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AP Computer Science A Textbook

1. Primitive Types

2. Using Objects

2.1 Objects: Instances of Classes

2.2 Creating and Storing Objects (Instantiation)

2.3 Calling a Void Method

2.4 Calling a Void Method with Parameters

2.5 Calling a Non-void Method

2.6 String Objects: Concatenation, Literals & More

2.7 String Methods

2.8 Wrapper Classes: Integers and Doubles

2.9 Using the Math Class

Basic Math Operations

Using the Math Class

Random Numbers

Generating Random Numbers

Static Methods: Rectangle

Check Your Understanding

Exercise: Circle Area

3. Boolean Expressions and if Statements

4. Iteration

5. Writing Classes

6. Arrays

7. ArrayLists

8. 2D Arrays

9. Inheritance

10. Recursion

Chapter 2

Using Objects

2.9 Using the Math Class

In this section, you are going to take a look at another class in the java.lang package, the Math class. The Math class is a class that only contains static methods that are used for specific purposes.

Basic Math Operations

Several of the methods that are used from the Math class perform basic math functions. These are listed in the table below.

Method Use
Math.abs(x) Returns the absolute value of x. This can take either an int or a double.
Math.pow(base, exponent) Returns the value of base raised to the power of exponent.
Math.sqrt(x) Returns the positive square root of x.

As a reminder, since the math class contains only static methods, you do not need to instantiate an object to use it. Static methods are called using the dot operator along with the class name.

Take a look at the Math class in action in this example.

Using the Math Class

Random Numbers

Another useful method from the Math class is the random method. The random method takes no input and returns a random number from zero to one, inclusive of zero, but not inclusive of one.

// Generate a random number
double ranNum = Math.random();
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Now you may be thinking that a random number between zero and one is not that useful, however, the number can be manipulated to generate a random number in any range. Using three basic operations, you can convert the output of Math.random() into any random number you need.

Multiply - By multiplying the results of Math.random(), you can create values from 0 up to (but not including) the number you multiple by. 

// Generates a random value from 0 to 10
double ranNum = Math.random() * 10;
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Add - Adding to the results offsets the range that you will get from the random number.

// Generates a random value from 100 to 110
double ranNum = Math.random() * 10 + 100;
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Casting to an integer - Finally, if you cast the value to an integer, you will truncate the value and only have random whole numbers.

// Generates random integers between 50 and 75 (not including 75)
int ranNum = (int)(Math.random() * 25) + 50;
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Watch out! Be careful, a common mistake that programers make is to cast the value at the wrong place. You must include the multiplier inside parentheses along with the random call. If you do not, you will end up casting only the Math.random() part, and since it is always less than one, your random function will always result in zero. The offset addition operation can either be inside or outside the parentheses.

// Same as above, but without the parentheses
// The result is zero every time
int ranNum = (int)Math.random() * 25 + 50;
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Generating Random Numbers

Static Methods: Rectangle

Check Your Understanding

  1. Incorrect Correct No Answer was selected Invalid Answer

    What range of numbers would be generated by using:

    int num  = (int) (Math.random() * 201);
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Exercise: Circle Area

Here is a Circle class.

Implement getArea and getCircumference by using methods from the Math class.