Reshma Bidikar

In my earlier article, I had explained the Map.compute, Map.computeIfPresent and Map.computeIfAbsent methods added by Java 8. In addition,Java 8 has added two other methods called putIfAbsent and getOrDefault to the Map interface. In this article, I will be explaining the these methods.

How putIfAbsent works

Prior to Java 8, the Map interface had a put method. This method accepts a key and a value as a parameter and adds the key-value pair to the Map. If the key passed to this method is present in the Map, its value gets overwritten with the new value. However, sometimes, we may wish to add the key-value pair only if the key is absent from the Map. The putIfAbsent method caters to such situations. Thus, it accepts a key and a value as a parameter and adds the key-value pair to the Map only if the specified key is not present in the Map. If the key is present in the Map, it does not do anything. The following code demonstrates the put and putIfAbsent methods:

Map<Integer,String> shapesMap = new HashMap<>();
shapesMap.put(1, "circle");
shapesMap.put(2, "square");
shapesMap.put(3, "triangle");
shapesMap.put(1, "rectangle");
System.out.println("Value for key 1:"+shapesMap.get(1));
shapesMap.putIfAbsent(2, "pentagon");
System.out.println("Value for key 2:"+shapesMap.get(2));
shapesMap.putIfAbsent(4, "pentagon");
System.out.println("Value for key 4:"+shapesMap.get(4));

• Line 1 creates a Map called shapesMap which has an Integer key and a String value
• Lines 2-4 add some values to it via the put method
• Line 6 again invokes put with the key-value pair 1, rectangle. Since the key 1 is already present in the Map, this line overwrites its value with rectangle
• Line 9 invokes the putIfAbsent method with 2, pentagon. Since the key 2 is already present in the Map, the putIfAbsent method does not overwrite its value.
• Line 12 again invokes the putIfAbsent method with 4, pentagon. Since the key 4 is not present in the Map, the putIfAbsent method adds it to the map.

This code produces the following output:

Value for key 1:rectangle
Value for key 2:square
Value for key 4:pentagon

How getOrDefault works

Prior to Java 8, the Map interface had a get method. This method accepts a key as a parameter and returns the value corresponding to the key. If the key passed to this method is not present in the Map, its returns a null. However, sometimes, we may wish to return a default value if the specified key is not present in the Map. The getOrDefault method caters to such situations. Thus, it accepts a key and a default value as a parameter. It returns the value corresponding to the key only if the key is present in the map. If the key is not present in the Map, it returns the specified default value. The following code demonstrates the get and getOrDefault methods:

Map<Integer,String> shapesMap = new HashMap<>();
shapesMap.put(1, "circle");
shapesMap.put(2, "square");
shapesMap.put(3, "triangle");
System.out.println("Value for key 1:"+shapesMap.get(1));
System.out.println("Value for key 5:"+shapesMap.get(5));
System.out.println("Value for key 2:"+shapesMap.getOrDefault(2,"pentagon"));
System.out.println("Value for key 5:"+shapesMap.getOrDefault(5,"pentagon"));

• Line 1 creates a Map called shapesMap which has an Integer key and a String value
• Lines 2-4 add some values to shapesMap via the put method
• Line 6 invokes get method with the key 1. Since the key 1 is present in the Map, this returns its value that is circle
• Line 7 invokes get method with the key 5. Since the key 5 is not present in the Map, this returns a null
• Line 9 then invokes the getOrDefault method with the key 2 and the value pentagon. Since the key 2 is present in the Map, the getOrDefault method returns the value corresponding to 2 that is square
• Line 10 then invokes the getOrDefault method with the key 5 and the value pentagon. Since the key 5 is not present in the Map, the getOrDefault method returns the default value that is pentagon.

This code produces the following output:

Value for key 1:circle
Value for key 5:null
Value for key 2:square
Value for key 5:pentagon

Conclusion

So, in this article, we saw how the putIfAbsent and getOrDefault methods work. Just to summarize, the putIfAbsent adds the a key-value pair to the Map only if the key is absent in the Map. The getOrDefault returns the value corresponding to a key if the key is present in the Map. Otherwise, it returns a default value.