String oldValue = ProductMap. Sample 05: Replace a Value for the Given Key (The HashMap class is roughly equivalent to Hashtable, except that it is unsynchronized and permits nulls.) This class makes no guarantees as to the order of the map in particular, it does not guarantee that the order will remain constant. The below picture shows the three views of the Java HashMap: This implementation provides all of the optional map operations, and permits null values and the null key. As each entry is unique because of the Key, the return value is again a Set. The Entry View is the collection of Entries which can be retrieved from the Java HashMap using the entrySet method. Since HashMap allows duplicates for Values, the return value is a Collection. Value View contains only the Values and we can get it via the method call values. The return value is a Set as keys does not allow duplicates. Key View contains only Keys and we can retrieve it using the method keyset. Examples include keySet (), values (), entrySet (), and asList (). We can extract three types of view from the Java HashMap. We can use a variety of methods to print its elements. But HashMap does not allow the duplicates in Key. Say, for example, it is possible for the Ke圓 to point to a Value, value 1, which is a duplicate. In some programming languages, this concept is called a dictionary. So, the above HashMap is having three Entries. Here, we can see a HashMap is holding three Key-Value pairs. The HashMap class implements the Map Interface and hence it represents the data items as Key-Value pairs. We will learn about them when we use its implementing class HashMap in this example. The put and get are the frequently used methods of the Map interface. Since key is the one which gets you to the Value, the Key must be unique, and value may not be. For example, we can use the Product Id as Key and store the entire Product object as a Value. So, when we have Key in hand, we can get the Value. In the end, the reference variable has the. Repeat this process for all the entries in the map. If the current entry’s value is greater than the reference entry’s value, then store the current entry as the reference entry. Store the first entry in a reference variable to compare to initially. The Map Interface is a contract for representing the Key-Value pairs. Approach: Iterate the map entry by entry. We will see how HashMap stores collection items using Key-Value pairs and how it offers different views on its internal data store. Then, we will move on to the implementing class HashMap. In this example, we will explore Map Interface and its functions. Unlike other collection classes, it represents Key-Value pairs. The Java HashMap is a special type of Collection. 5.7 Printing Key-Value Pairs of Java HashMap.5.6 Printing All Values of Java HashMap.5.4 The containsKey and containsValue Methods.5.1 Adding Entries to HashMap Via put Method.* in combination of for loop and iterator. * There are multiple ways to do this e.g. * Java Program to print all key-value pairs of ConcurrentHashMap Java Program to iterate and print keys and values of ConcurrentHashMap import To read more about it I strongly suggest reading the first few chapters of Head First object-oriented Analysis and design book. and you should always do that in Java or any other object-oriented programming language. This is also the beauty of programming for interfaces than implementations, one of the important object-oriented design principles. It's guaranteed that the new Map implementation will implement the interface hence it will have the keySet(), values(), and entrySet() method, which means this code will work there. How is that possible? because we are using the methods defined on Map interface and not on ConcurrentHashMap. You can use this technique to print all keys, values, or entries with any Map implementations including HashMap, Hashtable, LinkedHashMap, EnumMap, IdentityHashMap, WeakHashMap, or any other future implementations. This technique is also the standard way to iterate over Map and print all key-value pairs. Though you should remember that these views are backed by Map, so when you remove a key-value pair from the entry set it will also be removed by the ConcurrentHashMap.
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