Different Ways Of Iterating Map In Java – Including forEach() and stream() Of Java 8
Introduction
Map in Java is an interface. It stores data as key-value pair. We will learn different ways to iterate over Map including Java 8 stream() and forEach().
Basics of Map
The Map is an interface in Java which can be used to store data as key-value pairs. If we want to store details of a person such as First name, last name, address, and age then we can use Map here from Java. First name, last name, address, and age are keys and the person’s details for these keys are values.
Since Map is an interface, we can not instantiate it. We need to use its implemented classes such as HashMap, TreeMap etc. Please note that the Map interface is not part of the Collection interface.
Below is a sample program –
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package Maps; import java.util.HashMap; import java.util.Map; public class WaysOfInteratingMap { public static void main(String[] args) { Map<String,String> personData = new HashMap<>(); personData.put("FirstName", "Amod"); personData.put("LastName", "Mahajan"); personData.put("Address", "New York"); personData.put("Age", "30"); System.out.println(personData); } } |
Output
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{Address=New York, FirstName=Amod, LastName=Mahajan, Age=30} |
We can print the entire Map data i.e. key-value pair as shown above but in real-time we may be required to iterate the Map to perform different actions. In this post, we will see different ways of iterating over the Map including Java 8 methods.
Iterating using entrySet() method
Map interface provides a method called entrySet() which returns a Set view of the mappings contained in the map. Entry is an inner static interface of Map.
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// Iterating using entrySet() method Set<Entry<String, String>> entries = personData.entrySet(); for(Entry<String, String> entry : entries) { System.out.println(entry.getKey()+" : "+entry.getValue()); } |
Output
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Address : New York FirstName : Amod LastName : Mahajan Age : 30 |
Iterating using keySet() method
The above method gives us key and value mappings but we can also get all keys using the keySet() method and iterate over it. This method returns a Set of all keys in Map.
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// Iterating using keySet() method Set<String> allKeys = personData.keySet(); for(String key : allKeys) { System.out.println(key + " : "+personData.get(key)); } |
Output
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Address : New York FirstName : Amod LastName : Mahajan Age : 30 |
Iterating using forEach() method – Java 8
We can directly use the forEach() method of Map introduced in Java 8. With lambda, we can make it shorter.
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// Iterating using forEach() method with lambda personData.forEach((key,value) -> System.out.println(key + " : "+ value)); // Iterating using forEach() method without lambda personData.forEach(new BiConsumer<String, String>() { @Override public void accept(String key, String value) { System.out.println(key + " : "+ value); } }); |
Output
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Address : New York FirstName : Amod LastName : Mahajan Age : 30 |
Iterating using stream() – Java 8
We can also use the stream() method which may be an indirect way of the above method. This method will be useful if you want to perform some actions on data.
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// Iterating using stream() method personData.keySet().stream().forEach((key) -> System.out.println(key + " : "+ personData.get(key))); personData.entrySet().stream().forEach((keyValuePair) -> System.out.println(keyValuePair.getKey()+" : "+keyValuePair.getValue())); |
Output
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Address : New York FirstName : Amod LastName : Mahajan Age : 30 |
Complete Program
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package Maps; import java.util.HashMap; import java.util.Map; import java.util.Map.Entry; import java.util.Set; import java.util.function.BiConsumer; import java.util.function.Consumer; import java.util.function.Function; public class WaysOfInteratingMap { public static void main(String[] args) { Map<String,String> personData = new HashMap<>(); personData.put("FirstName", "Amod"); personData.put("LastName", "Mahajan"); personData.put("Address", "New York"); personData.put("Age", "30"); System.out.println(personData); // Iterating using entrySet() method Set<Entry<String, String>> entries = personData.entrySet(); for(Entry<String, String> entry : entries) { System.out.println(entry.getKey()+" : "+entry.getValue()); } System.out.println("========================================"); // Iterating using keySet() method Set<String> allKeys = personData.keySet(); for(String key : allKeys) { System.out.println(key + " : "+personData.get(key)); } System.out.println("========================================"); // Iterating using forEach() method with lambda personData.forEach((key,value) -> System.out.println(key + " : "+ value)); // Iterating using forEach() method without lambda personData.forEach(new BiConsumer<String, String>() { @Override public void accept(String key, String value) { System.out.println(key + " : "+ value); } }); // Iterating using stream() method personData.keySet().stream().forEach((key) -> System.out.println(key + " : "+ personData.get(key))); System.out.println("========================================"); personData.entrySet().stream().forEach((keyValuePair) -> System.out.println(keyValuePair.getKey()+" : "+keyValuePair.getValue())); } } |
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