java loop and add values to a map

Question

New to using map and was wondering how to add values and loop thur it to retrieve the values. Below is my code:

Map<String, List<String>> map = new LinkedHashMap<String, List<String>>();

for ( int i = 0, m = 1; i < visualcategory.size(); i = i+2, m = m+2) {
    String categoryName = visualcategory.get(m);
    map.put(categoryName , null);
}

for which i will be having this which is im assuming is correct even with the null

MAP {5=null, 11=null, 15=null, 24=null, 96=null, 98=null}

Its currently null as the next process (below) will fill up that list as it loops thru a for condition unless it can be done to add the categoryName and the List values at the same time?

Now, i need to loop and add a list from each string/map

String value1 = value;
String value2 = value;

for (Entry<String, List<String>> entry : map.entrySet()) {
    map.put(value1, entry.getValue());  --> doesn't work
    map.put(value2, entry.getValue());  --> doesn't work
}

I need something like this

MAP {5=[value1,value2,etc], 11=[value1,value2,etc], .......

Problem is, it doesn't work I can't seem to add inside the List. I need help on how to add values from a map > and loop thru to it?

The map contains null as values and no list, why not start there? — Smutje
– Smutje, Commented Jul 1, 2014 at 6:33
Can you modify MAP {5=null, 11=null, 15=null, 24=null, 96=null, 98=null} with your example values instead of null? It is very confusing.. as you are expecting MAP {5=[value1,value2,etc], 11=[value1,value2,etc], ....... and not null values.. — Ninad Pingale
– Ninad Pingale, Commented Jul 1, 2014 at 6:41
Its currently null as the next process (below) will fill up that list as it loops thru a for condition unless it can be done to add the categoryName and the List values at the same time? — user3736748
– user3736748, Commented Jul 1, 2014 at 6:46

JavaGoPro · Accepted Answer · 2016-07-07 15:39:46Z

1

This is what you need:

anotherMap.forEach((k, v) -> myMap.put(k.toString(), v.toString()));

Source: Looping Through a Map in Java

answered Jul 7, 2016 at 15:39

JavaGoPro

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Comments

Ninad Pingale · Accepted Answer · 2014-07-01 06:36:19Z

0

Use Iterator insted of EntrySet

    Map<String, List<String>> map = new LinkedHashMap<String, List<String>>();
    Iterator itr=map.keySet().iterator();
    while (itr.hasNext()) {
        String key =  itr.next().toString();
        String value=map.get(key).toString();
        System.out.println(key+"="+value);
    }

answered Jul 1, 2014 at 6:36

Ninad Pingale

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Comments

shmosel · Accepted Answer · 2014-07-01 07:13:20Z

When you create your map, instead of filling it with nulls, fill it with empty Lists:

Map<String, List<String>> map = new LinkedHashMap<String, List<String>>();

for ( int i = 0, m = 1; i < visualcategory.size(); i = i+2, m = m+2) {
    String categoryName = visualcategory.get(m);
    map.put(categoryName , new ArrayList<String>());
}

Now, when you're ready to populate the values, you can do:

String value1 = value;
String value2 = value;

for (List<String> list : map.values()) {
    list.add(value1);
    list.add(value2);
}

If you have the values when you're creating the map, you make it simpler:

String value1 = value;
String value2 = value;

Map<String, List<String>> map = new LinkedHashMap<String, List<String>>();

for ( int i = 0, m = 1; i < visualcategory.size(); i = i+2, m = m+2) {
    String categoryName = visualcategory.get(m);
    List<String> values = new ArrayList<String>();
    values.add(value1);
    values.add(value2);
    map.put(categoryName, values);
}

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