CS 46A - Lecture 19

Pre-class reading

• Sections 7.3 - 7.4

Wrapper Classes

• For each primitive type there is a wrapper class for storing values of that type:

  Double d = new Double(29.95);

  
• Wrapper objects can be used anywhere that objects are required instead of primitive type values:

  ArrayList<Double> values = new ArrayList<Double>();

Wrapper Classes

Auto-boxing

• Auto-boxing: Automatic conversion between primitive types and the corresponding wrapper classes:

  Double d = 29.95; // auto-boxing; same as Double d = new Double(29.95);
  double x = d; // auto-unboxing; same as double x = d.doubleValue();

• Auto-boxing even works inside arithmetic expressions:

  d = d + 1;

  Means:
  • auto-unbox d into a double
  • add 1
  • auto-box the result into a new Double
  • store a reference to the newly created wrapper object in d

Auto-boxing and Array Lists

• To collect numbers in an array list, use the wrapper type as the type parameter, and then rely on auto-boxing:

  ArrayList<Double> values = new ArrayList<Double>(); 
  values.add(29.95); 
  double x = values.get(0);

• Storing wrapped numbers is quite inefficient
  • Acceptable if you only collect a few numbers
  • Use arrays for long sequences of numbers or characters

Lecture 18 Clicker Question 1

1. values.set(0, values.get(0) + 1);
2. values.get(0)++;
3. values.set(0, values.get(0)++);
4. values[0]++;

The Enhanced for Loop

• Traverses all elements of a collection:

  double[] values = . . .;
  double sum = 0;
  for (double element : values)
  {
     sum = sum + element;
  }

• Read the loop as for each element in values
• Traditional alternative:

  double[] values = . . .;
  double sum = 0;
  for (int i = 0; i < values.length; i++)
  {
     double element = values[i];
     sum = sum + element;
  }

The Enhanced for Loop

• Works for ArrayLists too:

  ArrayList<BankAccount> accounts = . . . ;
  double sum = 0;
  for (BankAccount account : accounts)
  {
     sum = sum + account.getBalance();
  }

• Equivalent to the following ordinary for loop:

  double sum = 0;
  for (int i = 0; i < accounts.size(); i++)
  {
     BankAccount account = accounts.get(i);
     sum = sum + account.getBalance();
  }

The Enhanced for Loop

• The for each loop does not allow you to modify the contents of an array:

  for (double element : values) 
  { 
     element = 0; // ERROR—this assignment does not modify array elements 
  }

• Must use an ordinary for loop:

  for (int i = 0; i < values.length; i++) 
  { 
     values[i] = 0; // OK 
  }

Syntax 7.3 The for each Loop

Lecture 19 Clicker Question 2

for (double element : values) 
   System.out.print(element);
   System.out.print(",");

1. It prints all elements, separated by commas
2. It prints all elements, separated by commas, and an additional comma
3. It prints all elements, followed by a comma
4. Something else

Lecture 19 Clicker Question 3

What is true about this for each loop?

int counter = 0; 
for (BankAccount a : accounts) 
{ 
   if (a.getBalance() >= 0) { counter++; } 
}

1. It counts how many accounts are overdrawn
2. If accounts has length 0, counter is zero
3. accounts must be an array list
4. It gives the wrong answer if all balances are negative