SWEN20003
Object Oriented Software Development
Arrays and Strings
Bach Le
bach.le@unimelb.edu.au
University of Melbourne
© University of Melbourne 2023
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The Road So Far
Subject Introduction
Java Introduction
Classes and Objects
Software Tools
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Lecture Objectives
After this lecture you will be able to:
Understand how to use Arrays
Understand how to use Strings
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Arrays
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Motivation
Store a single integer value
int x;
Store two integer values
int x1, x2;
Store n integer values
int[] intArray;
Keyword
Array: A sequence of elements of the same type arranged in order in memory
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Array Declaration
basetype[] varName; \\ OR
basetype varName[];
Declares an array ([])
Each element is of type basetype
int[] intArray;
How many elements does this array have?
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Pitfall: Array Declaration
int[] intArray;
int x = intArray[0];
Program.java:13: error: variable intArray might not have been initialized
Arrays must be initialised, just like any other variable
Let’s look at how
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Array Initialization and Assignment
int[] intArray_1 = {0, 1, 2, 3, 4};
How many elements?
What are their values?
int[] intArray_2 = new int[100];
How many elements?
What are their values?
int[] intArray_1 = new int[n];
int[] intArray_2 = intArray_1;
How many elements?
What are their values?
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Assess Yourself
int[] intArray_1 = {10, 20, 30, 40};
int[] intArray_2 = intArray_1;
System.out.println(intArray_2[0]);
intArray_1[0] = 15;
System.out.println(intArray_2[0]);
Program Output: ?
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Assess Yourself
int[] intArray_1 = {10, 20, 30, 40};
int[] intArray_2 = intArray_1;
System.out.println(intArray_2[0]);
intArray_1[0] = 15;
System.out.println(intArray_2[0]);
Program Output:
10
15
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Pitfall: Array Assignment
Array is a data type, similar to data types you create by defining
Arrays are references!
Manipulating one reference affects all references
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Assess Yourself
Write a Java static method, computeDoublePowers, that accepts an integer n,
and returns an array of doubles of that size. Your method should then fill that
array with increasing powers of two (starting from 1.0).
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Assess Yourself
1 public class ComputeDoublePowers {
2
3 public static void main(String[] args) {
4 double x[];
5 x = computeDoublePowers(5);
6 System.out.println("The Element at index is: " + x[2]);
7 }
8
9 public static double[] computeDoublePowers(int n) {
10 double[] nums = new double[n];
11
12 for (int i = 0; i < n; i++) {
13 nums[i] = Math.pow(2, i);
14 }
15
16 // For sanity checking
17 for (int i = 0; i < n; i++) {
18 System.out.println(nums[i]);
19 }
20
21 return nums;
22 }
23
24 }
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Multi-Dimensional Arrays
Java permits “multi-dimensional” arrays
Technically exist as “array of arrays”
Declared just like 1D arrays
int[][] nums = new int[10][10]; // Square array
int[][] nums = new int[10][]; // Irregular array
Initialising 2D arrays slightly more complicated
for (int i = 0; i < nums.length; i++) {
nums[i] = new int[];
}
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Assess Yourself
Write a program that can generate the following 2D array:
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Assess Yourself
public class Main {
public static void main(String[] args) {
int HEIGHT = 5;
int MAX_WIDTH = HEIGHT;
int[][] triangleArray= new int[HEIGHT][];
for (int i = 0; i < HEIGHT; i++) {
triangleArray[i] = new int[HEIGHT - i];
for (int j = 0; j < HEIGHT - i; j++) {
triangleArray[i][j] = i + j + 1;
}
}
}
}
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Arrays of Objects
Arrays can be used to store objects.
Follow the steps below to create and store objects.
Declaration of the array:
Circle[] circleArray;
Allocation of Storage:
circleArray = new Circle[25];
▶ The above statement created an array that can store references to 25 Circle
objects.
▶ Circle objects are not created, you have to create them and store them in the
array - see next example.
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Arrays of Objects - Example
// CircleArray.java
class CircleArray{
public static void main (String[] args){
//declare an array for Circles
Circle[] circleArray = new Circle[3];
// create circle objects and store in array
for ( int i = 0; i < circleArray.length; i++) {
circleArray[i] = new Circle(i,i, i + 2);
}
for ( int i = 0; i < circleArray.length; i++) {
System.out.println("Circle " + i + " Radius = " +
circleArray[i].getR());
}
}
}
Program Output:
Circle 0 Radius = 2.0
Circle 1 Radius = 3.0
Circle 2 Radius = 4.0
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Array Methods
Indexing
int[] intArray = new int[10];
int x = intArray[0];
int x = intArray[10];%
// Gives out of bounds error
int x = intArray[-1];%// Gives out of bounds error
Length
int len = intArray.length
Equality
import java.util.Arrays;
int[] n1 = {1, 2, 3};
int[] n2 = {1, 2, 3};
Arrays.equals(n1, n2);
//true if the element values are the same, false otherwise
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Array Methods
Indexing
int[] intArray = new int[10];
int x = intArray[0];
int x = intArray[10];%// Gives out of bounds error
int x = intArray[-1];%
// Gives out of bounds error
Length
int len = intArray.length
Equality
import java.util.Arrays;
int[] n1 = {1, 2, 3};
int[] n2 = {1, 2, 3};
Arrays.equals(n1, n2);
//true if the element values are the same, false otherwise
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Array Methods
Indexing
int[] intArray = new int[10];
int x = intArray[0];
int x = intArray[10];%// Gives out of bounds error
int x = intArray[-1];%// Gives out of bounds error
Length
int len = intArray.length
Equality
import java.util.Arrays;
int[] n1 = {1, 2, 3};
int[] n2 = {1, 2, 3};
Arrays.equals(n1, n2);
//true if the element values are the same, false otherwise
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Array Methods
Resizing - arrays are fixed length; resizing requires creating a new array.
int[] intArray = new int[5];
intArray = new int[intArray.length + 3];
Sorting (“ascending”)
Arrays.sort(n1);
Printing
System.out.println(Arrays.toString(n1));
Output:
[1, 2, 3]
Full Array documentation here
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For Each Loop
for ( varName : ) {

}
A more convenient method of iteration
No indexing required
Useful when operating with/on the data, and not the array
for (Circle c : circleArray) {
System.out.println(c.getRadius());
}
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Strings
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Strings
You have already seen Strings in use:
public static void main(String[] args) {... }
final String STRING_CONSTANT = "Welcome to Java";
public String toString() {....}
System.out.println("arg[" + i + "]: " + args[i]);
But what is a String?
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Assess Yourself
A “String” is a(n) what?
1 Object
2 Class
3 Variable
4 Data Type
5 Method
6 Privacy Modifier
7 I have literally no clue
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Assess Yourself
A “String” is a(n) what?
1 Object (not technically correct)
2 Class
3 Variable
4 Data Type
5 Method
6 Privacy Modifier
7 I have literally no clue
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Strings
Strings store sequences of characters
String is a Java class
Used to represent messages, errors, and “character” related attributes like
name
Incredibly powerful for input and output
Keyword
String: A Java class made up of a sequence of characters.
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Strings
Some examples of String variables
String s1 = "This is a String";
String s2 = "This is " + "also a String";
String s3 = "10";
String s4 = "s3 is still a string, even though it's a number";
Java Strings are almost identical to Python, except you can’t use single quotes.
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Assess Yourself
What does this code output?
System.out.println("Game of Thrones season 8 was "good".");
1 “Game of Thrones season 8 was “good”.”
2 Game of Thrones season 8 was “good”.
3 Game of Thrones season 8 was good.
4 Error
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Assess Yourself
What does this code output?
System.out.println("Game of Thrones season 8 was "good".");
1 “Game of Thrones season 8 was “good”.”
2 Game of Thrones season 8 was “good”.
3 Game of Thrones season 8 was good.
4 Error
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Special Characters
Some characters (like ”) are “reserved”
Mean something special to Java
Need to “escape” them with “\” to use alternate meaning
Examples “\n” (newline), “\t” (tab) “\”” (quotation)
System.out.println("Game of Thrones season 8 was \"good\".");
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Special Characters
Some characters (like ”) are “reserved”
Mean something special to Java
Need to “escape” them with “\” to use alternate meaning
Examples “\n” (newline), “\t” (tab) “\”” (quotation)
System.out.println("Game of Thrones season 8 was \"good\".");
Keyword
Escaping: To include “special” characters in a string, use “\” to escape from that
character’s normal meaning.
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String Operations
You can use + (and +=) to append/concatenate two strings
▶ System.out.println("Hello " + "World");
▶ Prints "Hello World"
+ is clever: if either operand is a string, it will turn the other into a string
▶ System.out.println("a = " + a + ", b = " + b);
▶ If a = 1 and b = 2, this prints: "a = 1, b = 2"
Why is this useful?
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Assess Yourself
What does this print?
System.out.println("1 + 1 = " + 1 + 1);
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Assess Yourself
System.out.println("1 + 1 = " + 1 + 1);
Actually prints "1 + 1 = 11"
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Assess Yourself
System.out.println("1 + 1 = " + 1 + 1);
Actually prints "1 + 1 = 11"
System.out.println("1 + 1 = " + (1 + 1));
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Assess Yourself
System.out.println("1 + 1 = " + 1 + 1);
Actually prints "1 + 1 = 11"
System.out.println("1 + 1 = " + (1 + 1));
Prints "1 + 1 = 2"
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Assess Yourself
Name some “logical” things you might do with a String
Think about how you would do them in C and Python
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String Methods
Length
▶ C: Need a helper/buddy variable
▶ Python: len("Hello")
▶ Java: "Hello".length()
Upper/Lower case
▶ C: toupper(*s)
▶ Python: s.upper()
▶ Java: s.toUpperCase()
Split
▶ C: Stop
▶ Python: s.split()
▶ Java: s.split(" ")
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String Methods
Check substring presence
▶ C: Why
▶ Python: "Hell" in s
▶ Java: s.contains("Hell")
Find substring location
▶ C: Never mind
▶ Python: s.find("Hell")
▶ Java: s.indexOf("Hell")
Substring
▶ C: I’m out
▶ Python: s[2:7]
▶ Java: s.substring(2, 7)
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String Methods
The full String class documentation can be found here.
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Assess Yourself
What does this output?
String s = "Hello World";
s.toUpperCase();
s.replace("e", "i");
s.substring(0, 2);
s += " FIVE";
System.out.println(s);
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Assess Yourself
What does this output?
String s = "Hello World";
s.toUpperCase();
s.replace("e", "i");
s.substring(0, 2);
s += " FIVE";
System.out.println(s);
"Hello World FIVE"
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Immutability
Strings are immutable; once created, they can’t be modified, only replaced
This means that every String operation returns a new String
Let’s fix up that code
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Assess Yourself
What does this output?
String s = "Hello World";
s = s.toUpperCase();
s = s.replace("e", "i");
s = s.substring(0, 2);
s += " FIVE";
System.out.println(s);
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Assess Yourself
What does this output?
String s = "Hello World";
s = s.toUpperCase();
s = s.replace("e", "i");
s = s.substring(0, 2);
s += " FIVE";
System.out.println(s);
"HE FIVE"
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Assess Yourself
What does this output?
System.out.println("Hello" == "Hello");
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Assess Yourself
What does this output?
System.out.println("Hello" == "Hello");
true
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Assess Yourself
What does this output?
String s = "Hello";
System.out.println(s == "Hello");
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Assess Yourself
What does this output?
String s = "Hello";
System.out.println(s == "Hello");
true
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Assess Yourself
What does this output?
String s = "Hello";
String s2 = "Hello";
System.out.println(s == s2);
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Assess Yourself
What does this output?
String s = "Hello";
String s2 = "Hello";
System.out.println(s == s2);
true
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Assess Yourself
What does this output?
String s = "Hello";
String s2 = new String("Hello");
System.out.println(s == s2);
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Assess Yourself
What does this output?
String s = "Hello";
String s2 = new String("Hello");
System.out.println(s == s2);
false
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Assess Yourself
What does this output?
String s = "Hello";
String s2 = new String("Hello");
System.out.println(s == s2);
false
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Equality
In the previous example s and s2 are references to objects.
To check equality between two objects we must use the equals method.
▶ Remember the equals method from our previous topic - a standard method
every class should have
String s = "Hello";
String s2 = new String("Hello");
System.out.println(s.equals(s2));
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Equality
In the previous example s and s2 are references to objects.
To check equality between two objects we must use the equals method.
▶ Remember the equals method from our previous topic - a standard method
every class should have
String s = "Hello";
String s2 = new String("Hello");
System.out.println(s.equals(s2));
true
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Equality
In the previous example s and s2 are references to objects.
To check equality between two objects we must use the equals method.
▶ Remember the equals method from our previous topic - a standard method
every class should have
String s = "Hello";
String s2 = new String("Hello");
System.out.println(s.equals(s2));
Keyword
.equals: A method used to check two objects for equality
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Lecture Objectives
Upon completion of this topic you will be able to:
Use Arrays
Use Strings
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