Java Language TreeMap of a simple Java type


First, we create an empty map, and insert some elements into it:

Java SE 7
TreeMap<Integer, String> treeMap = new TreeMap<>();
Java SE 7
TreeMap<Integer, String> treeMap = new TreeMap<Integer, String>();
treeMap.put(10, "ten");
treeMap.put(4, "four");
treeMap.put(1, "one");
treeSet.put(12, "twelve");

Once we have a few elements in the map, we can perform some operations:

System.out.println(treeMap.firstEntry()); // Prints 1=one
System.out.println(treeMap.lastEntry()); // Prints 12=twelve
System.out.println(treeMap.size()); // Prints 4, since there are 4 elemens in the map
System.out.println(treeMap.get(12)); // Prints twelve
System.out.println(treeMap.get(15)); // Prints null, since the key is not found in the map

We can also iterate over the map elements using either an Iterator, or a foreach loop. Note that the entries are printed according to their natural ordering, not the insertion order:

Java SE 7
for (Entry<Integer, String> entry : treeMap.entrySet()) {
    System.out.print(entry + " "); //prints 1=one 4=four 10=ten 12=twelve 
Iterator<Entry<Integer, String>> iter = treeMap.entrySet().iterator();
while (iter.hasNext()) {
    System.out.print( + " "); //prints 1=one 4=four 10=ten 12=twelve