# 11 - Data Structures. Sets

# Goals

• Learn what sets are
• Learn when to use sets
• Compare Sets vs Lists
• Java Set Implementations(HashSet & TreeSet)
• Common used sets methods (Set theory)

# Slides

# Useful links

Sets

• https://docs.oracle.com/javase/9/docs/api/java/util/Set.html
• http://tutorials.jenkov.com/java-collections/set.html
• https://www.geeksforgeeks.org/set-in-java/

# Exercise 1

In main method

• Create a new HashSet of strings, add banana, apple, orange.
• Print content of sets using for loop
• Clear the set.
• Add one more element grapes.
• Check if set is empty, and if it’s not empty print “I am not empty!”. What is the order of elements after adding them to HashSet?

# Exercise 2

Write a static method randomSet that will return a HashSet containing a collection of n unique random numbers in range 0-100. In the main method iterate over the returned Set and print each number multiplied by 2. Where n is a number of random elements in set.

# Exercise 3

Change data type in previous exercise from HashSet to TreeSet. What is the difference?

# Exercise 4

Write a static method intersection that will return an intersection of two sets given by parameters. Note - sets given by parameters may not be modified.

# Exercise 5

Write a static method union that will return an union of two sets given by parameters. Note - sets given by parameters may not be modified.

# Exercise 6

Write a static method difference that will return a difference between two sets given by parameters. Note - sets given by parameters may not be modified.

# Exercise 7

Create a TreeSet containing integers that will maintain descending order of numbers. Hint: Look at alternative TreeSet constructors.

# Homework

# Exercise 1

Write a static method symetricDifference that will return the symetric difference between t wo sets given by parameters. Note - sets given by parameters may not be modified.

# Exercise 2

Write a method that takes an array with integers and return an array of unique values of the array

# Exercise 3

Write a method that takes a String and returns all unique characters of this String

# Exercise 4 (Optional)

Write a method that takes a String and returns all duplicate characters of this String