.*;
public class HashMap07 {
public static void main(String[] args) {
Map<Integer,String> hMap=new HashMap<Integer,String>();
hMap.put(1001,"Item1");
hMap.put(1002,"Item2");
hMap.put(1003,"Item3");
System.out.print("Number of Entry = " + hMap.size());
printMap(hMap);
hMap.putIfAbsent(1002,"New Item");
System.out.print("Number of Entry = " + hMap.size());
printMap(hMap);
hMap.putIfAbsent(1004,"New Item");
System.out.print("Number of Entry = " + hMap.size());
printMap(hMap);
}
public static void printMap(Map<Integer,String> hMap)
{
System.out.println("\n----------------------------------");
if(hMap.size() > 0) {
for(Map.Entry m:hMap.entrySet()){
System.out.println(m.getKey()+","+m.getValue());
}
System.out.println("");
} else {
System.out.println("No Entry");
};
}
}
|
Output :
Number of Entry = 3
----------------------------------
1001,Item1
1002,Item2
1003,Item3
Number of Entry = 3 <-- after hMap.putIfAbsent(1002,"New Item");
----------------------------------
1001,Item1
1002,Item2
1003,Item3
Number of Entry = 4 <-- after hMap.putIfAbsent(1004,"New Item");
----------------------------------
1001,Item1
1002,Item2
1003,Item3
1004,New Item
How to apply a function to every Entry in a Map ?
NOTE : forEtach() method is supported in Java 8 or later.
|
HashMap09.java
|
|
import java.util.*;
public class HashMap09 {
public static void main(String[] args) {
Map<Integer,String> hMap=new HashMap<Integer,String>();
hMap.put(1001,"Item1");
hMap.put(1002,"Item2");
hMap.put(1003,"Item3");
System.out.print("Number of Entry = " + hMap.size());
printMap(hMap);
hMap.forEach((k,v)->
{
String s = v.toUpperCase();
hMap.replace(k,s);
}
);
System.out.print("Number of Entry = " + hMap.size());
printMap(hMap);
}
public static void printMap(Map<Integer,String> hMap)
{
System.out.println("\n----------------------------------");
if(hMap.size() > 0) {
hMap.forEach((k,v)->System.out.println(k + "," + v));
System.out.println("");
} else {
System.out.println("No Entry");
};
}
}
|
Reference :
[1] Class HashMap<K,V> (Oravle)
|
|