Java arraylist map entry

How to Convert HashMap to ArrayList in Java?

HashMap is a part of Java’s collection since Java 1.2. It provides the basic implementation of the Map interface of Java. It stores the data in (Key, Value) pairs. Unlike Hashmap, an ArrayList is used to provides us with dynamic arrays in Java. It is an alternative to an array that provides a lot more flexibility in terms of storage. In this article, we will see how to convert a Hashmap to an ArrayList.
Since every element in the hashmap contains a key/value pair, it is not possible to store this key/value pair in a single ArrayList. Therefore, in order to convert a hashmap into an ArrayList, we need to maintain two separate ArrayList’s where one ArrayList is used to store the Keys and the other to store the values corresponding to those keys. The following are the various methods used to convert a hashmap to the ArrayList.

Method 1:

One way to convert is to use the constructor of the ArrayList. In order to do this, we can use the keySet() method present in the HashMap. This method returns the set containing all the keys of the hashmap. This set can be passed into the ArrayList while initialization in order to obtain an ArrayList containing all the keys. After obtaining the keys, we can use the values() method present in the hashmap to obtain a collection of all the values present in the hashmap. We can use this collection to initialize another array that contains all the values for the keys in the hashmap.

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Using the Map.Entry Java Class

The Kubernetes ecosystem is huge and quite complex, so it’s easy to forget about costs when trying out all of the exciting tools.

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Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.

The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.

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Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes:

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The Kubernetes ecosystem is huge and quite complex, so it’s easy to forget about costs when trying out all of the exciting tools.

To avoid overspending on your Kubernetes cluster, definitely have a look at the free K8s cost monitoring tool from the automation platform CAST AI. You can view your costs in real time, allocate them, calculate burn rates for projects, spot anomalies or spikes, and get insightful reports you can share with your team.

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We’re looking for a new Java technical editor to help review new articles for the site.

1. Overview

We often use maps to store a collection of key-value pairs. Then, at some point, we often need to iterate over them.

In this tutorial, we’ll compare different methods of map iteration, highlighting when it may be beneficial to use Map.Entry. Then, we’ll learn how Map.Entry can be used to create a tuple. Finally, we’ll create an ordered list of tuples.

2. Optimizing Map Iteration

Suppose that we have a map of book titles with the author’s name as the key:

Map map = new HashMap<>(); map.put("Robert C. Martin", "Clean Code"); map.put("Joshua Bloch", "Effective Java");

Let’s compare two methods of getting all the keys and values from our map.

2.1. Using Map.keySet

First, consider the following:

for (String key : bookMap.keySet())

Here, the loop iterates over keySet. For each key, we get the corresponding value using Map.get. While this is an obvious way to use all of the entries in the map, it requires two operations for each entry — one to get the next key and one to look up the value with get.

If we need just the keys in a map, keySet is a good option. However, there’s a faster way to get both the keys and values.

2.2. Using Map.entrySet Instead

Let’s rewrite our iteration to use entrySet:

for (Map.Entry book: bookMap.entrySet())

In this example, our loop is over a collection of Map.Entry objects. As Map.Entry stores both the key and value together in one class, we get them both in a single operation.

The same rules apply to using Java 8 stream operations. Streaming over the entrySet and working with Entry objects is more efficient and can require less code.

3. Working With Tuples

A tuple is a data structure that has a fixed number and order of elements. We can think of Map.Entry is a tuple that stores two elements – a key and a value. However, as Map.Entry is an interface, we require an implementation class. In this section, we’ll explore one implementation provided by the JDK: AbstractMap.SimpleEntry.

3.1. Creating a Tuple

First, consider the Book class:

Next, let’s create a Map.Entry tuple with the ISBN as the key and the Book object as the value:

Finally, let’s instantiate our tuple with AbstractMap.SimpleEntry:

tuple = new AbstractMap.SimpleEntry<>("9780134685991", new Book("Effective Java 3d Edition", "Joshua Bloch")); 

3.2. Creating an Ordered List of Tuples

When working with tuples, it’s often useful to have them as an ordered list.

First, we’ll define our list of tuples:

List> orderedTuples = new ArrayList<>(); 

Secondly, let’s add some entries to our list:

orderedTuples.add(new AbstractMap.SimpleEntry<>("9780134685991", new Book("Effective Java 3d Edition", "Joshua Bloch"))); orderedTuples.add(new AbstractMap.SimpleEntry<>("9780132350884", new Book("Clean Code","Robert C Martin")));

3.3. Comparing With a Map

In order to compare the differences with a Map, let’s add a new entry with a key that already exists:

orderedTuples.add(new AbstractMap.SimpleEntry<>("9780132350884", new Book("Clean Code", "Robert C Martin"))); 

Secondly, we’ll iterate over our list, displaying all the keys and values:

for (Map.Entry tuple : orderedTuples)

Finally, let’s see the output:

key: 9780134685991 value: Book key: 9780132350884 value: Book key: 9780132350884 value: Book

Notice that we can have duplicate keys, unlike a basic Map, where each key has to be unique. This is because we’ve used a List implementation to store our SimpleEntry objects, which means all the objects are independent of each other.

3.4. Lists of Entry Objects

We should note that the purpose of Entry is not to act as a generic tuple. Library classes often provide a generic Pair class for this purpose.

However, we may find that we need to temporarily work with lists of entries while preparing data for a Map or extracting data from one.

4. Conclusion

In this article, we looked at Map.entrySet as an alternative to iterating over a map’s keys.

We then looked at how Map.Entry can be used as a tuple.

Finally, we created a list of ordered tuples, comparing the differences to a basic Map.

As always, the example code is available over on GitHub.

Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.

The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.

Critically, it has very minimal impact on your server’s performance, with most of the profiling work done separately — so it needs no server changes, agents or separate services.

Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes:

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Convert Map to a List in Java 8 and above

This post will discuss how to convert map to a list in Java.

1. Converting Map to List

We know that Map.entrySet() returns a set view of the mappings contained in this map. In Java 8, we can easily get a list of key-value pairs by converting it to stream, as shown below:

Output:

[A=65, B=66, C=67]

Here’s an even shorter version of the above code that makes use of the ArrayList constructor to create a list of Map.Entry objects containing both key and value:

2. Converting Map to List

We can get a list of keys of the map using Map.entrySet() in Java 8 and above:

Output:

[A, B, C]

Here’s a better way to do it in Java 8 and above using Map.keySet() :

3. Converting Map to List

We can get a list of values of the map using Map.entrySet() in Java 8 and above:

Output:

[65, 66, 67]

Here’s a better way to do it in Java 8 and above using Map.values() :

That’s all about converting Map to a List in Java.

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