Looping through item occurences with same property (catalog_id)

问题:

I have a Catalog bean with the following properties:

catalog_id catalog_name catalog_price catalog_title 

catalog_id is not unique, there can be multiple Catalog object with the same id.

Having a list of Catalog objects, I want to collect all with the same id and in a second for loop I want to loop with through the number of occurance of that id.

Let say :

catalogList2.stream()         .collect(Collectors.groupingBy(Catalog::getId, Collectors.counting()))         .entrySet()         .stream()         .map(p->"catalog id--->"+p.getKey()+"    -"+p.getValue()).forEach(System.out::println); 

From the above code I am able to get the below output:

catalog id--->17553    -- 8 catalog id--->545    -- 8 catalog id--->546    -- 6 catalog id--->40962    -- 16 catalog id--->901    -- 12 

But, after that I want to do like below : ( its just a structure I want to implement)

for(Catalog cat1:p.getKey()){      for(int i=0;i<p.getValue();i++){          System.out.println("Name -->"+cat1.get(i)));         //Something i will do here.      }  } 

I am not sure how to implement this structure.

Update

Am getting the message in my eclipse Lambda expression's parameter catalog cannot redeclare another local variable defined in an enclosing scope.

when am trying to add 2nd sysout :

 catalogList2.stream()     .collect(Collectors.groupingBy(Catalog::getId))     .forEach((id, catalogs) -> {         System.out.println("Catalog id : " + id + " : " + catalogs.size());         catalogs.forEach(catalog -> {             System.out.println("Name -->"+catalog.getCatalog_attr_name());             System.out.println("Value --->"+catalog.getCatalog_attr_value());         });     }); 

Update 2

Am getting this message in eclipse Local variable catalog defined in an enclosing scope must be final or effectively final

at the line String catalogName=catalog.getCatalog_name(); // getting error on this line

Please find my code here ;

JSONObject jsonObj=null; JSONArray respArray=new JSONArray(); catalogList2.stream() .collect(Collectors.groupingBy(Catalog::getId)) .forEach((id, catalogs) -> {     System.out.println("Catalog id : " + id + " : " + catalogs.size());     String catalogName=catalog.getCatalog_name();  // getting error on this line     String longDescriptionStr = catalogName.concat("_long_description");     String descriptionStr = catalogName.concat("_description");     jsonObj.put("catalog_id", id);     catalogs.forEach(c -> {         String longDescription=null;         String description=null;         String catalogAttrName = c.getCatalog_attr_name();         if(StringUtils.equalsIgnoreCase(catalogAttrName, longDescriptionStr)) {              longDescription = c.getCatalog_attr_value();         }         if(StringUtils.equalsAnyIgnoreCase(catalogAttrName, descriptionStr)) {             description = c.getCatalog_attr_value();         }         jsonObj.put("description", description);         jsonObj.put("longDescription", longDescription);      });     respArray.put(jsonObj);  }); System.out.println("Printing JSON Array --->"+respArray.toString()); 

回答1:

Using groupingBy without a downstream collector will map the IDs to lists of the catalogs with those IDs. You can then ask the lists for their size to get the count and iterate through the lists to get each catalog:

catalogList2.stream()     .collect(Collectors.groupingBy(Catalog::getId))     .forEach((id, catalogs) -> {         System.out.println("Catalog id : " + id + " : " + catalogs.size());         catalogs.forEach(System.out::println);     }); 

回答2:

You can do it as below:

catalogList .stream() .collect(Collectors.groupingBy(Catalog::getId)) .forEach((key,value)->{     for(Catalog curValue:value)         System.out.println("Name -->"+curValue.catalog_name);     }); 

Output:

Name -->Catalog 2 Name -->Catalog 3 Name -->Catalog 4 Name -->Catalog 1 Name -->Catalog 7 Name -->Catalog 8 Name -->Catalog 9 Name -->Catalog 15 Name -->Catalog 16 Name -->Catalog 18 Name -->Catalog 19 Name -->Catalog 20 

文章来源: Looping through item occurences with same property (catalog_id)