Chapter Goals

• To become familiar with using array lists to collect objects 
• To learn about common array algorithms 
• To be able to use arrays 
• To understand when to choose array lists and arrays in your programs 
• To implement partially filled arrays 
• To learn how to use two-dimensional arrays
  
  

Array Lists

• Consider Purse class
• Purse doesn't remember individual Coin objects, just the total
• Don't know how many coins--can't have variables coin1...coin10
• Use ArrayList to store variable number of objects
  ArrayList coins = new ArrayList();
  coins.add(new Coin(0.1, "dime"));
  . . .
• size method yields number of elements
  

Retrieving Array List Elements

• Use get method
• Index starts at 0
• Must cast to correct type
• Coin c = coins.get(0); // gets first coin
• Bounds error if index is out of range
• Most common bounds error:
  int n = coins.size();
  c = (Coin)coins.get(n); // ERROR
  // legal index values are 0...n-1
  

Stepping Through all Elements

for (int i = 0; i < coins.size(); i++)
{
   Coin c = (Coin)coins.get(i);
   do something with c
}

Adding and Removing Elements

• set overwrites an existing value
  coins.set(4, aNickel);
  
• add adds a new value before the index
  add(i, c)
  
  
• remove removes an element at an index
       
  

File Purse.java

Linear Search Algorithm

public class Purse 
{ 
   public boolean find(Coin aCoin) 
   { 
      for (int i = 0; i < coins.size(); i++) 
      { 
         Coin c =(Coin)coins.get(i); 
         if (c.equals(aCoin)) return true; //found a match 
      } 
      return false; //no match in the entire array list 
   } 
   ... 
}

Counting

public class Purse 
{ 
   public int count(Coin aCoin) 
   { 
      int matches = 0; 
      for (int i = 0; i < coins.size(); i++) 
      {
         Coin c =(Coin)coins.get(i); 
         if (c.equals(aCoin)) matches++; //found a match 
      } 
      return matches; 
   } 
   ... 
}

Finding Maximum

public class Purse 
{ 
   public Coin getMaximum() 
   { 
      Coin max =(Coin)coins.get(0); 
      for (int i = 1; i <coins.size(); i++) // loop starts at 1
      { 
         Coin c =(Coin)coins.get(i); 
         if (c.getValue()>max.getValue()) max =c; 
      } 
      return max; 
   } 
   ... 
} 

File Purse.java

Storing Numbers in an Array List

• Array lists store objects
• Use wrapper classes to store numbers
• Double wrapper = new Double(29.95);
  double unwrapped = wrapper.doubleValue()
• ArrayList data = new ArrayList();
  data.add(wrapper);
  unwrapped = ((Double).data.get(i)).doubleValue(); 
• Also have  Integer and Boolean wrappers

Arrays

• Construct array:
  new double[10]
• Store in variable of type double[]
  double[] data = new double[10];
  
  
• Arrays have fixed length
• Arrays have element of specific type, not Object
• Use [] to access element:
  data[4] = 29.95;
• Get array length as data.length. (Not a method!)

Syntax 13.1: Array Construction

Syntax 13.2: Array Element Access

arrayReference[index]

Copying Arrays

• Copying an array reference yields a second reference to the same array
  double[] data = new double[10];
  // fill array . . .
  double[] prices = data;
  
  
• Use clone to make true copy
  double[] prices = (double[])data.clone();
  
  

Copying Array Elements

Adding and Removing Array Elements

• Add an element:
  System.arraycopy(data, i, data, i + 1, data.length - i - 1);
  data[i] = x;
  
• Remove an element:
  System.arraycopy(data, i + 1, data, i, data.length - i - 1);
  
  

Partially Filled Arrays

• Array length = maximum number of elements in array
• Usually, array is partially filled
• Need companion variable to keep track of current size
• Uniform naming convention:
  final int DATA_LENGTH = 100;
  double[] data = new double[DATA_LENGTH];
  int dataSize = 0;
• Update dataSize as array is filled:
  data[dataSize] = x;
  dataSize++;

Partially Filled Arrays

Partially Filled Arrays

• Remember to stop at dataSize when looking at array elements:
  for (int i = 0; i < dataSize; i++)
     sum = sum + data[i];
• Be careful not to overfill the array
  if (dataSize >= data.length)
     System.out.println("Sorry--array full");
  
• Or grow the array:
  double newData = new double[2 * data.length];
  System.arraycopy(data, 0, newData, 0, data.length);
  data = newData;
  

Growing an Array

File DataSet.java

File DataSetTest.java

Two-Dimensional Arrays

• Matrix with rows and columns
• Example: Tic Tac Toe board
  
  
• char[][] board = new char[3][3];
  board[i][j] = 'x';

File TicTacToe.java

File TicTacToeTest.java