Array of objects

Class under consideration

We will use the following class for this discussion.

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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

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Rectangle[] blocks = new Rectangle[5];

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

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

You’ll get the following output:

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null
null
null
null
null

This is because none of the items of the array, each a Rectangle object, were instantiated.

The memory diagram for the current state of the array is

Hence…

STEP 2 - Instantiating each object

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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:

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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:

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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

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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

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Rectangle[] bigRectangles = new Rectangle[count];

STEP 3: Populate the array

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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

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