Add multiple items

We can use V put(K key,V value):

Associates the specified value with the specified key in this map (optional operation). If the map previously contained a mapping for the key, the old value is replaced by the specified value.

String currentVal;
Map<Integer, String> map = new TreeMap<>();
currentVal = map.put(1, "First element.");
System.out.println(currentVal);// Will print null
currentVal = map.put(2, "Second element.");
System.out.println(currentVal); // Will print null yet again    
currentVal = map.put(2, "This will replace 'Second element'");
System.out.println(currentVal); // will print Second element.
System.out.println(map.size()); // Will print 2 as key having
// value 2 was replaced.

Map<Integer, String> map2 = new HashMap<>();
map2.put(2, "Element 2");
map2.put(3, "Element 3");

map.putAll(map2);

System.out.println(map.size());

Output:

3

To add many items you can use an inner classes like this:

Map<Integer, String> map = new HashMap<>() {{
    // This is now an anonymous inner class with an unnamed instance constructor
    put(5, "high");
    put(4, "low");
    put(1, "too slow");
}};

Keep in mind that creating an anonymous inner class is not always efficient and can lead to memory leaks so when possible, use an initializer block instead:

static Map<Integer, String> map = new HashMap<>();

static {
    // Now no inner classes are created so we can avoid memory leaks
    put(5, "high");
    put(4, "low");
    put(1, "too slow");
}

The example above makes the map static. It can also be used in a non-static context by removing all occurences of static.

In addition to that most implementations support putAll, which can add all entries in one map to another like this:

another.putAll(one);

Found a mistake? Have a question or improvement idea? Let me know.

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