![]() ![]() Letâs consider a pretty common problem in Java: retrieving values from a HashMap. This post will discuss various methods to initialize a map in Java. You can also find the sample code for this post over here in GitHub. Letâs start with a typical use-case for null references and examine a few different ways to use Optional as a replacement, and see how adding it to your repertoire can change the way you code. Overview The difference between Map and HashMap is that the first one is an interface, and the second is an implementation. It accepts function objects, including lambdas of course, which can make code more concise and easier to read. The HashMap class of the Java collections framework provides the functionality of the hash table data structure. let hashMap new Map() hashmap.get(1).Optional is also a great âgatewayâ class to using Java 8âs functional features. The hashmap contains the unique keys, and we can access its value using the get() method. Optional types have existed in functional languages for a long time, and when Java 8 introduced many functional features, adding in Optional made much sense. Optional is a parameterized class that encapsulates a value that may or may not be there. To help avoid these exceptions, Java 8 introduced the Optional class. First, well construct a new HashMap by copying all the entries from the map1: Map map3 new HashMap <> (map1) Next, well introduce the merge () function, along with a merging rule: rge (key, value, (v1, v2) -> new Employee (v1.getId (),v2.![]() ![]() This generates the curiously named Null Pointer Exception. 1 Answer Sorted by: 12 Less typing, Map is the Interface (same with List) for that class so it's easier to pass a Map around than a HashMap since the functionality is the same the only difference is the implementation. So why does it have the âNull Pointer Exception?âÄ«ecause Java really does have pointers: theyâre âreferences.â References give us some of the power that pointers in C and C++ give us, such as passing large objects between modules without having to copy them, but at the same time not allowing some of the more common mistakes, such as using a pointer to an object X as a pointer to object Y.Ä«ut itâs still possible to try to use a reference that doesnât point to anything, i.e., a null reference. ![]()
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