Assumed Knowledge
Learning Outcomes
  • Understand the underlying features of lists and how they differ from arrays.
  • Be able to use built-in Java lists
  • Be able to build a custom list class
  • Understand the time costs of various list operations

Author: Gaurav Gupta

What are lists?

Lists are data structures, much like arrays. The differences being,

1. Lists must hold objects

Arrays can hold a collection of primitive data types or a collection of objects, while lists can hold a collection of objects, not primitive data types.

2. Lists grow as required

The size of array needs to be specified at the time of creating an array. The size of a list need not be specified. You can add as many items as you want to a list (permitting system memory).

3. Lists have a range of instance methods

With arrays (assuming array name is arr), the only operators you have to work with are arr.length and arr[i]. Anything and everything you need to do must be done using these two operators. Several life-saving methods are applicable on list objects, such as:

  • get(int) //similar to arr[i]
  • size() //similar to arr.length
  • add(Object) //add item at the end of the list

Why are arrays not good enough?

Example - copying over a subset

Consider an array src holding 100 integers. Some negative, some positive. We need to copy all negative items over to a new array dest.

As an example, if

src is {10, 20, 50, 0, -40, 30, 90, 60, -10, -50, 80}

dest should be {-40, -10, -50}

In order to do this, we need to,

  1. Count the number of required (negative) values in the array src
  2. Create an array dest of that size
  3. Copy items over to dest.

Step 1

int count = 0;
for(int i=0; i < src.length; i++) {
	if(src[i] < 0) {
		count++;
	}
}

Step 2

int[] dest = new int[count];

Step 3

We are copying,

src[4] into dest[0]

src[8] into dest[1]

src[9] into dest[2]

So, in addition to the current index of src, we also need to keep track of the current index of dest into which the item must be copied.

int idx = 0; //index where item must be copied
for(int i=0; i < src.length; i++) {
	if(src[i] < 0) {
		dest[idx] = src[i]; //another item copied
		idx++; //move destination index forward
	}
}

Final solution

int count = 0;
for(int i=0; i < src.length; i++) {
	if(src[i] < 0) {
		count++;
	}
}

int[] dest = new int[count];

int idx = 0; //index where item must be copied

for(int i=0; i < src.length; i++) {
	if(src[i] < 0) {
		dest[idx] = src[i]; //another item copied
		idx++; //move destination index forward
	}
}

Solution using List (just focus on how easy and intuitive it is)

A solution to the same problem when src and dest are lists instead of arrays is,

ArrayList<Integer> dest = new ArrayList<Integer>();
for(int item: src) {
	if(item < 0) {
		dest.add(item);
	}
}

Creating an ArrayList object

The syntax to create an ArrayList object is:

ArrayList<Object> name = new ArrayList<Object>();

Some examples:

ArrayList<Integer> list1 = new ArrayList<Integer>(); //list of integers
ArrayList<String> list2 = new ArrayList<String>(); //list of String objects
ArrayList<Rectangle> list3 = new ArrayList<Rectangle>(); //list of our beloved user-defined Rectangle objects

List of selected methods in ArrayList class

Method Description
int size()                                 It is used to return the number of elements present in the list.
E get(int index) It is used to fetch the element from the particular position of the list.
boolean add(E e) It is used to append the specified element at the end of a list.
void add(int index, E element) It is used to insert the specified element at the specified position in a list.
void clear() It is used to remove all of the elements from this list.
boolean isEmpty() It returns true if the list is empty, otherwise false.
int indexOf(Object o) It is used to return the index in this list of the first occurrence of the specified element, or -1 if the List does not contain this element.
int lastIndexOf(Object o) It is used to return the index in this list of the last occurrence of the specified element, or -1 if the list does not contain this element.
boolean contains(Object o) It returns true if the list contains the specified element
E remove(int index) It is used to remove the element present at the specified position in the list.
boolean remove(Object o) It is used to remove the first occurrence of the specified element.
E set(int index, E element) It is used to replace the specified element in the list, present at the specified position.

Use of methods in examples

These examples assume the existence of the following Rectangle class:

class Rectangle {
	public int length, breadth;
	public Rectangle(int len, int bre) {
		length = len;
		breadth = bre;
	}

	public String toString() {
		return length + " by " + breadth;
	}

	public int area() {
		return length * breadth;
	}

	public boolean isSquare() {
		return length == breadth;
	}
}

Each example builds on top of the previous example.

Example 1 - size() and add(Object)

ArrayList<Integer> data = new ArrayList<Integer>();
int a = data.size(); //a will be 0 since data doesn't contain any items
data.add(10); //data becomes [10]
data.add(70); //data becomes [10, 70]
data.add(20); //data becomes [10, 70, 20]
data.add(90); //data becomes [10, 70, 20, 90]
int b = data.size(); //b will be 4

Example 2 - add(int, Object)

//note data = [10, 70, 20, 90] already
data.add(0, 50); //data becomes [50, 10, 70, 20, 90]
data.add(data.size(), 40); //data becomes [50, 10, 70, 20, 90, 40]
data.add(-1, 100); //INVALID - throws IndexOutOfBoundsException
data.add(data.size()+1, 100); //INVALID - throws IndexOutOfBoundsException

Example 3 - set(int, Object)

//note data = [50, 10, 70, 20, 90, 40] already
data.set(0, 60); //data becomes [60, 10, 70, 20, 90, 40]
data.set(data.size()-1, 60); //data becomes [60, 10, 70, 20, 90, 60]
data.set(-1, 100); //INVALID - throws IndexOutOfBoundsException
data.set(data.size(), 100); //INVALID - throws IndexOutOfBoundsException

Example 4 - contains(Object), indexOf(Object), lastIndexOf(Object)

//note data = [60, 10, 70, 20, 90, 60] already
data.
boolean status = data.contains(70); //status is true
boolean status = data.contains(50); //flag is false
int a = data.indexOf(60); //a is 0
int b = data.lastIndexOf(60); //b is 5
int c = data.indexOf(70); //c is 2
int d = data.lastIndexOf(70); //d is 2
int e = data.indexOf(80); //e is -1 (80 not found)
int f = data.lastIndexOf(80); //e is -1 (80 not found)

Example 5 - remove(int), remove(Object)

//note data = [60, 10, 70, 20, 90, 60] already
data.remove(10);
//IndexOutOfBoundsException since 10 is treated as int, and hence remove(int) is called
data.remove((Integer)10); //data = [60, 70, 20, 90, 60]
data.remove((Integer)60); //data = [70, 20, 90, 60]
data.remove((Integer)60); //data = [70, 20, 90]

//removing all occurrences of a specific item -

for(int i=0; i < 5; i++) {
	data.add(80);
}
//data = [70, 20, 90, 80, 80, 80, 80, 80]
while(data.contains(80)) {
	data.remove(80);
}
//data = [70, 20, 90]

Example 6 - isEmpty(), clear()

//note data = [70, 20, 90[ already
boolean g = data.isEmpty(); //g is false
data.clear(); //data = [] now
boolean h = data.isEmpty(); //h is true

Passing ArrayList to functions

ArrayLists are objects and a reference copy of the actual parameter is made into the formal parameter when they are passed to functions.

import java.util.ArrayList;

public class ListToFunctionClient {
	public static void main(String[] args) {
		ArrayList<Integer> data = new ArrayList<Integer>();
		data.add(10);
		data.add(70);
		data.add(20);
		data.add(90);
		int total = sum(data); //total = 190
		increment(data); //data = [11, 71, 21, 91]
		System.out.println(total+" "+data); //190 [11, 71, 21, 91]
	}

	public static int sum(ArrayList<Integer> list) {
		int result = 0;
		for(int i=0; i < list.size(); i++) {
			result+=list.get(i);
		}
		return result;
	}

	public static void increment(ArrayList<Integer> list) {
		for(int i=0; i < list.size(); i++) {
			list.set(i, list.get(i)+1);
		}
	}
}

For-each loop

The for-each loop helps simplify list (or array) traversal where the index is not required besides accessing the item. Syntax:

for(Object obj: ArrayList) {
	//use obj
}

Example:

ArrayList<Integer> data = new ArrayList<Integer>();
data.add(10);
data.add(70);
data.add(20);
data.add(90);
int total = 0;
for(Integer item: data) {
	total+=item;
}
//total = 190

In the above example, item becomes a reference copy of,

  • First item (10) in the first iteration of the loop
  • Second item (70) in the second iteration of the loop
  • Third item (20) in the third iteration of the loop
  • Fourth item (90) in the fourth iteration of the loop

Since java handles casting between Integer and int, you can also write:

for(int item: data) {
	total+=item;
}

Here, the value of the Integer object is copied into item during each iteration.

So, are we going to use the Rectangle class at all?

Yes, we will :)

Take the following example -

ArrayList<Rectangle> rects = new ArrayList<Rectangle>();
for(int i=0; i < 5; i++) {
	if(i!=3) {
		Rectangle temp = new Rectangle(1+i/3, 1+i%3);
		rects.add(temp);
	}
	else {
		rects.add(null);
	}
}
System.out.println(rects);

At the end of the execution of the above code, the output will be:

[1 by 1, 1 by 2, 1 by 3, null, 2 by 2]

The memory diagram is provided below: