如何在 java HashMap 中根据产品类别获取产品详细信息？

Question

有HashMap。我在 hashmap 中添加了所有产品详细信息。我想根据类别显示产品详细信息。这意味着应该显示所有相同类别的产品。我正在尝试为此创建方法。方法名称是

public List<Product> getProductsBasedOnCategory(String category)
{

}

请查找以下代码。

Product.java

public class Product {

    private long pid;
    private String pname;
    private String category;
    private float price;
    private long stock;
    private String remarks;

    public Product()
    {

    }

    public Product(long pid,String pname,String category,float price,long stock,String remarks){
        this.pid=pid;
        this.pname=pname;
        this.category=category;
        this.price=price;
        this.stock=stock;
        this.remarks=remarks;
    }
    public long getPid() {
        return pid;
    }
    public void setPid(long pid) {
        this.pid = pid;
    }
    public String getPname() {
        return pname;
    }
    public void setPname(String pname) {
        this.pname = pname;
    }
    public String getCategory() {
        return category;
    }
    public void setCategory(String category) {
        this.category = category;
    }
    public float getPrice() {
        return price;
    }
    public void setPrice(float price) {
        this.price = price;
    }
    public long getStock() {
        return stock;
    }
    public void setStock(long stock) {
        this.stock = stock;
    }
    public String getRemarks() {
        return remarks;
    }
    public void setRemarks(String remarks) {
        this.remarks = remarks;
    }

}

DatabaseClass.java

public class DatabaseClass {

    private static Map<Long, Product> products=new HashMap<>();

    public static Map<Long, Product> getProduct()
    {
        return products;
    }

}

ProductDao.java

private Map<Long, Product> products=DatabaseClass.getProduct();

public ProductDaoImpl()
{
    products.put(1L, new Product(1L,"TV","Entertinement",10000F,250L,"This is best TV!"));
    products.put(2L, new Product(2L,"Computer","Technology",20000F,350L,"My Computer name Hp and SONY ViVo!"));
    products.put(3L, new Product(3L,"DeskTopComputer","Technology",15000F,150L,"My Desktop Computer name Accer and SONY ViVo!"));
}

//Get All products
public List<Product> getAllProducts() {

    return new ArrayList<Product>(products.values());
}

//Get product by product id
public Product getProduct(long pid) {

    return products.get(pid);
}

//To Add the products 
public Product addProduct(Product product) {
    product.setPid(products.size()+1);
    products.put(product.getPid(), product);
    return product;
}

//Update the product
public Product updateProduct(Product product) {
    if(product.getPid()<=0)
    {
        return null;
    }
    products.put(product.getPid(), product);
    return product;
}

// Delete the product
public Product deleteProduct(long pid) {

    return products.remove(pid);

}



//Get the product by category
public List<Product> getProductByCategory(String category) {

    if(products.size()<=0)
    {
        return null;
    }

    else if(category.equals(products.get(Product))
    {



    }

我尝试了很多如何编写代码来获取 HashMap 中模型 class 的值。以及如何为上面的 getProductByCategory(String category) 编写代码。

Answer 1

您可以遍历映射中的值集，并过滤到 return 匹配产品列表：

public List<Product> getProductByCategory(String category) {

    if(products.size() == 0){
        return new ArrayList<>();
    }

    return this.products.values().stream()
       .filter(product -> product.getCategory().equals(category))
       .collect(Collectors.toList());
}

您也可以为此使用 for 循环：

public List<Product> getProductByCategory(String category) {

    List<Product> ret = new ArrayList<>();

    if(products.size() == 0){
        return ret;
    }

    for(Product p: this.products.values()) {
        if(p.getCategory().equals(category))
            ret.add(p);
    }

    return ret;
}

请注意，如果产品地图为空，我 return 为空 ArrayList。这是集合 return 类型的更好做法（而不是 returning null）

Answer 2

一种方法是按如下方式迭代 Hashmap :

public List<Product> getProductsBasedOnCategory(String category)
{
    List<Product> list = new ArrayList<Product>();

    if (products.size()<=0) {
        return list;
    }

    products.entrySet().stream().forEach((entry) -> {
        if (((Product) entry.getValue()).getCategory().equals(category)) {
            list.add(entry.getValue())
        }
    });

    return list;

}

Answer 3

您至少有 2 个选项，例如。

为您要搜索的每个字段准备一张特定的地图。这是如果你有很多对象会更快 map 需要 O(1) 而遍历整个集合需要在）。 Big O Cheat Sheet

声明地图

private Map<Long, Product> productsByID = new HashMap();
private Map<String, Product> productsByCategory = new HashMap();

初始化地图

public ProductDaoImpl()
{
    // Create the objects
    Product p1 = new Product(1L,"TV","Entertinement",10000F,250L,"This is best TV!");
    Product p2 = new Product(2L,"Computer","Technology",20000F,350L,"My Computer name Hp and SONY ViVo!");
    Product p3 = new Product(3L,"DeskTopComputer","Technology",15000F,150L,"My Desktop Computer name Accer and SONY ViVo!");

    //Assign the objects into the map by ids
    productsByID.put(1L, p1);
    productsByID.put(2L, p2);
    productsByID.put(3L, p3);

    //Assign the objects into the map by category
    productsByCategory.put(p1.getCategory(), p1);
    productsByCategory.put(p2.getCategory(), p2);
    productsByCategory.put(p3.getCategory(), p3);
}

使用你拥有的同一张地图并且漂亮正如其他答案所解释的那样，多次遍历所有值。

最后，我不知道这是否只是一个练习代码，你的值很少，在这种情况下，性能并不重要，或者这是你的生产代码的开始，在这种情况下，你想要期待很多价值。如果最新的是真的，你可能想将其建模到数据库中（sql 或不是 sql）并通过你 want/need 的特定字段进行查询。

如何在 java HashMap 中根据产品类别获取产品详细信息？

How to get product details based on product category, in java HashMap?

java

dao

model

hashmap