Question

Can someone please explain this piece of java code line by line as to what they are doing and the purpose of each line (you can put it in the code comments).

Code:

import java.util.*;

public class Array
{
private Integer[] array; // NOTE: Integer is an Object.

Array()
{
    super();
    array = new Integer[0];
}

Array(Array other)
{
    super();
    array = other.array.clone(); // NOTE: All arrays can be cloned.
}

void add(int value)
{
    Integer[] newarray = new Integer[array.length+1];
    int i;
    for (i=0; i < array.length; ++i)
      newarray[i] = array[i];
    newarray[i] = value;
    array = newarray;
    return;
}

public Iterator<Integer> iterator()
{
    class Iter implements Iterator<Integer>
    {
      int pos = 0;

      public boolean hasNext()
      {
        return pos < array.length;
      }

      public Integer next() throws NoSuchElementException
      {
        if (hasNext())
        {
          Integer retval = array[pos];
          ++pos;
          return retval;
        }
        else
          throw new NoSuchElementException();
      }

      public void remove() throws UnsupportedOperationException
      {
        throw new UnsupportedOperationException();
      }
    }
    return new Iter();
}

public static final void main(String[] args)
{
    Array a = new Array();
    a.add(1);
    a.add(2);
    a.add(3);
    for (Iterator<Integer> i=a.iterator(); i.hasNext(); )
    {
      Integer value = i.next();
      System.out.println(value);
    }
}
}

Answer 1

// Feel free to post a comment if you have any queries (attaching screenshots of the code below if you'd like to know the meaning of a specific line)

// Upvote the answer if you like it!

import java.util.*;

public class Array {
private Integer[] array; // NOTE: Integer is an Object.
// Integer is an object, it hasn't been initialized till now
// Only a reference named "array" is created

Array() {
// This just creates a new object
// super() is used to call the constructor of the parent class explicitly
// Every class in Java inherits from Object class
super();
// An integer array is made of length 0
array = new Integer[0];
}

// This is a copy constructor
Array(Array other) {
super();
// It just clones the array that we have, please note the lowercase 'a'
// This constructor can be called in main as:
// Array b = new Array(a);
// After we have defined the array a that is (in main)
array = other.array.clone(); // NOTE: All arrays can be cloned.
}

// This method is used to append values to the array
void add(int value) {
// Everytime this method is called a new array is created with length 1 greater than the previous array
Integer[] newarray = new Integer[array.length + 1];
int i;
// Each element in the old array is copied to the new array
// This can be expensive because consider you have a million elements and a million iterations will take place when
// adding a new element
for (i = 0; i < array.length; ++i)
newarray[i] = array[i];
  
// Append the value to be added at the end
newarray[i] = value;

// This is the most important step, to make changes to the original array, we just assign the value
array = newarray;
  
// return back to calling function when done
return;
}

// Create a method called iterator() which return an Iterator<Integer>
public Iterator<Integer> iterator() {
// Define a class inside it which is just used to override the default behaviour of the Iterator
class Iter implements Iterator<Integer> {
// pos is used to store the current position of the iterator
int pos = 0;

// Check if there are elements ahead of the current position of the iterator
// we return pos < array.length which is a boolean value which is true if the condition is satisfied
public boolean hasNext() {
return pos < array.length;
}
  
// get the element from the position
// and increment the position by 1
// "next" is in terms of the Iterator, it just moves onto the next element
// while also returning the current element
public Integer next() throws NoSuchElementException {
// Check if there are more elements
if (hasNext()) {
// save the value to be returned to a variable
Integer retval = array[pos];
++pos;
return retval;
} else
// if no elements are ahead of pos
throw new NoSuchElementException();
}
  
// This just overrides the method remove() from Iterator
// and throws an exception if it is called
public void remove() throws UnsupportedOperationException {
throw new UnsupportedOperationException();
}
}
// return an object of the Iter class that you just defined
return new Iter();
}

public static final void main(String[] args) {
// Create an object of Array class
Array a = new Array();

// Add element elements to the array
a.add(1);
a.add(2);
a.add(3);

// Iterate over the array using an iterator which in simple terms is like an index
// a.iterator() means that we are calling our own "iterator" method
// inside the iterator method, we have defined a class name Iter which implements the Iterator interface
for (Iterator<Integer> i = a.iterator(); i.hasNext();) {
// This part is only executed if there are more elements in the Array
// We have seen 0 elements till now so i.next() returns the first element which is at index 0 [for first iteration]
Integer value = i.next();
// value = 1 [for first iteration]
// print the value
System.out.println(value);
}
}
}

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