将 HashMap 转换为字符串逗号分隔

Question

I have a class ShoppingCartHelper and a method.

private static Map<Product, ShoppingCartEntry> cartMap = new HashMap<Product, ShoppingCartEntry>();

And method that stores data product to cartMap:

public static List<Product> getCartList() {
    List<Product> cartList = new Vector<Product>(cartMap.keySet().size());
    for(Product p : cartMap.keySet()) {
        cartList.add(p);
    }
    return cartList;
}

In other class I call stored data on map:

private List<Product> mCartList;
mCartList = ShoppingCartHelper.getCartList();

and print it in comma separated:

StringBuilder commaSepValueBuilder = new StringBuilder();
for ( int i = 0; i< mCartList.size(); i++) {
   commaSepValueBuilder.append(mCartList.get(i));
   if ( i != mCartList.size()-1) {
       commaSepValueBuilder.append(", ");
   }
}
System.out.println(commaSepValueBuilder.toString());

Its printed like com.android.test@34566f3,com.android.test@29f9042

How do I print data on Map to string (human readable)?

Answer 1

Make your Product class overriding the toString() method or in your custom logic, make a string builder appending not the element itself, but it's fileds, which you wish to recieve as text description of the product instance. I mean something like this:

//since I don't know, what is the Product class, I supposed it has a name filed
commaSepValueBuilder.append(mCartList.get(i).getName());

Answer 2

Don't use Vector , use ArrayList . Quoting javadoc:

If a thread-safe implementation is not needed, it is recommended to use ArrayList in place of Vector

The shorter version of getCartList() is:

public static List<Product> getCartList() {
    return new ArrayList<>(cartMap.keySet());
}

As for how to build comma-separated list of products, the best way is to implement the Product method toString() . This will also help when debugging.

public class Product {

    // lots of code here

    @Override
    public String toString() {
        return getName(); // Assuming Product has such a method
    }
}

Then you can use StringBuilder in a simple for-each loop:

StringBuilder buf = new StringBuilder();
for (Product product : ShoppingCartHelper.getCartList()) {
    if (buf.length() != 0)
        buf.append(", ");
    buf.append(product); // or product.getName() if you didn't implement toString()
}
System.out.println(buf.toString());

In Java 8 that can be simplified by using StringJoiner :

StringJoiner sj = new StringJoiner(", ");
for (Product product : ShoppingCartHelper.getCartList())
    sj.add(product);
System.out.println(sj.toString());

Answer 3

You can override toString method in your Product class like below,

public class Product {

//sample properties
private String name;
private Long id;

public String getName() {
    return name;
}

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

public Long getId() {
    return id;
}

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

@Override
public String toString() {
    return "Product{" +
            "name='" + name + '\'' +
            ", id=" + id +
            '}';
}

}

it can print string in human readable format.

Answer 4

Override toString in Product class and use Java 8 Streams to make it more simpler, as below:

 mCartList.stream().map(e -> e.toString())
                .collect(Collectors.joining(","));

Usage:

List<Product> mCartList = new ArrayList<>();
        mCartList.add(new Product(1, "A"));
        mCartList.add(new Product(2, "B"));

        String commaSepValue = mCartList.stream().map(e -> e.toString())
                .collect(Collectors.joining(","));
        System.out.println(commaSepValue);

Product.java

final class Product {
        private final int id;
        private final String name;

        public Product(int id, String name) {
            this.id = id;
            this.name = name;
        }

        public String getName() {
            return name;
        }

        public int getId() {
            return id;
        }

        @Override
        public String toString() {
            return "Product [id=" + id + ", name=" + name + "]";
        }
    }

Answer 5

Below code works for me by using Java 8 Streams:

For Hashmap :

hashmapObject.entrySet().stream().map(e -> e.toString())
                .collect(Collectors.joining(","));

For List :

listObject.stream().map(e -> e.toString())
                .collect(Collectors.joining(","));