This article show few ways of loop Map in Java.
- 1. Using
entrySet() - 2. Using
forEach(Java 8 and later) - 3. Using
stream()(Java 8 and later) - 4. Using
iterator() - 5. Using
keySet()andget() - 6. References
1. Using `entrySet()`
Before Java 8, this is the most common method to loop a Map in Java. We use the map’s entry set entrySet() to loop and access to each key-value pair.
LoopMap1.java
package com.mkyong.examples;
import java.util.HashMap;
import java.util.Map;
public class LoopMap1 {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(1, "Jan");
map.put(2, "Feb");
map.put(3, "Mar");
for (Map.Entry<Integer, String> entry : map.entrySet()) {
System.out.println("ID: " + entry.getKey() + ", Name: " + entry.getValue());
}
}
}
output
ID: 1, Name: Jan
ID: 2, Name: Feb
ID: 3, Name: Mar
2. Using `forEach` (Java 8 and later)
Java 8 introduced the forEach method, which provides a more simple way to iterate over maps using lambda expressions.
LoopMap2.java
package com.mkyong.examples;
import java.util.HashMap;
import java.util.Map;
public class LoopMap2 {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(1, "Jan");
map.put(2, "Feb");
map.put(3, "Mar");
// map.forEach((key, value) -> System.out.println("ID: " + key + ", Name: " + value));
map.forEach((id, name) -> {
System.out.println("ID: " + id + ", Name: " + name);
});
}
}
output
ID: 1, Name: Jan
ID: 2, Name: Feb
ID: 3, Name: Mar
3. Using `stream()` (Java 8 and later)
We can use stream() to loop, filtering and collect in Java 8.
LoopMap3.java
package com.mkyong.examples;
import java.util.HashMap;
import java.util.Map;
public class LoopMap3 {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(1, "Jan");
map.put(2, "Feb");
map.put(3, "Mar");
map.entrySet().stream()
.forEach(entry -> System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue()));
}
}
output
ID: 1, Name: Jan
ID: 2, Name: Feb
ID: 3, Name: Mar
In below example, we use stream() to get the map’s entry if it’s value is equals to Jan.
LoopMap3.java
package com.mkyong.examples;
import java.util.HashMap;
import java.util.Map;
public class LoopMap3 {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(1, "Jan");
map.put(2, "Feb");
map.put(3, "Mar");
map.entrySet().stream()
.filter(entry -> "Jan".equals(entry.getValue()))
.forEach(entry -> System.out.println("Key : " + entry.getKey() + " Value : " + entry.getValue()));
// Map stream, filter and collect to a Map.
/*
Map<Integer, String> result = map.entrySet().stream()
.filter(entry -> "Jan".equals(entry.getValue()))
.collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));*/
}
}
output
ID: 1, Name: Jan
4. Using `iterator()`
The iterator did the jobs but not a common way to loop Map in Java.
LoopMap4.java
package com.mkyong.examples;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
public class LoopMap4 {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(1, "Jan");
map.put(2, "Feb");
map.put(3, "Mar");
// Map -> Set -> Iterator -> Map.Entry
Iterator<Map.Entry<Integer, String>> iterator = map.entrySet().iterator();
while (iterator.hasNext()) {
Map.Entry<Integer, String> entry = iterator.next();
System.out.println("Key : " + entry.getKey() + " Value :" + entry.getValue());
}
}
}
5. Using `keySet()` and `get()`
This method involves iterating over the key set and then using each key to get its corresponding value. It’s slightly less efficient than using entrySet() because of the additional get() call. It might be useful for conditionally fetch values.
LoopMap5.java
package com.mkyong.examples;
import java.util.HashMap;
import java.util.Map;
public class LoopMap5 {
public static void main(String[] args) {
Map<Integer, String> map = new HashMap<>();
map.put(1, "Jan");
map.put(2, "Feb");
map.put(3, "Mar");
for (Integer key : map.keySet()) {
System.out.println("Key : " + key + " Value : " + map.get(key));
}
}
}
