java 8 filter a ListOfMap for checking a key exists, if exists then collect a map

Question

Below is the code for collecting listofmap from resultset and checking the listofmap for a perticular product, if product exists then get that map. Please help me to do it in java8

List<Map<String,Object>>    lmProducts  =   new ArrayList<Map<String, Object>>();
Map<String, Object>         map         =   null;
try {
    if(resultSet.next()){
        System.out.println("PRODUCT\t PRICE\t QUANTITY");
        do{
            map  =   new HashMap<String, Object>();
            map.put("NAME", resultSet.getString("NAME"));
            map.put("PRICE", resultSet.getString("PRICE"));
            map.put("QUANTITY", resultSet.getString("QUANTITY"));
            System.out.println(resultSet.getString("NAME")+"\t "+resultSet.getInt("PRICE")+"\t "+resultSet.getInt("QUANTITY"));
            lmProducts.add(map);
        }while (resultSet.next());
    }

}catch (SQLException e){
    System.out.println("Exception while processing Resultset: "+e.toString());
}

//Trying to get mProduct map by checking key condition

Map<String, Object> mProductMap     =   new HashMap<String, Object>();
mProductMap     =   lmProducts.stream().filter(m -> m.get("NAME").toString().equalsIgnoreCase(sProductName)).collect(to)

engin · Accepted Answer · 2020-05-29 16:41:52Z

2

You can collect all of the entries combined from the map using Collectors.toMap.

Map<String, Object> mProductMap = lmProducts.stream()
        .flatMap(a -> a.entrySet().stream()) // stream of entries of all maps
        // entries with specific key and value combination
        .filter(m -> m.getKey().equals("NAME") && m.getValue().toString().equalsIgnoreCase(sProductName)) 
        .collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));

edited May 29, 2020 at 16:41

engin

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answered Feb 27, 2019 at 4:09

Naman

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Comments

Ryuzaki L · Accepted Answer · 2019-02-27 05:21:16Z

You can do in this way also, Since Map is of unique key value pair check Map contains that key and value, Then collect it to Map

Map<String,Object> result = list.stream()
    .filter(m->m.containsKey("NAME") && m.get("NAME").toString().equalsIgnoreCase(sProductName))    // check map contains that key and value
         .collect(Collectors.toMap(key->"NAME",value->value.get("NAME")));  //collect it to Map

Note Suppose if you have two map object in list with same key then it will throw an exception

java.lang.IllegalStateException: Duplicate key NAME

So to ignore that case use merge function toMap(Function keyMapper, Function valueMapper, BinaryOperator mergeFunction)

.collect(Collectors.toMap(key->"NAME",value->value.get("NAME"), (v1,v2)->v1));

ETO · Accepted Answer · 2019-02-27 05:50:24Z

If I get your code correctly, you're looking for something like this:

Predicate<Map<String, Object>> hasExpectedName = 
    prod -> prod.containsKey("NAME") && prod.get("NAME").toString().equalsIgnoreCase(sProductName);

Map<String, Object> mProductMap = 
    lmProducts.stream()
              .filter(hasExpectedName)
              .findFirst()
              .orElse(null);    // <= return null or a default value
           // .orElseThrow(...) // <= or you can throw an exception if you want

Although I would do in a bit more OOP style:

Define a class for holding product data:

static class Product {
    private String name;
    private String price;
    private String quantity;

    // Constructor & setters & getters
}

then fetch the data into Product objects:

// ...
System.out.println("PRODUCT\t PRICE\t QUANTITY");
do {
    product = new Product(
        resultSet.getString("NAME"),
        resultSet.getString("PRICE"),
        resultSet.getString("QUANTITY")
    );
    // System.out.println(...);
    products.add(product);
} while (resultSet.next());
// ...

From now on the stream operations are pretty straightforward:

Product product = products.stream()
                          .filter(p -> p.getName().equalsIgnoreCase(sProductName))
                          .findFirst()   // or .findAny()
                          .orElse(null); // or a default value

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