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Java sets

A set stores an unordered collection of unique values.

There is three implementations:

• HashSet
• TreeSet
• LinkedHashSet

Comparison

	HashSet	TreeSet	LinkedHashSet
Elements order	random	numerical or alpahbetical	order added
can store null value	Yes	No	Yes
add, remove, contains complexity	O(1)	O(log n)	O(1)

Creating a Set

Specify the data type of the elements and the type of set. It can only contain reference type values.

// import the desired set type
import Java.util.Set;
import Java.util.HashSet;
import Java.util.TreeSet;
import Java.util.LinkedHashSet;

// Create a string HashSet
Set<String> colors = new HashSet<String>();

// Create an integer Tree set
TreeSet<Integer> myNumbers = new TreeSet<Integer>();

Methods

Adding

colors.add("red");
colors.add("red"); // duplicate is ignored

Removing

colors.remove("red");

Checking a value

colors.contains("red"); // true or false

Size

colors.size()

Iteration

for (String item : colors) {
  System.out.println(item);
}

Set operations

The set classe provide methods to perform mathematical operations on two different sets. Allowing to create or modify existing ones.

Union

Use .addAll() to add all element of one set to another.

Set<String> colors = new HashSet<String>();
Set<String> favColors = new HashSet<String>();

// add colors to them

// add the favColors to the colors set
colors.addAll(favColors);

Intersection

Use .retainAll() to keep only in a set, the value that are common to another one.

// only keep in colors, the ones that are also in favColors
colors.retainAll(favColors);

Complement

Use .removeAll() to remove from one set, the value that are also in another one.

// remove from colors, the one present in favColors
colors.removeAll(favColors);