interface in Java is a subinterface of the List
interface, representing an ordered set, allowing duplicate elements. Collection
is a commonly used implementation class for the ArrayList
interface. It is a dynamic array without pre-specifying the size. List
is a class in the Java collection framework, used to store key-value pairs. Keys cannot be repeated, and values ??can be repeated. HashMap<k v></k>
to ArrayList
: HashMap
- By iteration
-
ArrayList
Using Java 8 Stream API
<code>輸入列表:[1="1", 2="2", 3="3"] 輸出映射:{1=1, 2=2, 3=3} 輸入列表:[1="Java", 2="for", 3="JavaScript"] 輸出映射:{1=Java, 2=for, 3=JavaScript} 輸入:Mercedes, Audi, BMW, Harley Davidson, Triumph 輸出:{Car=[Mercedes, Audi, BMW], Bike=[Harley Davidson, Triumph]}</code>
Algorithm: Convert ArrayList to HashMap
This algorithm describes how to convert to ArrayList
. HashMap
- Step 1: Start.
- Step 2: Declare and import the necessary Java packages.
- Step 3: Create a public list.
- Step 4: Declare key-value pairs.
- Step 5: Create a constructor for the referenced value.
- Step 6: Assign values ??to the declared key.
- Step 7: Return the private variable id.
- Step 8: Declare a public class and method.
- Step 9: Declare the parameter string.
- Step 10: Create a .
ArrayList
- Step 11: Use data elements to fill the list value.
- Step 12: Create and declare a mapping value.
- Step 13: Declare the object method.
- Step 14: Create an object map value.
- Step 15: Add each data element to the map.
- Step 16: Print the map value and end.
Syntax: Convert ArrayList to HashMap
The following are some sample code snippets:
ArrayList<product> productList = new ArrayList<product>(); productList = getProducts(); Map<string, product> urMap = yourList.stream().collect(Collectors.toMap(Product::getField1, Function.identity())); HashMap<string, product> productMap = new HashMap<string, product>(); for (Product product : productList) { productMap.put(product.getProductCode(), product); } for (Product p: productList) { s.put(p.getName(), p); } for(Product p : productList){ s.put(p.getProductCode() , p); }These snippets show how to convert
to ArrayList
. HashMap
Method
We will introduce two methods:
Method 1: Use iteration and Collectors.toMap()
This method iterates directly
. ArrayList
HashMap
// 示例代碼 (假設(shè)ArrayList包含自定義對象) class Color { private String name; private String code; // ... getters and setters ... } // ... List<Color> colors = new ArrayList<>(); // ... populate colors list ... Map<String, String> colorMap = colors.stream() .collect(Collectors.toMap(Color::getName, Color::getCode));Method 2: Use Java 8 Stream API and
Collectors.groupingBy()
If you need to group, you can use the method.
Collectors.groupingBy()
Through the above methods, you can choose the appropriate method to convert
// 示例代碼 (假設(shè)ArrayList包含自定義對象,需要按類別分組) class Product { private String category; private String name; // ... getters and setters ... } // ... List<Product> products = new ArrayList<>(); // ... populate products list ... Map<String, List<Product>> productMap = products.stream() .collect(Collectors.groupingBy(Product::getCategory));according to actual needs. Remember to deal with potential duplicate keys, such as using the third parameter of
to specify the merge function. Select ArrayList
to maintain the insertion order. HashMap
The above is the detailed content of Convert ArrayList to HashMap in Java. For more information, please follow other related articles on the PHP Chinese website!

Hot AI Tools

Undress AI Tool
Undress images for free

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Hot Topics

The key to writing PHP comments is to clarify the purpose and specifications. Comments should explain "why" rather than "what was done", avoiding redundancy or too simplicity. 1. Use a unified format, such as docblock (/*/) for class and method descriptions to improve readability and tool compatibility; 2. Emphasize the reasons behind the logic, such as why JS jumps need to be output manually; 3. Add an overview description before complex code, describe the process in steps, and help understand the overall idea; 4. Use TODO and FIXME rationally to mark to-do items and problems to facilitate subsequent tracking and collaboration. Good annotations can reduce communication costs and improve code maintenance efficiency.

Comments cannot be careless because they want to explain the reasons for the existence of the code rather than the functions, such as compatibility with old interfaces or third-party restrictions, otherwise people who read the code can only rely on guessing. The areas that must be commented include complex conditional judgments, special error handling logic, and temporary bypass restrictions. A more practical way to write comments is to select single-line comments or block comments based on the scene. Use document block comments to explain parameters and return values at the beginning of functions, classes, and files, and keep comments updated. For complex logic, you can add a line to the previous one to summarize the overall intention. At the same time, do not use comments to seal code, but use version control tools.

The key to writing good comments is to explain "why" rather than just "what was done" to improve the readability of the code. 1. Comments should explain logical reasons, such as considerations behind value selection or processing; 2. Use paragraph annotations for complex logic to summarize the overall idea of functions or algorithms; 3. Regularly maintain comments to ensure consistency with the code, avoid misleading, and delete outdated content if necessary; 4. Synchronously check comments when reviewing the code, and record public logic through documents to reduce the burden of code comments.

The key to writing PHP comments is clear, useful and concise. 1. Comments should explain the intention behind the code rather than just describing the code itself, such as explaining the logical purpose of complex conditional judgments; 2. Add comments to key scenarios such as magic values, old code compatibility, API interfaces, etc. to improve readability; 3. Avoid duplicate code content, keep it concise and specific, and use standard formats such as PHPDoc; 4. Comments should be updated synchronously with the code to ensure accuracy. Good comments should be thought from the perspective of others, reduce the cost of understanding, and become a code understanding navigation device.

The first step is to select the integrated environment package XAMPP or MAMP to build a local server; the second step is to select the appropriate PHP version according to the project needs and configure multiple version switching; the third step is to select VSCode or PhpStorm as the editor and debug with Xdebug; in addition, you need to install Composer, PHP_CodeSniffer, PHPUnit and other tools to assist in development.

The basic PHP syntax includes: 1. Use wrapping code; 2. Use echo or print to output content, where echo supports multiple parameters; 3. Variables do not need to declare types, start with $. Common types include strings, integers, floating-point numbers, booleans, arrays and objects. Mastering these key points can help you get started with PHP development quickly.

PHP has 8 variable types, commonly used include Integer, Float, String, Boolean, Array, Object, NULL and Resource. To view variable types, use the gettype() or is_type() series functions. PHP will automatically convert types, but it is recommended to use === to strictly compare the key logic. Manual conversion can be used for syntax such as (int), (string), etc., but be careful that information may be lost.

PHP variables start with $, and the naming must follow rules, such as they cannot start with numbers and are case sensitive; the scope of the variable is divided into local, global and hyperglobal; global variables can be accessed using global, but it is recommended to pass them with parameters; mutable variables and reference assignments should be used with caution. Variables are the basis for storing data, and correctly mastering their rules and mechanisms is crucial to development.
