《SimpleJava》Problems 37th: Frequently Used Methods Of Java HashMap

HashMap is very useful when a counter is required.

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HashMap<String, Integer> countMap = new HashMap<String , Integer>();

// ... a lot of a’s like the following
if (countMap.keySet().contains(a)) {
countMap.put(a, countMap.get(a) + 1);
} else {
countMap.put(a, 1);
}

loop through hashmap

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Iterator it = mp.entrySet().iterator();
while (it.hasNext()) {
Map.Entry pairs = (Map.Entry) it.next();
System.out.println(pairs.getKey() + " = " + pairs.getValue());
}

Map<Integer, Integer> map = new HashMap<Integer, Integer>();
for (Map.Entry<Integer, Integer> entry : map.entrySet()) {
System.out.println("Key = " + entry.getKey() + " , Value = " + entry.getValue());
}

2 print hashmap

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public static void printMap(Map mp) {
Iterator it = mp.entrySet().iterator();

while (it.hasNext()) {
Map.Entry pairs = (Map.Entry) it.next();

System.out.println(pairs.getKey() + " = " + pairs.getValue());
it.remove(); // avoids a Concurrent Modification Exception
}
}

3 sort hashmap by value

The following code example take advantage of a constructor of TreeMap here.

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class ValueComparator implements Comparator<String> {
Map<String, Integer> base;

public ValueComparator(Map<String, Integer> base) {
this.base = base;
}

public int compare(String a, String b) {
if (base.get(a) >= base.get(b)) {
return -1;
} else {
return 1 ;
} // returning 0 would merge keys
}
}
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HashMap<String, Integer> countMap = new HashMap<String, Integer>();

// add a lot of entries
countMap.put("a", 10);
countMap.put("b", 20);

ValueComparator vc = new ValueComparator(countMap);

TreeMap<String, Integer> sortedMap = new TreeMap<String, Integer>(vc);
sortedMap.putAll(countMap);
printMap(sortedMap);

There are different ways of sorting HashMap, this way has been voted the most in stackoverflow