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The Complete Java Crash Course

Fractional values of numbers with `double`

Explore how to work with double in Java to store fractional values. Understand declaring and using double variables through practical coding exercises like calculating circle areas and scaling drawings. Gain skills in handling precise decimal numbers and applying them effectively in Java programs.

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The int data type only represents numbers without a fractional part. What if we want to store an approximation for π in a mathematics program? We can use a different data type, called a double, to store numbers that have values after the decimal point. double is short for double-precision floating point. Floating point means that there is a decimal point that can be placed at different locations (or float), in the number.

Computer programming languages evolve over time; double-precision just means that this representation allows double the precision that an earlier representation of floating-point numbers used. There is also a floating point type called float in Java, which uses less memory and has less precision than a double. It is rarely used.

Exercise: circle area

Objective: Create and use values and variables of the double type.

Declare and give initial values to variables r and pi representing the radius of a circle, and an approximation of the mathematical constant π. Use those values to compute and print the area of the circle. You can square a number in Java by multiplying it by itself.

CircleArea.java
Sample Solution
class CircleArea {
  public static void main(String args[]) {
  	double area;
  }
}

Notice from the sample solution that you can declare multiple variables of the same type using a single line of code.

Exercise: big smile

Objective: use variables to allow the behavior of code to be changed easily.

The code below will draw a smiley face on the screen, and you can use the variables x and y to change where the smiley face is drawn. But the smiley face is always the same size. Add a new variable, scale, that allows you to change the size of the smiley face to make the face either larger or smaller. For example, if scale had the value 2, then the code would draw the smiley face twice as large (but still centered on x and y).

Any time you create a variable, you should first ask what type of data it will hold, since you will need that information in order to declare the variable. Should scale be an integer? No, not if you’d like it to hold a value like 0.5 to make the smiley smaller.

Test your code by changing variable values a few times to draw the smiley face at different locations at both small and large scales.

Hint: Some values should be scaled; others should not be. For example, if the x location of the left eye is x - 20, then the 20 should be scaled, but the x should not be, since x will still be the x location of the center of the entire smiley, even if the smiley is scaled.

BigSmile.java
Sample solution
import com.educative.graphics.*;
class BigSmile {
  public static void main(String[] args) {
	  Canvas c;
    c = new Canvas(200, 200);
     
    int x;
    int y;
    x = 100;
    y = 100;
    // Draw the outline of the face
    c.fill("yellow");
    c.stroke("black");
    c.circle(x, y, 50);
    // draw the mouth
    c.stroke("black");
    c.fill("yellow");
    c.circle(x, y, 30);
    c.stroke("yellow");
    c.rect(x - 32, y - 32, 62, 40);
    // draw the eyes
    c.stroke("black");
    c.fill("black");
    c.circle(x - 20, y - 10, 5);
    c.circle(x + 20, y - 10, 5);
  }
}