Java Foundations: Lists, ArrayList<T>

Java Foundations
Lists, List<T>,
ArrayList<T>

Your Course
Instructors
Svetlin Nakov
George Georgiev

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Lists in Java
Processing Variable Length
Sequences of Elements

Table of Contents
1. Lists: Overview
2. List Manipulating: Add, Delete, Insert, Clear
3. Reading Lists from the Console. Printing Lists
4. Sorting Lists and Arrays
7

List<E> – Overview
 List<E> holds a list of elements of any type
9
List<String> names = new ArrayList<>();
// Create a list of strings
names.add("Peter");
names.add("Maria");
names.add("George");
names.remove("Maria");
for (String name : names)
System.out.println(name);
// Peter, George

List<E> – Overview (2)
10
List<Integer> nums = new ArrayList<>(
Arrays.asList(10, 20, 30, 40, 50, 60));
nums.remove(2);
nums.remove(Integer.valueOf(40));
nums.add(100);
nums.add(0, -100);
for (int i = 0; i < nums.size(); i++)
System.out.print(nums.get(i) + " ");
-100 10 20 50 60 100
Inserts an element to index
Items count
Remove by index
Remove by value (slow)

 List<E> holds a list of elements (like array, but extendable)
 Provides operations to add / insert / remove / find elements:
 size() – number of elements in the List<E>
 add(element) – adds an element to the List<E>
 add(index, element) – inserts an element to given position
 remove(element) – removes an element (returns true / false)
 remove(index) – removes element at index
 contains(element) – determines whether an element is in the list
 set(index, item) – replaces the element at the given index
List<E> – Data Structure
11

add – Appends an Element
12
10
5
2
0
Count: 1
2
3
List<Integer>
10
5
2

10
remove – Deletes an Element
13
2
5
Count:
List<Integer>
10
2
3
10

add (index, el) – Inserts an Element at Position
14
3
2
2
5
Count:
List<Integer>
-5
-5

Reading Lists from the Console
Using for Loop or String.split()
15

 First, read from the console the array length:
 Next, create a list of given size n and read its elements:
Reading Lists From the Console
Scanner sc = new Scanner(System.in);
int n = Integer.parseInt(sc.nextLine());
List<Integer> list = new ArrayList<>();
for (int i = 0; i < n; i++) {
int number = Integer.parseInt(sc.nextLine());
list.add(number);
}

 Lists can be read from a single line of space separated values:
Reading List Values from a Single Line
17
2 8 30 25 40 72 -2 44 56
String values = sc.nextLine();
List<String> items = Arrays.stream(values.split(" "))
.collect(Collectors.toList());
List<Integer> nums = new ArrayList<>();
for (int i = 0; i < items.size(); i++)
nums.add(Integer.parseInt(items.get(i)));
Convert a collection
into List
List<Integer> items = Arrays.stream(values.split(" "))
.map(Integer::parseInt).collect(Collectors.toList());

 Printing a list using a for-loop:
 Printing a list using a String.join():
Printing Lists on the Console
18
List<String> list = new ArrayList<>(Arrays.asList(
"one", "two", "three", "four", "five", "six"));
for (int index = 0; index < list.size(); index++)
System.out.printf
("arr[%d] = %s%n", index, list.get(index));
List<String> list = new ArrayList<>(Arrays.asList(
"one", "two", "three", "four", "five", "six"));
System.out.println(String.join("; ", list));
Gets an element
at given index

 Write a program to sum all adjacent equal numbers in a list of
decimal numbers, starting from left to right
 Examples:
Problem: Sum Adjacent Equal Numbers
19
3 3 6 1 12 1
8 2 2 4 8 16 16 8 16
5 4 2 1 1 4 5 8 4

List<Double> numbers = Arrays.stream(sc.nextLine().split(" "))
.map(Double::parseDouble).collect(Collectors.toList());
for (int i = 0; i < numbers.size() - 1; i++)
if (numbers.get(i).equals(numbers.get(i + 1))) {
numbers.set(i, numbers.get(i) + numbers.get(i + 1));
numbers.remove(i + 1);
i = -1;
}
// Continues on the next slide
Solution: Sum Adjacent Equal Numbers (1)
0

Solution: Sum Adjacent Equal Numbers (2)
21
static String joinElementsByDelimiter
(List<Double> items, String delimiter) {
String output = "";
for (Double item : items)
output += (new DecimalFormat("0.#").format(item) + delimiter);
return output;
}
String output = joinElementsByDelimiter(numbers, " ");
System.out.println(output);

 Write a program that sums all numbers in a list in the
following order:
 first + last, first + 1 + last - 1, first + 2 + last - 2, … first + n, last – n
 Examples:
Problem: Gauss' Trick
22
1 2 3 4 5 6 6 3
1 2 3 4 5 5

Solution: Gauss' Trick
3
List<Integer> numbers = Arrays.stream(sc.nextLine().split(" "))
int size = numbers.size();
for (int i = 0; i < size / 2; i++) {
numbers.set(i, numbers.get(i) + numbers.get(numbers.size() - 1));
numbers.remove(numbers.size() - 1);
}
System.out.println(numbers.toString().replaceAll("[[],]", ""));

 You receive two lists with numbers. Print a result list which
contains the numbers from both lists
 If the length of the two lists is not equal, just add the
remaining elements at the end of the list
 list1[0], list2[0], list1[1], list2[1], …
Problem: Merging Lists
24
1 2 3 4 5
6 7 8
1 6 2 7 3 8 4 5

// TODO: Read the input
List<Integer> resultNums = new ArrayList<>();
for (int i = 0; i < Math.min(nums1.size(), nums2.size()); i++) {
// TODO: Add numbers in resultNums
}
if (nums1.size() > nums2.size())
resNums.addAll(getRemainingElements(nums1, nums2));
else if (nums2.size() > nums1.size())
resNums.addAll(getRemainingElements(nums2, nums1));
System.out.println(resNums.toString().replaceAll("[[],]", ""));
Solution: Merging Lists (1)
25

public static List<Integer> getRemainingElements
(List<Integer> longerList, List<Integer> shorterList) {
List<Integer> nums = new ArrayList<>();
for (int i = shorterList.size(); i < longerList.size(); i++)
nums.add(longerList.get(i));
return nums;
}
Solution: Merging Lists (2)
26

Live Exercises
Reading and Manipulating Lists

 Sorting a list == reorder its elements incrementally: Sort()
 List items should be comparable, e.g. numbers, strings, dates, …
Sorting Lists
List<String> names = new ArrayList<>(Arrays.asList(
"Peter", "Michael", "George", "Victor", "John"));
Collections.sort(names);
System.out.println(String.join(", ", names));
// George, John, Michael, Peter, Victor
Collections.sort(names);
Collections.reverse(names);
System.out.println(String.join(", ", names));
// Victor, Peter, Michael, John, George
Sort in natural
(ascending) order
Reverse the sorted result

 Read a number n and n lines of products. Print a numbered list
of all the products ordered by name
 Examples:
Problem: List of Products
30
4
Potatoes
Tomatoes
Onions
Apples
1.Apples
2.Onions
3.Potatoes
4.Tomatoes
A
Z

int n = Integer.parseInt(sc.nextLine());
List<String> products = new ArrayList<>();
for (int i = 0; i < n; i++) {
String currentProduct = sc.nextLine();
products.add(currentProduct);
}
Collections.sort(products);
for (int i = 0; i < products.size(); i++)
System.out.printf("%d.%s%n", i + 1, products.get(i));
Solution: List of Products

 Read a list of integers, remove all negative numbers from it
 Print the remaining elements in reversed order
 In case of no elements left in the list, print "empty"
Problem: Remove Negatives and Reverse
32
10 -5 7 9 -33 50 50 9 7 10
7 -2 -10 1 1 7
-1 -2 -3 empty

List<Integer> nums = Arrays.stream(sc.nextLine().split(" "))
for (int i = 0; i < nums.size(); i++)
if (nums.get(i) < 0)
nums.remove(i--);
Collections.reverse(nums);
if (nums.size() == 0)
System.out.println("empty");
else
System.out.println(nums.toString().replaceAll("[[],]", ""));
Solution: Remove Negatives and Reverse

 …
 …
 …
Summary
35
 Lists hold a sequence of elements
(variable-length)
 Can add / remove / insert elements
at runtime
 Creating (allocating) a list:
new ArrayList<E>()
 Accessing list elements by index
 Printing list elements: String.join(…)

 …
 …
 …
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Java Foundations: Lists, ArrayList<T>

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