Map interface
In this chapter you will learn:
What is Map interface
A map is a group of key/value pairs. A map cannot contain duplicate keys. Each key can map to at most one value.
Maps are described by the Map interface.
Map interface has no parent interface and is declared as
Map<K,V>
- K is the key's type
- V is the value's type
Unlike List, Set, and
Queue, Map does not extend Collection.
It is possible to get a Collection instance
by calling Map's keySet(), values(),
and entrySet() methods,
which respectively return a Set of keys,
a Collection of values, and a Set of key/value pair entries.
The following code creates a Map from HashMap. We will talk about
HashMap later. HashMap is one of the implementation Java platform
provided.
Also the Map is defined in a generic way that it only accepts string pair.
import java.util.HashMap;
import java.util.Map;
/*from j av a 2 s . co m*/
public class Main {
public static void main(String[] a) {
Map<String,String> map = new HashMap<String,String>();
map.put("key1", "value1");
map.put("key2", "value2");
map.put("key3", "value3");
System.out.println(map);
}
}
The code above generates the following result.
The toString implementation from HashMap is very readable and good for
debug.
Get the size of a Map
The size of a map is the count of key-value pairs.
import java.util.HashMap;
import java.util.Map;
//from j a va2 s. c om
public class Main{
public static void main(String[] a) {
Map<String,String> map = new HashMap<String,String>();
map.put("key1", "value1");
map.put("key2", "value2");
map.put("key3", "value3");
System.out.println(map.size());
}
}
The code above generates the following result.
Next chapter...
What you will learn in the next chapter: