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This chapter is from the book

Generic Array Lists

In many programming languages—in particular, in C—you have to fix the sizes of all arrays at compile time. Programmers hate this because it forces them into uncomfortable trade-offs. How many employees will be in a department? Surely no more than 100. What if there is a humongous department with 150 employees? Do we want to waste 90 entries for every department with just 10 employees?

In Java, the situation is much better. You can set the size of an array at runtime.

int actualSize = . . .;
Employee[] staff = new Employee[actualSize];

Of course, this code does not completely solve the problem of dynamically modifying arrays at runtime. Once you set the array size, you cannot change it easily. Instead, the easiest way in Java to deal with this common situation is to use another Java class, called ArrayList. The ArrayList class is similar to an array, but it automatically adjusts its capacity as you add and remove elements, without your needing to write any code.

As of Java SE 5.0, ArrayList is a generic class with a type parameter. To specify the type of the element objects that the array list holds, you append a class name enclosed in angle brackets, such as ArrayList<Employee>. You will see in Chapter 13 how to define your own generic class, but you don't need to know any of those technicalities to use the ArrayList type.

Here we declare and construct an array list that holds Employee objects:

ArrayList<Employee> staff = new ArrayList<Employee>();

You use the add method to add new elements to an array list. For example, here is how you populate an array list with employee objects:

staff.add(new Employee("Harry Hacker", . . .));
staff.add(new Employee("Tony Tester", . . .));

The array list manages an internal array of object references. Eventually, that array will run out of space. This is where array lists work their magic: If you call add and the internal array is full, the array list automatically creates a bigger array and copies all the objects from the smaller to the bigger array.

If you already know, or have a good guess, how many elements you want to store, then call the ensureCapacity method before filling the array list:

staff.ensureCapacity(100);

That call allocates an internal array of 100 objects. Then, the first 100 calls to add do not involve any costly reallocation.

You can also pass an initial capacity to the ArrayList constructor:

ArrayList<Employee> staff = new ArrayList<Employee>(100);

The size method returns the actual number of elements in the array list. For example,

staff.size()

returns the current number of elements in the staff array list. This is the equivalent of

a.length

for an array a.

Once you are reasonably sure that the array list is at its permanent size, you can call the trimToSize method. This method adjusts the size of the memory block to use exactly as much storage space as is required to hold the current number of elements. The garbage collector will reclaim any excess memory.

Once you trim the size of an array list, adding new elements will move the block again, which takes time. You should only use trimToSize when you are sure you won't add any more elements to the array list.

Accessing Array List Elements

Unfortunately, nothing comes for free. The automatic growth convenience that array lists give requires a more complicated syntax for accessing the elements. The reason is that the ArrayList class is not a part of the Java programming language; it is just a utility class programmed by someone and supplied in the standard library.

Instead of using the pleasant [] syntax to access or change the element of an array, you use the get and set methods.

For example, to set the ith element, you use

staff.set(i, harry);

This is equivalent to

a[i] = harry;

for an array a. (As with arrays, the index values are zero-based.)

To get an array list element, use

Employee e = staff.get(i);

This is equivalent to

Employee e = a[i];

You can sometimes get the best of both worlds—flexible growth and convenient element access—with the following trick. First, make an array list and add all the elements:

ArrayList<X> list = new ArrayList<X>();
while (. . .)
{
   x = . . .;
   list.add(x);
}

When you are done, use the toArray method to copy the elements into an array:

X[] a = new X[list.size()];
list.toArray(a);

Sometimes, you need to add elements in the middle of an array list. Use the add method with an index parameter:

int n = staff.size() / 2;
staff.add(n, e);

The elements at locations n and above are shifted up to make room for the new entry. If the new size of the array list after the insertion exceeds the capacity, then the array list reallocates its storage array.

Similarly, you can remove an element from the middle of an array list:

Employee e = staff.remove(n);

The elements located above it are copied down, and the size of the array is reduced by one.

Inserting and removing elements is not terribly efficient. It is probably not worth worrying about for small array lists. But if you store many elements and frequently insert and remove in the middle of a collection, consider using a linked list instead. We explain how to program with linked lists in Chapter 13.

As of Java SE 5.0, you can use the "for each" loop to traverse the contents of an array list:

for (Employee e : staff)
   do something with e

This loop has the same effect as

for (int i = 0; i < staff.size(); i++)
{
   Employee e = staff.get(i);
   do something with e
}

Listing 5-4 is a modification of the EmployeeTest program of Chapter 4. The Employee[] array is replaced by an ArrayList<Employee>. Note the following changes:

  • You don't have to specify the array size.
  • You use add to add as many elements as you like.
  • You use size() instead of length to count the number of elements.
  • You use a.get(i) instead of a[i] to access an element.

Listing 5-4. ArrayListTest.java

 1. import java.util.*;
 2.
 3. /**
 4.  * This program demonstrates the ArrayList class.
 5.  * @version 1.1 2004-02-21
 6.  * @author Cay Horstmann
 7.  */
 8. public class ArrayListTest
 9. {
10.    public static void main(String[] args)
11.    {
12.       // fill the staff array list with three Employee objects
13.       ArrayList<Employee> staff = new ArrayList<Employee>();
14.
15.       staff.add(new Employee("Carl Cracker", 75000, 1987, 12, 15));
16.       staff.add(new Employee("Harry Hacker", 50000, 1989, 10, 1));
17.       staff.add(new Employee("Tony Tester", 40000, 1990, 3, 15));
18.
19.       // raise everyone's salary by 5%
20.       for (Employee e : staff)
21.          e.raiseSalary(5);
22.
23.       // print out information about all Employee objects
24.       for (Employee e : staff)
25.          System.out.println("name=" + e.getName() + ",salary=" + e.getSalary() + ",hireDay="
26.                + e.getHireDay());
27.    }
28. }
29.
30. class Employee
31. {
32.    public Employee(String n, double s, int year, int month, int day)
33.    {
34.       name = n;
35.       salary = s;
36.       GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
37.       hireDay = calendar.getTime();
38.    }
39.
40.    public String getName()
41.    {
42.       return name;
43.    }
44.
45.    public double getSalary()
46.    {
47.       return salary;
48.    }
49.
50.    public Date getHireDay()
51.    {
52.       return hireDay;
53.    }
54.
55.    public void raiseSalary(double byPercent)
56.    {
57.       double raise = salary * byPercent / 100;
58.       salary += raise;
59.    }
60.
61.    private String name;
62.    private double salary;
63.    private Date hireDay;
64. }

Compatibility between Typed and Raw Array Lists

When you write new code with Java SE 5.0 and beyond, you should use type parameters, such as ArrayList<Employee>, for array lists. However, you may need to interoperate with existing code that uses the raw ArrayList type.

Suppose that you have the following legacy class:

public class EmployeeDB
{
   public void update(ArrayList list) { ... }
   public ArrayList find(String query) { ... }
}

You can pass a typed array list to the update method without any casts.

ArrayList<Employee> staff = ...;
employeeDB.update(staff);

The staff object is simply passed to the update method.

Conversely, when you assign a raw ArrayList to a typed one, you get a warning.

ArrayList<Employee> result = employeeDB.find(query); // yields warning

Using a cast does not make the warning go away.

ArrayList<Employee> result = (ArrayList<Employee>)
   employeeDB.find(query); // yields another warning

Instead, you get a different warning, telling you that the cast is misleading.

This is the consequence of a somewhat unfortunate limitation of generic types in Java. For compatibility, the compiler translates all typed array lists into raw ArrayList objects after checking that the type rules were not violated. In a running program, all array lists are the same—there are no type parameters in the virtual machine. Thus, the casts (ArrayList) and (ArrayList<Employee>) carry out identical runtime checks.

There isn't much you can do about that situation. When you interact with legacy code, study the compiler warnings and satisfy yourself that the warnings are not serious.

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