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Java 8 Collection API changes Part1- Map

Published Feb 23, 2017
Java 8 Collection API changes Part1- Map

It is been a quite a long time ever since Java8 released. Java programmers around the world started using it already in their new projects. However I recently came across an article saying that most using feature of Java8 is streams which are on Demand Collections. On the other hand, the native Collection API also got improved and added few useful and powerful features. Looking at open source projects on GitHub and some enterprise project codes which built on Java8, I rarely see new methods of Collections framework being used and people mostly using the old school methods.

So I decided to explore the Collections framework changes one by one. Since there are a lot of changes,let us see the changes happened to Map first. A lot of internal architecture/performance changes been happend but I am just going to discuss about the useful methods been added.

Sorting Map directly with Comparators.

As we know Map is in order, it is a lot of struggle to get it sorted. Now Map interface added default methods which gives you comparators for different styles like comparingByKey, comparingByValue.

    Map<String, String> map = new HashMap<>();
    map.put("C", "c");
    map.put("B", "b");
    map.put("Z", "z");
    List<Map.Entry<String, String>> sortedByKey = map.entrySet().stream().sorted(Map.Entry.comparingByKey())
        .collect(Collectors.toList());
    sortedByKey.forEach(System.out::println);

output : 

B=b
C=c
Z=z

Iterate over map easily with forEach.

If you observe the above example code, while printing I used the method forEach method. This is very revealing feature so far in Map. We all know how ugly is the old fashioned way of iterating and finally implementers added a default method forEach. Now it is super easy to iterate over map just like List.

    Map<String, String> map = new HashMap<>();
    map.put("C", "c");
    map.put("B", "b");
    map.put("Z", "z");
    map.forEach((k, v) -> System.out.println("Key : " + k + " Value : " + v));

output : 

Key : B Value : b
Key : C Value : c
Key : Z Value : z


Get rid off ugly if-else condition, use getOrDefault method.

Legacy code for checking containsKey got moved to default method getOrDefault. This method returns the value to which the specified key is mapped, otherwise returns the given defaultValue if this map contains no mapping for the key.

    Map<String, String> map = new HashMap<>();
    map.put("C", "c");
    String val = map.getOrDefault("B", "Nah!");
    System.out.println(val); // prints Nah!

Replace and Remove utilities.

New utility default methods have been added now. replaceAll Can replace all the values in a single attempt

    Map<String, String> map = new HashMap<>();
    map.put("C", "c");
    map.put("B", "b");
    map.replaceAll((k, v) -> "x"); // values is "x" for all keys.  

And replace(K key, V oldValue, V newValue) method replaces the entry for the specified key only if currently mapped to the specified value. In the same way you can use replace, remove methods to check by key and values pairs together.

Do not override keys accidentally use putIfAbsent

As the method name is self explanatory, here is an example.

    Map<String, String> map = new HashMap<>();
    map.put("C", "c");
    map.put("B", "b");
    map.putIfAbsent("B", "x");
    System.out.println(map.get("B")); // prints "b"

operate directly on values.

Gone are the days when you needed to get the value for specific keys, process it and put them back. Now you can directly modify with help of compute method.

    Map<String, String> map = new HashMap<>();
    map.put("C", "c");
    map.put("B", "b");
    map.compute("B", (k, v) -> v.concat(" - new "));
    System.out.println(map.get("B")); // prints "b - new"

Conditional computes are also available. Look at computeIfPresent, computeIfAbsent methods.

To merge maps use merge method.

This is little tricky and more useful when you are combining maps or appending values for duplicated keys.

Docs says

If the specified key is not already associated with a value or is associated with null, associates it with the given non-null value. Otherwise, replaces the associated value with the results of the given remapping function, or removes if the result is null.

To demonstrate it simply, just merge the values for a key with old and new, see the below example.

    Map<String, String> map = new HashMap<>();
    map.put("C", "c");
    map.put("B", "b");
    map.merge("B", "NEW", (v1, v2) -> v1 + v2);
    System.out.println(map.get("B")); // prints bNEW

That is all for now and lets meet in next article exploring another Collection changes with Java8.

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