An Iterator for Arrays

Traditional Method of Iterating Through an Array

int [] arr = new int [...];
...			
for (int i = 0; i < arr.length; i++) {
	// Do something with arr[i]
} 

• Have to know about the initial condition (first element is at 0)
• Have to know about terminating condition (last element is at arr.length-1
• Have to know how to get the current element (arr[i])

• Pascal programmers don't know about 0-based arrays
• C programmers don't know about arr.length
• PL/I programmers don't know about arr[i]

Revisiting the Iterator Interface

• Recall Iterator has two method specs:

  interface Iterator {
  	boolean hasNext();
  	Object next();
  }
  		

• hasNext eliminates the need to know the terminating condition
• nexteliminates the need to know how to access the next element
• Remember that Iterator is an interface
  • A set of method specs, not an actual class
  • A class will implement the interface
  • It makes sense for the class' constructor to take care of the initial condition

An Iterator for Arrays

• Simple, understandable example of an iterator
• Our 'traditional' method of array iteration will drive the implementation of our iterator
  • Even though the user of the iterator should have a uinform mechanism, the designer will be taking into account the specifics of the collection being iterated over

  class ArrayIterator implement Iterator{
  }
  		

• Maintaining the State
  • Will use an integer to hold the current index
  • Corresponds to the loop index in the 'traditional' method

  class ArrayIterator implement Iterator{
  	private int currentIndex;
  }
  		

• Handling the Initial Condition - The Constructor
  • Will set currentIndex to 0
  • Corresponds to the i = 0 of the traditional method

  class ArrayIterator implement Iterator{
  	ArrayIterator() {
  		currentIndex = 0;
  	}
  	private int currentIndex;
  }
  		

• Handling the Termination Condition - The hasNext Method
  • hasNext tests if we've reached the last element
  • Corresponds to the i < arr.length

    class ArrayIterator implement Iterator{
    	ArrayIterator() {
    		currentIndex = 0;
    	}
    	boolean hasNext() {return currentIndex < arr.length;}
    	private int currentIndex;
    }
    				

  • This implies that we need to maintain which array we're iterating over as part of the state

    class ArrayIterator implement Iterator {
    	ArrayIterator(int [] arr) {
    		this.arr = arr;
    		currentIndex = 0;
    	}
    	boolean hasNext() {return currentIndex < arr.length;}
    	private int [] arr;
    	private int currentIndex;
    }
    				

  • This seems right -- when we create an iterator for an array, it makes sense we should specify WHICH array
• Accessing the Next Element - The next Method
  • next retrieves the next element
  • Corresponds to a[i]

  class ArrayIterator implement Iterator {
  	ArrayIterator(int [] arr) {
  		this.arr = arr;
  		currentIndex = 0;
  	}
  	boolean hasNext() {return currentIndex < arr.length;}
  	int next() {
  		int hold = arr[currentIndex];
  		currentIndex++;
  		return hold;
  	}
  	
  	private int [] arr;
  	private int currentIndex;
  }
  				

An Iterator for DynArr

• DynArr is really nothing more than an array with a resize method
• Same basic idea holds, except
  • the int [] now becomes a DynArr
  • the []'s must be replaced with get
  • the length field becomes an invocation of the length method
• Your next part of the project

Why a Separate Object for the Iterator?

• Why not have methods hasNext and next be part of the DynArr class itself-- why introduce a new class
• What if we are iterating through our array with a nested loop
  • For example, for each element, find how many elements in the rest of the array are equal to it

    for (int i = 0; i < arr.length; i++) {
    	count = 0;
    	for (j = i+1; j < arr.length; j++)
    		if (arr[i] == arr[j]) count++;
    	System.out.println("Element " + arr[i] + " at index " + i + " is repeated " + count + " times");  
    } 
    

  • Suppose we wrote instead:

    for (int i = 0; i < arr.length; i++) {
    	count = 0;
    	for (i = i+1; j < arr.length; i++) 		// ???
    		if (arr[i] == arr[i]) count++;
    	System.out.println("Element " + arr[i] + " at index " + i + " is repeated " + count + " times");  
    } 
    

    Remember that the state of the iterator (the currentIndex instance variable) corresponds to the loop index