How to Loop next element in hashmap

Question

I have a set of strings like this

A_2007-04, A_2007-09, A_Agent, A_Daily, A_Execute, A_Exec, B_Action, B_HealthCheck

I want output as:

Key = A, Value = [2007-04,2007-09,Agent,Execute,Exec]
Key = B, Value = [Action,HealthCheck]

I'm using HashMap to do this

pckg:{A,B}
count:total no of strings
reports:set of strings

Logic I used is nested loop:

for (String l : reports[i]) {
    for (String r : pckg) {
        String[] g = l.split("_");
        if (g[0].equalsIgnoreCase(r)) {
            report.add(g[1]);
            dirFiles.put(g[0], report);
        } else {
            break;
        }
    }
}

I'm getting output as

Key = A, Value = [2007-04,2007-09,Agent,Execute,Exec]

How to get second key? Can someone suggest logic for this?

First, your variable names are confusing. You should give them meaningful names. Second, what is the type of pckg? Seems weird that you are looping through the pckg variable while in the for loop of reports[i] — Orin
– Orin, Commented Sep 9, 2016 at 13:15
with P_Action and P_HealthCheck, you cannot get Key = B, Value = [Action,HealthCheck], you can only get it with Key = P so pckg should be:{A,P} not {A,B} — Nicolas Filotto
– Nicolas Filotto, Commented Sep 9, 2016 at 13:19

Nicolas Filotto · Accepted Answer · 2016-09-09 19:57:51Z

1

Assuming that you use Java 8, it can be done using computeIfAbsent to initialize the List of values when it is a new key as next:

List<String> tokens = Arrays.asList(
    "A_2007-04", "A_2007-09", "A_Agent", "A_Daily", "A_Execute", 
    "A_Exec", "P_Action", "P_HealthCheck"
);
Map<String, List<String>> map = new HashMap<>();
for (String token : tokens) {
    String[] g = token.split("_");
    map.computeIfAbsent(g[0], key -> new ArrayList<>()).add(g[1]);
}

edited Sep 9, 2016 at 19:57

answered Sep 9, 2016 at 14:06

Nicolas Filotto

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1 Comment

Bobulous Over a year ago

+1 for bringing computeIfAbsent to my attention. I'd been wondering for some time why Java core hadn't made it easier to handle a map whose values were themselves a collection, and this fills the gap nicely.

Bobulous · Accepted Answer · 2016-09-09 18:19:22Z

In terms of raw code this should do what I think you are trying to achieve:

// Create a collection of String any way you like, but for testing
// I've simply split a flat string into an array.
String flatString = "A_2007-04,A_2007-09,A_Agent,A_Daily,A_Execute,A_Exec,"
        + "P_Action,P_HealthCheck";
String[] reports = flatString.split(",");

Map<String, List<String>> mapFromReportKeyToValues = new HashMap<>();

for (String report : reports) {
    int underscoreIndex = report.indexOf("_");
    String key = report.substring(0, underscoreIndex);
    String newValue = report.substring(underscoreIndex + 1);
    List<String> existingValues = mapFromReportKeyToValues.get(key);
    if (existingValues == null) {
        // This key hasn't been seen before, so create a new list
        // to contain values which belong under this key.
        existingValues = new ArrayList<>();
        mapFromReportKeyToValues.put(key, existingValues);
    }
    existingValues.add(newValue);
}

System.out.println("Generated map:\n" + mapFromReportKeyToValues);

Though I recommend tidying it up and organising it into a method or methods as fits your project code.

Shahid · Accepted Answer · 2016-09-09 13:29:43Z

Doing this with Map<String, ArrayList<String>> will be another good approach I think:

    String reports[] = {"A_2007-04", "A_2007-09", "A_Agent", "A_Daily", 
                        "A_Execute", "A_Exec", "P_Action", "P_HealthCheck"};
    Map<String, ArrayList<String>> map = new HashMap<>();

    for (String rep : reports) {

        String s[] = rep.split("_");
        String prefix = s[0], suffix = s[1];
        ArrayList<String> list = new ArrayList<>();

        if (map.containsKey(prefix)) {
            list = map.get(prefix);
        }

        list.add(suffix);
        map.put(prefix, list);
    }

    // Print
    for (Map.Entry<String, ArrayList<String>> entry : map.entrySet()) {
        String key = entry.getKey();
        ArrayList<String> valueList = entry.getValue();
        System.out.println(key + " " + valueList);
    }

Balkrishna Rawool · Accepted Answer · 2016-09-09 13:54:28Z

0

for (String l : reports[i]) {
        String[] g = l.split("_");
        for (String r : pckg) {
            if (g[0].equalsIgnoreCase(r)) {
                report = dirFiles.get(g[0]);
                if(report == null){ report = new ArrayList<String>(); } //create new report
                report.add(g[1]);
                dirFiles.put(g[0], report);
            }
        }
}

Removed the else part of the if condition. You are using break there which exits the inner loop and you never get to evaluate the keys beyond first key.
Added checking for existing values. As suggested by Orin2005.
Also I have moved the statement String[] g = l.split("_"); outside inner loop so that it doesn't get executed multiple times.

edited Sep 9, 2016 at 13:54

answered Sep 9, 2016 at 13:15

Balkrishna Rawool

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2 Comments

Orin Over a year ago

In the line dirFiles.put(g[0], report);, I think you need to account for everything that is already in the HashMap for that key aswell. so it should be dirFiles.put(g[0], dirFiles.get(g[0]).addAll(report)); or something, depending on what the value of report is.

Balkrishna Rawool Over a year ago

I assume report is List of Strings and dirFiles is HashMap with key-type String and value-type List of Strings. In that case it'd work as expected.

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