CS 46A - Lecture 17

Pre-class reading

• Sections 7.1 - 7.2

Arrays

• Array: Sequence of values of the same type
• Construct array:

  new double[10]

• Store in variable of type double[]:

  double[] values = new double[10];

• When array is created, all values are initialized depending on array type:
  • Numbers: 0
  • boolean: false
  • Object References: null

Arrays

Arrays

values[2] = 29.95;

Arrays

• Using the value stored:

  System.out.println("The value of at index is " + values[2]);

• Get array length as values.length (Not a method!)
• Index values range from 0 to length - 1
• Accessing a nonexistent element results in a bounds error:

  double[] values = new double[10];
  values[10] = 29.95; // ERROR

• Limitation: Arrays have fixed length

Declaring Arrays

Syntax 7.1 Arrays

Lecture 17 Clicker Question 1

double[] values = new double[10];
for (int i = 0; i < values.length; i++) values[i] = (i + 1) * (i + 1);

1. 0, 1, 4, 9, 16, 25, 36, 49, 64, 81, but not 100
2. 1, 4, 9, 16, 25, 36, 49, 64, 81, 100
3. 4, 9, 16, 25, 36, 49, 64, 81, 100, 121
4. An exception has been thrown

Lecture 17 Clicker Question 2

What is true about the following code segments?

1. double[] a = new double[10];
   System.out.println(a[0]);

2. double[] b = new double[10];
   System.out.println(b[10]);

3. double[] c;
   System.out.println(c[0]);

1. #1 and #2 throw an exception, #3 doesn't compile
2. All three compile and throw an exception
3. #1 prints 0, #2 and #3 don't compile
4. #1 prints 0, #2 throws an exception, and #3 doesn't compile

Make Parallel Arrays into Arrays of Objects

// Don't do this
int[] accountNumbers;
double[] balances;

Make Parallel Arrays into Arrays of Objects

BankAccount[] accounts;

Array Lists

• ArrayList class manages a sequence of objects
• Can grow and shrink as needed
• ArrayList class supplies methods for many common tasks, such as inserting and removing elements
• ArrayList is a generic class:

  ArrayList<T>

  collects objects of type parameter T:

  ArrayList<String> names = new ArrayList<String>();
  names.add("Emily");
  names.add("Bob");
  names.add("Cindy");

• size method yields number of elements

Adding Elements

names.add("Emily"); 
names.add("Bob"); 
names.add("Cindy"); 

Retrieving Array List Elements

• To obtain the value an element at an index, use the get method
• Index starts at 0
• String name = names.get(2); // gets the third element of the array list
• Bounds error if index is out of range
• Most common bounds error:

  int i = names.size();
  name = names.get(i); // Error
  //legal index values are 0. . .i-1

Setting Elements

• To set an element to a new value, use the set method:

  names.set(2, “Carolyn”);

Removing Elements

names.remove(1);

Adding and Removing Elements

names.add("Emily"); 
names.add("Bob");
names.add("Cindy");
names.set(2, "Carolyn"); 
names.add(1, "Ann"); 
names.remove(1); 

Working with Array Lists

ArrayList<String> names =
   new ArrayList<String>();

names.add("Ann");
names.add("Cindy");

System.out.println(names);

names.add(1, "Bob");

names.remove(0);

names.set(0, "Bill");

String name = names.get(i);

String last =
   names.get(names.size() - 1);

ArrayList<Integer> squares =
   new ArrayList<Integer>();
for (int i = 0; i < 10; i++)
{
   squares.add(i * i);
}

Syntax 7.2 Array Lists

ch07/arraylist/ArrayListTester.java

ch07/arraylist/BankAccount.java

• Size: 3
  Expected: 3
  First account number: 1008
  Expected: 1008
  Last account number: 1729
  Expected: 1729

Lecture 17 Clicker Question 3

ArrayList<String> names = new ArrayList<String>();
names.add("A");
names.add(0, "B");
names.add("C");
names.remove(1);

1. [A, B, C]
2. [B, A, C]
3. [B, C]
4. Something else