Array of objects

Assumed Knowledge:

• Classes as Types
• Copying objects
• Instance methods

Learning Outcomes:

• Creating and operating on an array of objects.

Author: Gaurav Gupta

Array of objects

Class under consideration

We will use the following class for this discussion:

public class Rectangle {
	public double width, height;

	public Rectangle(double w, double h) {
		width = w;
		height = h;
	}

	public String toString() {
		return width + " by " + height;
	}
}

Creating an array of objects

STEP 1 - Instantiate the array

public class Client {
	public static void main(String[] args) {
		Rectangle[] blocks = new Rectangle[5];
	}
}

You can go through each item of the array and display it.

for(int i=0; i < blocks.length; i++) {
	System.out.println(blocks[i]);
}

You’ll get the following output:

null
null
null
null
null

Each item of the array is a Rectangle reference, and initialized to the default value (which, for objects, is null).

The memory diagram for the current state of the array is

Any attempt to access an instance variable or instance method on any of the items of the array will raise a NullPointerException.

blocks[0].width = 5; //NullPointerException
String str = blocks[0].toString(); //NullPointerException

Hence…

STEP 2 - Instantiating each object

for(int i=0; i < blocks.length; i++) {
	blocks[i] = new Rectangle(i+1, i*2); //instantiate item at index i
	System.out.println(blocks[i]); //display it
}

This time, you will get the following output:

1.0 by 0.0
2.0 by 2.0
3.0 by 4.0
4.0 by 6.0
5.0 by 8.0

Exercise: Creating (filtered) deep copy of an object array

Consider the array source populated as:

Rectangle[] source = new Rectangle[20];
for(int i=0; i < source.length; i++) {
	double randWidth = 1 + rand.nextInt(5);
	double randHeight = 1 + rand.nextInt(5);
	source[i] = new Rectangle(randWidth, randHeight);
}

Create a second array containing Rectangles that have an area of 10 or more.

STEP 1: Count the number of rectangles with area of 10 or more

int count = 0;
for(int i=0; i < source.length; i++) {
	if(source[i].area() >= 10) {
		count++;
	}
}

STEP 2: Create an array of required size

Rectangle[] bigRectangles = new Rectangle[count];

STEP 3: Populate the array

int destIndex = 0;
for(int i=0; i < source.length; i++) {
	if(source[i].area() >= 10) {
		bigRectangles[destIndex] = source[i];
		destIndex++;
	}
}

Given an array data of Rectangle objects, create an array wider with those rectangles whose width is more than their height

SOLUTION

int count = 0;
for(int i=0; i < data.length; i++) {
	if(data[i].width > data[i].height) {
		count++;
	}
}
Rectangle[] wider = new Rectangle[count];
int destIndex = 0;
for(int i=0; i < data.length; i++) {
	if(data[i].width > data[i].height) {
		wider[destIndex] = data[i];
		destIndex++;
	}
}

You can also access an instance variable or instance method on any of the items of the array.

blocks[0].width = 5; //change the width of the first item to 5
int h = blocks[3].height; //store height of fourth item into h

Complete code is provided in ArrayOfObjects.java