ArrayList计数相等的对象

Question

我有一个名为class的产品。 此类中的所有字段都是从数据库填充的：

 public class Product {
            private Long id;
            private String code;
            private String name;
            private String type;
            private Integer quantity;
            private Integer numOfPieces;
            private Integer sumOfPieces;
            private String savingPeriod;
            private Double costPrice;
            private Double price;

            public Product() {
            }

            public Product(Long id, String code, String name, String type, Integer 
            quantity, Integer numOfPieces, Integer sumOfPieces, String savingPeriod, 
            Double costPrice, Double price) {
                this.id = id;
                this.code = code;
                this.name = name;
                this.type = type;
                this.quantity = quantity;
                this.numOfPieces = numOfPieces;
                this.sumOfPieces = sumOfPieces;
                this.savingPeriod = savingPeriod;
                this.costPrice = costPrice;
                this.price = price;
        }

        public Long getId() {
            return id;
        }

        public void setId(Long id) {
            this.id = id;
        }

        public String getCode() {
            return code;
        }

        public void setCode(String code) {
            this.code = code;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public String getType() {
            return type;
        }

        public void setType(String type) {
            this.type = type;
        }

        public Integer getQuantity() {
            return quantity;
        }

        public void setQuantity(Integer quantity) {
            this.quantity = quantity;
        }

        public Integer getNumOfPieces() {
            return numOfPieces;
        }

        public void setNumOfPieces(Integer numOfPieces) {
            this.numOfPieces = numOfPieces;
        }

        public Integer getSumOfPieces() {
            return sumOfPieces;
        }

        public void setSumOfPieces(Integer sumOfPieces) {
            this.sumOfPieces = sumOfPieces;
        }

        public String getSavingPeriod() {
            return savingPeriod;
        }

        public void setSavingPeriod(String savingPeriod) {
            this.savingPeriod = savingPeriod;
        }

        public Double getCostPrice() {
            return costPrice;
        }

        public void setCostPrice(Double costPrice) {
            this.costPrice = costPrice;
        }

        public Double getPrice() {
            return price;
        }

        public void setPrice(Double price) {
            this.price = price;
        }

        @Override
        public String toString() {
            return "Product{" + "id=" + id + ", code=" + code + ", name=" + name + ", type=" + type + ", quantity=" + quantity + ", numOfPieces=" + numOfPieces + ", sumOfPieces=" + sumOfPieces + ", savingPeriod=" + savingPeriod + ", costPrice=" + costPrice + ", tax=" + price + '}';
        }
    @Override
    public int hashCode() {
        int hash = 7;
        hash = 83 * hash + Objects.hashCode(this.id);
        hash = 83 * hash + Objects.hashCode(this.code);
        hash = 83 * hash + Objects.hashCode(this.name);
        hash = 83 * hash + Objects.hashCode(this.type);
        hash = 83 * hash + Objects.hashCode(this.quantity);
        hash = 83 * hash + Objects.hashCode(this.numOfPieces);
        hash = 83 * hash + Objects.hashCode(this.sumOfPieces);
        hash = 83 * hash + Objects.hashCode(this.savingPeriod);
        hash = 83 * hash + Objects.hashCode(this.costPrice);
        hash = 83 * hash + Objects.hashCode(this.price);
        return hash;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Product other = (Product) obj;
        if (!Objects.equals(this.code, other.code)) {
            return false;
        }
        if (!Objects.equals(this.name, other.name)) {
            return false;
        }
        if (!Objects.equals(this.type, other.type)) {
            return false;
        }
        if (!Objects.equals(this.savingPeriod, other.savingPeriod)) {
            return false;
        }
        if (!Objects.equals(this.id, other.id)) {
            return false;
        }
        if (!Objects.equals(this.quantity, other.quantity)) {
            return false;
        }
        if (!Objects.equals(this.numOfPieces, other.numOfPieces)) {
            return false;
        }
        if (!Objects.equals(this.sumOfPieces, other.sumOfPieces)) {
            return false;
        }
        if (!Objects.equals(this.costPrice, other.costPrice)) {
            return false;
        }
        if (!Objects.equals(this.price, other.price)) {
            return false;
        }
        return true;
    }
    }

我有一个ProductsController类可与数据库一起使用：

ProductsController productsController = new ProductsController();

我创建一个新的ArrayList并添加一些具有相同信息的产品对象：

Product p1 = productsController.findOneById(1); //p1.getName() = "Analgin"
Product p2 = productsController.findOneById(1); //p2.getName() = "Analgin"
Product p3 = productsController.findOneById(1); //p3.getName() = "Analgin"
Product p4 = productsController.findOneById(1); //p4.getName() = "Analgin"
Product p5 = productsController.findOneById(2); //p5.getName() = "Pikovid"
Product p6 = productsController.findOneById(2); //p6.getName() = "Pikovid"
Product p7 = productsController.findOneById(3); //p7.getName() = "Bolnol"
List<Product> productsList = new ArrayList<>();

如何为以下输出计算ArrayList中的相同元素：

1. 4安乃近
2. 2皮科维奇
3. 1 Bolnol

Answer 1

Javlonbek，

我建议您使用Java Streams的“ groupingBy”功能。

productsList.stream().collect(
   Collectors.groupingBy(product -> product.getName(),
   Collectors.counting())
);

在您的示例中，输出按频率降序排列。 我不确定这是意外还是故意的。 streamApi还允许您进行排序。 参见mkyong网站上的示例[1]。

干杯，马可

1： https ： //www.mkyong.com/java8/java-8-collectors-groupingby-and-mapping-example/

Answer 2

我看到您已经实现了equals和hashCode，因此足以构建一个像这样的映射：

HashMap <String, Product> productsByName = new HashMap <>();
productsByName.put(product1.getName(), product1);
productsByName.put(product2.getName(), product2);
// add all the products

并且此映射已经包含了此类信息，足以通过给定名称查询产品列表，例如：

List <Product> analginProducts = productsByName.get("Analgin");