Java map examples

John D K -

In this post about java map / dictionary:

  • Define a java map
  • Adding new element to a map
  • Remove element from a Java map
  • Get element by key, get all keys
  • Iterating elements of Java map
  • Performance test

Video explaining java maps : java map examples

Define a java map

In java you can define new map with different types of keys and values(Primitive types like int can't be used in java map:

import java.util.HashMap;
import java.util.Map;

Map<String, String> stringMap = new HashMap<>();
Map<Integer, String> mixedMap = new HashMap<>();
Map<Integer, Integer> intMap = new HashMap<>();

you need to import the classes from java.util one by one or to import java.util.* for all.

Adding new element to a map

If you want to add new elements to a map you can use method put which takes two arguments:

  • key
  • value

You have to be careful with the key and value types:

Map<String, String> stringMap = new HashMap<>();
Map<Integer, String> mixedMap = new HashMap<>();
Map<Integer, Integer> intMap = new HashMap<>();

stringMap.put("1", "First element");
mixedMap.put(1, "First element");
intMap.put(1, 1);

you need to import the classes from java.util one by one or to import java.util.* for all.

Note: if you put wrong type you will get error like:

Error:(13, 23) java: incompatible types: int cannot be converted to java.lang.String

or

Error:(15, 23) java: incompatible types: java.lang.String cannot be converted to java.lang.Integer

Note: Adding new element with the same key will override the first element!

mixedMap.put(1, "First element");
mixedMap.put(1, "First element2");
mixedMap.forEach((key, value) -> System.out.printf("%d %s%n", key, value));

result:

1 First element2

Remove element from a Java map

Removing element can be done by providing a key to method remove:

Map<String, String> stringMap = new HashMap<>();
Map<Integer, String> mixedMap = new HashMap<>();
Map<Integer, Integer> intMap = new HashMap<>();

stringMap.remove("1");
mixedMap.remove(1);
intMap.remove(1);

Note: In case of missing element nothing will happen!

Get element by key, get all keys

You can use two methods:

  • mixedMap.keySet() - to get a list of all keys
  • mixedMap.get(key) - to get an element by key
for (Integer key : mixedMap.keySet()) {
    System.out.printf("%d %s%n", key, mixedMap.get(key));
}

Iterating elements of Java map

Since java * you can use for_each to iterate over map elements:

mixedMap.put(1, "First element");
mixedMap.put(2, "First element2");
mixedMap.put(3, "First element3");
mixedMap.forEach((key, value) -> System.out.printf("%d %s%n", key, value));

result:

1 First element
2 First element2
3 First element3

prior java 8 or without for_each example:

mixedMap.put(1, "First element");
mixedMap.put(2, "First element2");
mixedMap.put(3, "First element3");
for (Integer key : mixedMap.keySet()) {
    System.out.printf("%d %s%n", key, mixedMap.get(key));
}

result:

1 First element
2 First element2
3 First element3

Performance test

Several test are done in java 8 with creating a map and adding 1 million elements. Removing an random element or getting value of such:

  • Integer keys - less than a second
  • String keys - about 1.735100 seconds

So for most operation java maps are fast and efficient data structure when you need to have your value in a dictionary pairs - key and value.
Even writing all elements to console takes about 10 seconds.