Java Collections Framework - Building Java Programs: A Back to Basics Approach - page 695

B

A

A B
Union

A B
Intersect

A - B
Difference

Figure 11.3

Set operation Venn diagrams

However, a more elegant way to perform this calculation is to compute a union
between the sets. The union of two sets A and B is the set of all elements contained in
either A, B, or both. Union is an example of a set operation . Other examples of set
operations are intersection (the set of all elements that are in both A and B) and
difference (the set of all elements that are in A but not in B). Many set operations
combine two sets to produce a new set as their result.
Set operations are often depicted by drawings called Venn diagrams, which depict
sets as circles and set operations as shaded overlapping between the circles. Figure
11.3 shows some examples.
You can write code to perform set operations by calling methods whose names end
with “All” using the relevant pair of sets. Table 11.4 summarizes which methods cor-
respond to which set operations.

Table 11.4 Common Set Operations, Given Two Sets A and B

Set operation Method Description

union Set of all elements that are in A, B, or both addAll

intersection Set of all elements that are in both A and B

retainAll

difference Set of all elements that are in A but not in B removeAll

superset, subset Returns true if A is a superset of (contains all

elements of ) B

containsAll