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Home Java javaTutorial Convert ArrayList to HashMap in Java

Convert ArrayList to HashMap in Java

Feb 07, 2025 am 11:23 AM
java

Convert ArrayList to HashMap in Java

The

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>

This article will introduce several methods to convert

to ArrayList: HashMap

    By iteration
  • ArrayList
    • Using Java 8 Stream API
The following are some examples: 

<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

  1. Step 1: Start.
  2. Step 2: Declare and import the necessary Java packages.
  3. Step 3: Create a public list.
  4. Step 4: Declare key-value pairs.
  5. Step 5: Create a constructor for the referenced value.
  6. Step 6: Assign values ??to the declared key.
  7. Step 7: Return the private variable id.
  8. Step 8: Declare a public class and method.
  9. Step 9: Declare the parameter string.
  10. Step 10: Create a . ArrayList
  11. Step 11: Use data elements to fill the list value.
  12. Step 12: Create and declare a mapping value.
  13. Step 13: Declare the object method.
  14. Step 14: Create an object map value.
  15. Step 15: Add each data element to the map.
  16. 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

and adds each element to

. ArrayList HashMap 

// 示例代碼 (假設(shè)ArrayList包含自定義對(duì)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

to 
// 示例代碼 (假設(shè)ArrayList包含自定義對(duì)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

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