Map within map in java. Jan 8, 2024 · Java 9 introduced the Map.

Map within map in java In the JDK, java. However, my manager said it is a good practice to use Map of Map because it improves the efficiency of using the Map Container. A map contains unique keys. A Map is useful when you want to associate a key (like a name or ID) with a value (like an age or description). May 23, 2017 · You'll need to complete a few actions and gain 15 reputation points before being able to upvote. To keep it simple there is only one now but there could be more than one command. Keys are unique values associated with individual values. This interface takes the place of the Dictionary class, which was a totally abstract class rather than an interface. This guide will illustrate how to add values to a List that is mapped to a key by utilizing the `computeIfAbsent` method, ensuring that you do not encounter a NullPointerException and effectively manage your entries. Jul 6, 2024 · What is a Map in Java? In Java, a Map is an interface that represents a collection of key-value pairs where each key is unique within the Map, and it maps to exactly one value. Feb 14, 2019 · In this tutorial, we’ll discuss some examples of how to use Java Stream s to work with Map s. When they fit the need, I apply them. First question: what should be sorted? Please provide an example of the expected output. Feb 15, 2017 · 0 I have a HashMap which in turn has a HashMap within, Map<Long, Map<String, Double>> map = new HashMap<>(); I will have multiple inner maps for the same Long value, how do I make sure that the value field isn't replaced and rather merged (appended)? Feb 7, 2024 · Here, the map method takes a Function as input. The Map interface provides three collection views, which allow a map's contents to be viewed as a set of keys, collection of values, or set of key-value mappings. util. Hence, sharing the learnings in this article. We can put either simple values or complex objects in these data structures. I have a Spring Boot application with the following application. If a method returns a collection of objects that implement an interface, you don't need to worry about what the underlying object is. Learn to create a nested HashMap and add, remove and iterate over the elements with examples. It models the mathematical function abstraction. Let’s create a User class and List of users, which we will use in the examples of this tutorial:- Aug 15, 2024 · Maps in Java are the core building blocks of the Collections framework. Search Tips Index Status Default search is case insesitive, and looks for keyword within name For exact search, prefix = to the keyword ( Example =hashmap ) For search within frameworks / Libraries, use framework name or framework type name within round parenthesis, Example map (spring) - Will give all map classes within Spring Frameworks / Libraries =map (java) - Will give map classes ( exact Java Map Interface The Map interface is a part of the Java Collections Framework and is used to store key-value pairs. Below is the java program to demonstrate it. In addition, if you know that the inner map instance came from the outer map, you don't have to spend time putting it back where it came from. Need for Map Data Structure Map data structures are important In one of my projects, I tried to set values in a nested map passed in to the parameter and return an updated map. In this lesson we look at the java Map hierarchy, Map interface and four concrete implementations. If the value is als In this tutorial, we will explore the powerful features of Java Maps combined with Streams, a core aspect of Java's functional programming capabilities introduced in Java 8. I'm using snakeyaml as a dependency. The Map interface includes methods for basic operations (such as put, get, remove, containsKey, containsValue, size, and empty), bulk operations (such as putAll and clear), and collection views (such as keySet, entrySet, and Sep 23, 2021 · Map<Long, Employee. stream() . When working with maps, you often need to retrieve both the keys and values for further processing. util package and implements the Map interface. It is part of the Nov 11, 2015 · Map<String, String> ht = new HashMap(); and I would like to know how to search through it and find anything matching a particular string. Maps are fundamental data structures in Java, allowing developers to store and retrieve data based on a unique key. An understanding of Java data types is discussed in our tutorial Understanding Data Types in Java. SimpleImmutableEntry but the Map interface has an additional static method Map. Syntax : <R> Stream<R> map Feb 13, 2025 · Introduction Welcome to “Searching in Java Maps: Effective Techniques with Collections Framework. A map cannot contain duplicate keys: Each key can map to at most one value. In this step-by-step guide, we’ll explore various methods for getting keys and values from a Map in Java, making it accessible to beginners and non-techies. On this page, we provide descriptions of the map processing functions from the standard library. name} description Java HashMap A HashMap stores items in key/value pairs, where each key maps to a specific value. Also, a List holds a sequence of objects of the same type. Sep 30, 2023 · Java 8 streams API is a widely used feature to write code in a functional programming way. Jan 8, 2024 · Explore various approaches such as reflection, Jackson, and Gson to convert Java Objects into Java Maps. The Oct 23, 2023 · Ever found yourself struggling with Map in Java? You're not alone. Jan 8, 2024 · In our previous tutorial, A Guide to Java HashMap, we showed how to use HashMap in Java. of () Learn how to effectively collect a map within a map using functional programming in Kotlin and Java. At the second iteration commandName is the key and {dataKey1 Sep 21, 2020 · The Java Map interface is a collection containing key + value pairs. There are various Jan 8, 2024 · 1. # java Map Filter Examples It is an example for List of String which contains numbers as input and output returns even numbers. This includes for example: It generates an additional class which increases memory consumption, disk space consumption and startup-time In case of a non-static method: It holds a reference to the object the creating In the vast landscape of Java programming, the `Map. It simplifies data handling, promotes cleaner code, and bridges the gap between different data representations. In this tutorial, we’ll discuss how to use Streams API for Map creation, iteration and sorting. Grouping and Aggregating Data Oct 14, 2011 · I have a Map within a Map within a Map. The NASA WorldWind Java SDK (WWJ) is for building cross-platform 3D geospatial desktop applications in Java. Jul 9, 2021 · map. Aug 20, 2022 · A nested Map is Map inside another Map. Map interface is one of the most fundamental data structures in Java programming. ofEntries () method, which has slightly different functionality. Aug 19, 2024 · In Java, a Map is an interface that represents a collection of key-value pairs where each key is unique within the Map, and it maps to exactly one value. Sep 7, 2024 · Explore the various ways of initializing a Map, particularly to create empty, singleton, immutable and mutable maps. May 26, 2018 · 2 I found this old code snippet which uses a Map of Map of Map: Map<String, Map<Long, Map<String, String>>> sMap = null; Besides adding unnecessary complexity to the code, it is making it difficult to unit test the class. While many developers are familiar with basic operations like put() and get(), Java 8 and later versions introduced powerful new methods that can dramatically simplify your code and make it more expressive. Another solution is to implement a simple domain model. You can look at Composite pattern which is widely used for implementing tree structures (if their components has the same type which is not the case as I feel). Dive into detailed examples and best practices. Creating Maps When creating a map, you must first decide which map implementation you will use. Sep 29, 2023 · Java provides the Map interface, a versatile data structure for storing key-value pairs. util and it allows collections to store items as key-value pairs. Let’s assume we have the following list of maps: May 23, 2022 · How to check if all Keys of the given Map are present in a one of the Maps contained within a List<Map<String, Object>> in Java efficiently? Asked 3 years, 5 months ago Apr 3, 2025 · Java provides us with different data structures with various properties and characteristics to store objects. artifactId} name: ${project. Mar 7, 2025 · Before we explore iteration techniques, let’s grasp the concept of a List of Map s. What entries should be eventually in the sorted array/list? Maybe you code an object which contains the named contents of your map to get a better understanding on your problem. Dec 4, 2015 · Map of maps is actually a tree-type structure without single root node (as well as map of maps of maps). Apr 9, 2015 · Java - is adding a key/value to a map within a map in 1 line of code possible? Asked 16 years, 2 months ago Modified 10 years, 7 months ago Viewed 26k times Jul 2, 2025 · In Java applications, java. This Java Map tutorial explains how to use the Map implementations found in Java. Entry` interface provides a way to access and manipulate individual key-value pairs within a map. Includes examples, key methods, and best practices for working with Map in Java. It is part of the java. These structures, unlike other collections, demand unique techniques for traversal. In Java, elements of Map are stored in key/value pairs. Whether you’re managing configuration files, forming API responses, or calculating grouped statistics, working with structured data is essential. keySet () method returns a Set view of the keys contained in this map and Map. ” In this eBook, we delve into the various search mechanisms available within Java’s Collections Framework, focusing specifically on searching within Map collections. It might be simpler to create an AccountInfo class which has three strings as members. The question is: assuming that I didn't know the map structure, how can I set valu Feb 28, 2018 · I'm trying to do grouping by (to map) and then transform list of values to different list. The `Map. A Map is an object that maps keys to values. These pairs are called map entries. Understanding how to work with the built-in Map interface can significantly improve your data manipulation capabilities within collections. In this short tutorial, we’ll learn how to get a submap from a HashMap based on a list of keys. Jun 28, 2017 · What is a Map? A map is a data structure that’s designed for fast lookups. Learn about the Java Map Interface, its implementation, and common use cases in Java programming. Let’s explore some examples of using the map () method in Java 8 Streams. In this particular case the object implementing the interface Map. The Map. Therefore, it behaves a bit differently from the rest of the collection types. Before Java 8, typical methods for iterating over a map involved using entrySet(), keySet(), or values() in conjunction with loops. An object that maps keys to values. Entry` interface plays a crucial role in handling key-value pairs within maps. Maps provide an efficient way to store and retrieve data based on a unique identifier (the key). First, we’ll explain the basic idea we’ll be using to work with Maps and Stream s. map-interface Implemented Classes of Map Interafe HashMap: Stores key-value pairs using hashing for fast access, insertion, and deletion. 877Z+00:00 0 London London Over a year ago A hashmap of hashmaps might not be the best datastructure for what you are doing here. Entry is an inner class in whatever Map implementation you're using (in this case it's built into HashMap). Mastering the technique of casting maps to POJOs is essential for any Java developer dealing with data transmission or transformation. Mar 26, 2018 · How to Iterate List<Map<String, String>> using streams. Aug 28, 2024 · Introduction Iterating over a Map is a common task in Java, especially when you need to access or manipulate the key-value pairs stored within it. The Function interface is a functional interface with one abstract method, accepting input of type T and returning output of type R. But Jul 23, 2025 · 2. toMap(employeeRecord::getEmloyeeID, employeeRecord::getEmployeeStatus)); I already have the above code, I need to add a similar operation but instead of creating a new Map I want to add the result to the existing map. Created a This tutorial provides a comprehensive guide on how to effectively cast maps to POJOs in Java. In this tutorial, we’ll take a closer look at these two static factory methods for immutable maps and explain which one is suitable for which purpose. Map. And if it is a match store it into an arraylist. A Map allows its contents to be accessed as a set of keys, a set of values, or a set of key-value pairs. Alt: a red “Enter” button on the keyboard with the word “Java” inscribed on it This article sheds light on a range of methods to navigate through Java maps, ensuring clarity and efficacy in your development process Oct 11, 2025 · Learn what Java Map is, how it works, and when to use it. Each key must be unique, but values can be duplicated. I have List of DistrictDocuments: List&lt;DistrictDocument&gt; docs = new ArrayList&lt;&gt;(); Then I'm Jun 21, 2018 · Learn how to use the map() function in Java 8 to transform an object to another and how to use the filter() to select an object based upon condition. (Let's not be too general, let's say two keys) Keys are guaranteed to be unique. I tried the following : int number = ((Intege In Java, appending data to a Map involves modifying an existing key's value or adding new key-value pairs. Learn how to efficiently apply map and filter operations together using Java 8 Streams with code examples and best practices. See full list on baeldung. Jan 21, 2021 · Edit page 21 January 2021 In maps, types of both keys and values are user-defined. of () method, which makes it easier to create immutable maps, and the Map. Learn about Java's Map. Introduction We often make use of maps in our programs, as a means to associate keys with values. Unlike arrays and lists that store single elements, maps store data in key-value pairs, making them perfect for scenarios where you need to quickly look up values based on a unique identifier Apr 30, 2022 · The Map interface is found in java. Discover the syntax, benefits, and common use cases for maps within maps. It’s worth noting that some of these exercises could be solved using a bidirectional Map data structure, but we’re interested here in a functional approach. So for I have this: May 25, 2025 · The map method is a powerful tool for applying transformations to data within Java streams, making it essential for functional programming and modern Java development. Among those, HashMap is a collection of key-value pairs that maps a unique key to a value. The Oct 20, 2024 · Java Streams provide a flexible and powerful way to manipulate Map data, allowing for operations such as filtering, transforming, sorting, merging, and grouping. ofEntries are the static factory methods of java. Map and has been introduced in Java 9. There are three main types of maps in java HashMap LinkedHashMap TreeMap These interfaces extend the Map interface. To explain why changing it is a problem you need to know how hashmaps work. entrySet () method for working with key-value pairs, efficient iteration, modifications, and comparisons with other map techniques. Typically in our Java programs, especially since the introduction of generics, we will have all of the keys be the same type and all of the values be the same type. In Java, a Map is an interface within the Java Collections Framework that represents a collection of key-value pairs. A HashMap can store many different combinations, such as: String keys and Integer values String keys and String values I have a HashMap in Java, the contents of which (as you all probably know) can be accessed by HashMap. ofEntries are static factory methods that return unmodifiable Map containing specified mapping. For example, a map of IDs to values in a data store. e. Step-by-step process with code examples. What's reputation and how do I get it? Instead, you can save this post to reference later. For developers familiar with the Java Collections Framework, the phrase "map inside a map" might raise questions. All you need to know is that the object meets the contract specified by the interface Jun 27, 2019 · It's not yet clear, what you are up to: You provide a datastructure which should be sorted. The map put method in Java is a fundamental function for map manipulation. for (Map<String, String> temp1 : maps) { for May 5, 2009 · I need a data structure which behaves like a Map, but uses multiple (differently-typed) keys to access its values. util package represents a mapping between a key and a value. In this quick tutorial, we’ll explore various practical approaches to achieving this goal. entrySe. In this tutorial, we will learn about the Java Map interface and its methods. Let Jul 11, 2025 · Read More Map Methods Whether you are a beginner starting Java programming or an experienced looking to brush up on your Java skills, this tutorial will provide you with a deep understanding of the map put function and its uses in Java. Learn how to convert a String to a Map in Java with practical examples and best practices in this detailed tutorial. So, If you need only keys or values from the map, you can iterate over keySet or values using for-each loops. values () method returns a collection-view of the values contained in this map. One common requirement is the ability to store maps within maps in Java. Each key in a Map must be unique, and it maps to a corresponding value. Instead of accessing elements by an index (like with ArrayList), you use a key to retrieve its associated value. map) doesn't exist for the key, it is added and then returned. The Map interface is not a subtype of the Collection interface. of and Map. Data is stored in key-value pairs with every key being unique. The data types of the keys and values are set when the Map is declared, and each can be any data type. Using keySet () and values () Methods Map. Example: Jun 17, 2013 · How to put values in Map which is within a Map Asked 11 years, 3 months ago Modified 11 years, 3 months ago Viewed 103 times The key to a map needs to be constant and its equals and hashcode need to be set up to give the correct behavior. Store HashMap<String, ArrayList<String>> Inside a List Jul 23, 2025 · Map interface present in java. Map<String, String> myMap = new HashMap<String, String>() {{ put("a", "b"); put("c", "d"); }}; However, the anonymous subclass might introduce unwanted behavior in some cases. Map is the interface that includes method signatures for insertion, removal, and retrieval. Many developers find it challenging to grasp the concept and usage of Map in Java. i have tried without streams and i was able to iterate. entry which can also create an entry which can be used for this usecase. A List of Map s comprises multiple Map objects, each capable of holding key-value pairs where the keys remain unique within each Map. You'll learn how to manipulate map entries efficiently, apply transformations, and filter data using Stream operations, which can lead to cleaner and more readable code. These operations are always lazy. Then you could just store a list of AccountInfo objects - if all you need is a list of values to pass into a SQL insert. Discover how to efficiently store and retrieve key-value pairs. SnakeYAML wraps key-value pairs in a map. How do I determine if the pol Mar 27, 2016 · I'm new to Java, so sorry if this is pretty obvious, but I can't quite understand how to work with 2 HashMaps inside each other I have my main, where I want to add some words to a Map, and then, I Mar 7, 2025 · When we manipulate data structures in Java, one common scenario involves extracting values and keys from a HashMap and organizing them into an ArrayList. Learn how to effectively use nested maps in Java. Common classes that implement Map: HashMap - fast and unordered TreeMap - sorted by key LinkedHashMap Practice Map forEach and Replace Operations Learn how to iterate through maps and replace values with lambdas: Search Tips Index Status Default search is case insesitive, and looks for keyword within name For exact search, prefix = to the keyword ( Example =hashmap ) For search within frameworks / Libraries, use framework name or framework type name within round parenthesis, Example map (spring) - Will give all map classes within Spring Frameworks / Libraries =map (java) - Will give map classes ( exact Oct 3, 2023 · Within Java’s vast landscape, maps stand out as sophisticated repositories. With this guide, you can easily add new key-value pairs using the map put function. so I want to retrieve all values such as host,port,db based on bank_name (ex: 'sbi'). The map contains strings like this: 1,2,3,4,5,5,5 and the matching string would be 5. Intermediate operations are invoked on a Stream instance and after they finish their processing, they give a Stream instance as output. Understanding how to work with Maps and Streams is Aug 12, 2013 · Assuming that you have the following : List&lt;Map&lt;String,Object&gt;&gt; test; How can you retrieve a value from test, when the key is "key1". 2. status> prevStatus = empRecords. It is implemented in the HashMap and TreeMap classes. Jul 16, 2014 · Whenever we need to read/manipulate something generic in Java, particularely JSON files, you'd need nested maps, unless you want to hard code the structure of your file in your application. 3. A HashMap can be modified so should not be used as a key. If you modify a key after adding it into the map then you invalidate the map. Nov 1, 2015 · I have a map where the values are strings and the keys are lists: Map&lt;String, List&lt;BoMLine&gt;&gt; materials I'd like to filter this map by its values; something like this: materials. Mar 30, 2014 · From Java 9 onwards it is even easier to do the transformation within the map part of the stream. Learn how to effectively merge multiple lists of maps containing list values in Java using the Streams API. Is was already possible to use a new AbstractMap. collect(Collectors. This blog post will delve deep into the Jan 19, 2024 · Familiarity with Java and object-oriented programming, which you can find in our tutorial How To Write Your First Program in Java. This structure offers remarkable versatility and finds common applications in representing tabular data, configurations, or any other data requiring key-to-value mappings. Then we’ll present a couple of different problems Jan 8, 2024 · Java 9 introduced the Map. These tutorials will teach the basics of working with different maps. Jul 18, 2024 · The comprehensive and detailed tutorial and code examples about Java Map collection in the Java Collection Framework. Map is far more than a simple key-value store. Key-based access to map entries enables various map-specific processing capabilities from getting a value by key to separate filtering of keys and values. Apr 6, 2016 · I have a big list of polygons (consisting of google maps polygon options) which I would like to check if they are within the bounds of the screen before drawing them. The Nov 2, 2012 · Here sbi is a bank_name which is a key,then as a value there is another map. Oct 12, 2016 · Alright, thanks! does the map have to be created with that code? is properties more than just a text file? because the idea was people would be able to configure the contents of the map within the text file, if they need to run code to do it there's no point in including it as they might as well change the put statements in the code May 7, 2025 · I was creating a library in java that manages YAML objects, sets and changes data in a given path. Introduction to the Problem May 5, 2024 · Learn how to convert a Map to an Object or POJO in Java. Aug 2, 2023 · In this blog post, we will explore different techniques to iterate through a list of maps in Java, providing you with the knowledge and tools to efficiently work with such data structures. Stream map (Function mapper) is an intermediate operation. Java 8 introduced the Stream API and other features that … Java 8 – Iterate Over a Map Read More » Jul 23, 2025 · What is Map Data Structure? Map data structure (also known as a dictionary , associative array , or hash map ) is defined as a data structure that stores a collection of key-value pairs, where each key is associated with a single value. Learn how to effectively flatten a nested map in Java utilizing Stream API. The format is HashMap<String, Map> A sample data: deviceName={commandName={dataKey1=[dataValue1], dataKey2=[dataValue2]}} The way it's setup is deviceName is the key and commandName is the value for it. yml - taken basically from here: info: build: artifact: ${project. 🗺️ Java Map Interface: Complete Guide with Implementations and Examples 📚 Introduction to Maps in Java Maps are one of the most powerful and frequently used data structures in Java programming. a nested HashMap, how would i Feb 3, 2020 · The Map. You can store key + value pairs by their key, and use the key to lookup the value later. As part of the Java Collections Framework Jan 4, 2025 · Stream map (Function mapper) returns a stream consisting of the results of applying the given function to the elements of this stream. A map cannot contain duplicate keys; each key can map to at most one value. put("AAA", innerMap); But as of Java 8 you can create the map as a value if it doesn't exist which is then returned using computeIfAbsent If the value (i. Upvoting indicates when questions and answers are useful. Jan 25, 2018 · You can improve the code by avoiding the creation of a new inner map eagerly, until the point when you know that you must create it. It'll be much clearer to read and easy to maintain something like Jun 2, 2014 · How to convert the values of a Map to an array, List or Set using plain Java. At work, I often need to use advanced Map operations. What You'll Learn By the end of this guide May 9, 2014 · 6 Comments Add a comment London I'm trying to create method to create XML element, where element name and value are in one hashmap, and attributes and values of those attributes in other hashmap, do you think it is a good idea, or pass 4 arraylists to a method each containing only elem names, values, attributes, attribute values 2010-07-14T22:06:07. com Oct 27, 2025 · Hierarchy of Map It is part of the Java Collections Framework, and its key implementation classes include HashMap, LinkedHashMap, TreeMap, and Hashtable. . Sep 15, 2025 · Introduction to Modern Java Maps {#introduction} The java. Explore detailed examples and best practices. In Java 8, you can efficiently manage collections like Lists within a Map using built-in methods. Each key maps to a value hence the name. get ("keyname"); If a have a HashMap inside another HashMap i. kvvu mnydkan ktebny qixzempe mfettmz xzikrl pfon qaos rjkho jeif sfjw zfp lqsb laifsz fkamm