9.5 Array Lists

The classic problem with the Array type is its fixed size. If you do not know in advance how many objects an array will hold, you run the risk of declaring either too small an array (and running out of room) or too large an array (and wasting memory).

Your program might be asking the user for input, or gathering input from a web site. As it finds objects (strings, books, values, etc.), you will add them to the array, but you have no idea how many objects you'll collect in any given session. The classic fixed-size array is not a good choice, as you can't predict how large an array you'll need.

The ArrayList class is an array whose size is dynamically increased as required. ArrayLists provide a number of useful methods and properties for their manipulation. Some of the most important are shown in Table 9-3.

Table 9-3. ArrayList methods and properties

Method or property	Purpose
Adapter( )	Public static method that creates an ArrayList wrapper for an IList object.
FixedSize( )	Overloaded public static method that returns a list object as a wrapper. The list is of fixed size; elements can be modified but not added or removed.
ReadOnly( )	Overloaded public static method that returns a list class as a wrapper, allowing read-only access.
Repeat( )	Public static method that returns an ArrayList whose elements are copies of the specified value.
Synchronized( )	Overloaded public static method that returns a list wrapper that is thread-safe.
Capacity	Property to get or set the number of elements the ArrayList can contain.
Count	Property to get the number of elements currently in the array.
IsFixedSize	Property to get to find out if the ArrayList is of fixed size.
IsReadOnly	Property to get to find out if the ArrayList is read-only.
IsSynchronized	Property to get to find out if the ArrayList is thread-safe.
Item( )	Gets or sets the element at the specified index. This is the indexer for the ArrayList class.
SyncRoot	Public property that returns an object that can be used to synchronize access to the ArrayList.
Add( )	Public method to add an object to the ArrayList.
AddRange( )	Public method that adds the elements of an ICollection to the end of the ArrayList.
BinarySearch( )	Overloaded public method that uses a binary search to locate a specific element in a sorted ArrayList.
Clear( )	Removes all elements from the ArrayList.
Clone( )	Creates a shallow copy.
Contains( )	Determines if an element is in the ArrayList.
CopyTo( )	Overloaded public method that copies an ArrayList to a one-dimensional array.
GetEnumerator( )	Overloaded public method that returns an enumerator to iterate an ArrayList.
GetRange( )	Copies a range of elements to a new ArrayList.
IndexOf( )	Overloaded public method that returns the index of the first occurrence of a value.
Insert( )	Inserts an element into ArrayList.
InsertRange( )	Inserts the elements of a collection into the ArrayList.
LastIndexOf( )	Overloaded public method that returns the index of the last occurrence of a value in the ArrayList.
Remove( )	Removes the first occurrence of a specific object.
RemoveAt( )	Removes the element at the specified index.
RemoveRange( )	Removes a range of elements.
Reverse( )	Reverses the order of elements in the ArrayList.
SetRange( )	Copies the elements of a collection over a range of elements in the ArrayList.
Sort( )	Sorts the ArrayList.
ToArray( )	Copies the elements of the ArrayList to a new array.
TrimToSize( )	Sets the capacity to the actual number of elements in the ArrayList.

When you create an ArrayList, you do not define how many objects it will contain. Add to the ArrayList using the Add( ) method, and the list takes care of its own internal bookkeeping, as illustrated in Example 9-12.

Example 9-12. Working with an ArrayList

namespace Programming_CSharp {    using System;    using System.Collections;       // a simple class to store in the array    public class Employee    {       private int empID;       public Employee(int empID)       {          this.empID = empID;       }       public override  string ToString( )       {          return empID.ToString( );       }       public int EmpID       {          get          {             return empID;          }          set          {             empID = value;          }       }    }    public class Tester    {       static void Main( )       {          ArrayList empArray = new ArrayList( );          ArrayList intArray = new ArrayList( );          // populate the array          for (int i = 0;i<5;i++)          {             empArray.Add(new Employee(i+100));             intArray.Add(i*5);          }                    // print all the contents          for (int i = 0;i<intArray.Count;i++)          {             Console.Write("{0} ", intArray[i].ToString( ));          }          Console.WriteLine("\n");          // print all the contents of the Employee array          for (int i = 0;i<empArray.Count;i++)          {             Console.Write("{0} ", empArray[i].ToString( ));          }          Console.WriteLine("\n");          Console.WriteLine("empArray.Capacity: {0}",             empArray.Capacity);       }    } } Output: 0 5 10 15 20 100 101 102 103 104 empArray.Capacity: 16

With an Array class, you define how many objects the array will hold. If you try to add more than that, the Array class will throw an exception. With an ArrayList, you do not declare how many objects the ArrayList will hold. The ArrayList has a property, Capacity, which is the number of elements the ArrayList is capable of storing:

public int Capcity {virtual get; virtual set; }

The default capacity is 16. When you add the 17th element, the capacity is automatically doubled to 32. If you change the for loop to:

for (int i = 0;i<17;i++)

the output looks like this:

0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 empArray.Capacity: 32

You can manually set the capacity to any number equal to or greater than the count. If you set it to a number less than the count, the program will throw an exception of type ArgumentOutOfRangeException.

9.5.1 Implementing IComparable

Like all collections, the ArrayList implements the Sort( ) method, which allows you to sort any objects that implement IComparable. In the next example, you'll modify the Employee object to implement IComparable:

public class Employee : IComparable

To implement the IComparable interface, the Employee object must provide a CompareTo( ) method:

public int CompareTo(Object rhs) {    Employee r = (Employee) rhs;    return this.empID.CompareTo(r.empID); }

The CompareTo( ) method takes an object as a parameter; the Employee object must compare itself to this object and return -1 if it is smaller than the object, 1 if it is greater than the object, and 0 if it is equal to the object. It is up to Employee to determine what smaller than, greater than, and equal to mean. For example, cast the object to an Employee and then delegate the comparison to the empId member. The empId member is an int and uses the default CompareTo( ) method for integer types, which will do an integer comparison of the two values.

Because int derives from object, it has methods, including the method CompareTo( ). Thus int is an object to which you may delegate the responsibility of comparison.

You are now ready to sort the array list of employees, empList. To see if the sort is working, you'll need to add integers and Employee instances to their respective arrays with random values. To create the random values, you'll instantiate an object of class Random; to generate the random values, you'll call the Next( ) method on the Random object, which returns a pseudorandom number. The Next( ) method is overloaded; one version allows you to pass in an integer that represents the largest random number you want. In this case, you'll pass in the value 10 to generate a random number between 0 and 10:

Random r = new Random( ); r.Next(10);

Example 9-13 creates an integer array and an Employee array, populates them both with random numbers, and prints their values. It then sorts both arrays and prints the new values.

Example 9-13. Sorting an integer and an employee array

namespace Programming_CSharp {    using System;    using System.Collections;       // a simple class to store in the array    public class Employee : IComparable    {       private int empID;       public Employee(int empID)       {          this.empID = empID;       }       public override  string ToString( )       {          return empID.ToString( );       }       // Comparer delegates back to Employee       // Employee uses the integer's default       // CompareTo method              public int CompareTo(Object rhs)       {          Employee r = (Employee) rhs;          return this.empID.CompareTo(r.empID);       }    }    public class Tester    {       static void Main( )       {          ArrayList empArray = new ArrayList( );          ArrayList intArray = new ArrayList( );          // generate random numbers for           // both the integers and the          // employee id's                    Random r = new Random( );          // populate the array          for (int i = 0;i<5;i++)          {             // add a random employee id             empArray.Add(new Employee(r.Next(10)+100));             // add a random integer             intArray.Add(r.Next(10));          }                    // display all the contents of the int array          for (int i = 0;i<intArray.Count;i++)          {             Console.Write("{0} ", intArray[i].ToString( ));          }          Console.WriteLine("\n");          // display all the contents of the Employee array          for (int i = 0;i<empArray.Count;i++)          {             Console.Write("{0} ", empArray[i].ToString( ));          }          Console.WriteLine("\n");          // sort and display the int array          intArray.Sort( );          for (int i = 0;i<intArray.Count;i++)          {             Console.Write("{0} ", intArray[i].ToString( ));          }          Console.WriteLine("\n");          // sort and display the employee array          //Employee.EmployeeComparer c = Employee.GetComparer( );          //empArray.Sort(c);                    empArray.Sort( );          // display all the contents of the Employee array          for (int i = 0;i<empArray.Count;i++)          {             Console.Write("{0} ", empArray[i].ToString( ));          }          Console.WriteLine("\n");                 }    } } Output: 8 5 7 3 3 105 103 102 104 106 3 3 5 7 8 102 103 104 105 106

The output shows that the integer array and Employee array were generated with random numbers. When sorted, the display shows the values have been ordered properly.

9.5.2 Implementing IComparer

When you call Sort( ) on the ArrayList, the default implementation of IComparer is called, which uses QuickSort to call the IComparable implementation of CompareTo( ) on each element in the ArrayList.

You are free to create your own implementation of IComparer, which you might want to do if you need control over how the sort is accomplished. In the next example, you will add a second field to Employee: yearsOfSvc. You want to be able to sort the Employee objects in the ArrayList on either field, empID or yearsOfSvc.

To accomplish this, you will create a custom implementation of IComparer, which you will pass to the Sort( ) method of ArrayList. This IComparer class, EmployeeComparer, knows about Employee objects and knows how to sort them.

EmployeeComparer has a property, WhichComparison, of type Employee.EmployeeComparer.ComparisonType:

public Employee.EmployeeComparer.ComparisonType     WhichComparison {    get    {       return whichComparison;    }    set    {       whichComparison=value;    } }

ComparisonType is an enumeration with two values, empID or yearsOfSvc (indicating that you want to sort by employee ID or years of service, respectively):

public enum ComparisonType {    EmpID,    Yrs };

Before invoking Sort( ), you will create an instance of EmployeeComparer and set its ComparisionType property:

Employee.EmployeeComparer c = Employee.GetComparer( );  c.WhichComparison=Employee.EmployeeComparer.ComparisonType.EmpID; empArray.Sort(c);

When you invoke Sort( ), the ArrayList will call the Compare method on the EmployeeComparer, which in turn will delegate the comparison to the Employee.CompareTo( ) method, passing in its WhichComparison property.

public int Compare(object lhs, object rhs) {    Employee l = (Employee) lhs;    Employee r = (Employee) rhs;    return l.CompareTo(r,WhichComparison); }

The Employee object must implement a custom version of CompareTo( ), which takes the comparison and compares the objects accordingly:

public int CompareTo(     Employee rhs,      Employee.EmployeeComparer.ComparisonType which) {    switch (which)    {       case Employee.EmployeeComparer.ComparisonType.EmpID:          return this.empID.CompareTo(rhs.empID);       case Employee.EmployeeComparer.ComparisonType.Yrs:          return this.yearsOfSvc.CompareTo(rhs.yearsOfSvc);    }    return 0; }

The complete source for this example is shown in Example 9-14. The integer array has been removed to simplify the example, and the output of the employee's ToString( ) method enhanced to enable you to see the effects of the sort.

Example 9-14. Sorting an array by employees' IDs and years of service

namespace Programming_CSharp {    using System;    using System.Collections;       // a simple class to store in the array    public class Employee : IComparable    {       private int empID;              private int yearsOfSvc = 1;       public Employee(int empID)       {          this.empID = empID;       }              public Employee(int empID, int yearsOfSvc)       {          this.empID = empID;          this.yearsOfSvc = yearsOfSvc;       }       public override  string ToString( )       {          return "ID: " + empID.ToString( ) +           ". Years of Svc: " + yearsOfSvc.ToString( );       }       // static method to get a Comparer object       public static EmployeeComparer GetComparer( )       {          return new Employee.EmployeeComparer( );       }       // Comparer delegates back to Employee       // Employee uses the integer's default       // CompareTo method       public int CompareTo(Object rhs)       {          Employee r = (Employee) rhs;          return this.empID.CompareTo(r.empID);       }              // Special implementation to be called by custom comparer       public int CompareTo(          Employee rhs,           Employee.EmployeeComparer.ComparisonType which)       {          switch (which)          {             case Employee.EmployeeComparer.ComparisonType.EmpID:                return this.empID.CompareTo(rhs.empID);             case Employee.EmployeeComparer.ComparisonType.Yrs:                return this.yearsOfSvc.CompareTo(rhs.yearsOfSvc);          }          return 0;       }       // nested class which implements IComparer       public class EmployeeComparer : IComparer       {          // private state variable          private Employee.EmployeeComparer.ComparisonType              whichComparison;          // enumeration of comparsion types          public enum ComparisonType          {             EmpID,             Yrs          };          // Tell the Employee objects to compare themselves          public int Compare(object lhs, object rhs)          {             Employee l = (Employee) lhs;             Employee r = (Employee) rhs;             return l.CompareTo(r,WhichComparison);          }          public Employee.EmployeeComparer.ComparisonType              WhichComparison          {             get             {                return whichComparison;             }             set             {                whichComparison=value;             }          }       }    }    public class Tester    {       static void Main( )       {          ArrayList empArray = new ArrayList( );          // generate random numbers for           // both the integers and the          // employee id's          Random r = new Random( );          // populate the array          for (int i = 0;i<5;i++)          {             // add a random employee id                          empArray.Add(                new Employee(                   r.Next(10)+100,r.Next(20)                )             );          }                    // display all the contents of the Employee array          for (int i = 0;i<empArray.Count;i++)          {             Console.Write("\n{0} ", empArray[i].ToString( ));          }          Console.WriteLine("\n");                    // sort and display the employee array          Employee.EmployeeComparer c = Employee.GetComparer( );          c.WhichComparison=Employee.EmployeeComparer.ComparisonType.EmpID;          empArray.Sort(c);          // display all the contents of the Employee array          for (int i = 0;i<empArray.Count;i++)          {             Console.Write("\n{0} ", empArray[i].ToString( ));          }          Console.WriteLine("\n");                    c.WhichComparison=Employee.EmployeeComparer.ComparisonType.Yrs;          empArray.Sort(c);          for (int i = 0;i<empArray.Count;i++)          {             Console.Write("\n{0} ", empArray[i].ToString( ));          }          Console.WriteLine("\n");                           }    } } Output: ID: 103. Years of Svc: 11 ID: 108. Years of Svc: 15 ID: 107. Years of Svc: 14 ID: 108. Years of Svc: 5 ID: 102. Years of Svc: 0 ID: 102. Years of Svc: 0 ID: 103. Years of Svc: 11 ID: 107. Years of Svc: 14 ID: 108. Years of Svc: 15 ID: 108. Years of Svc: 5 ID: 102. Years of Svc: 0 ID: 108. Years of Svc: 5 ID: 103. Years of Svc: 11 ID: 107. Years of Svc: 14 ID: 108. Years of Svc: 15

The first block of output shows the Employee objects as they are added to the ArrayList. The employee ID values and the years of service are in random order. The second block shows the results of sorting by the employee ID, and the third block shows the results of sorting by years of service.