- How to tell what you need
to do to
invoke (= use) a method
- How many parameters to pass ?
- What type of parameters ?
- Does the method return a value ?
- What happens when you invoke a method
- Terminology:
- Invoking a method =
executing a method
- Other phrases with exactly the same meaning:
- calling a method
- running a method
- Invoking a method =
executing a method
- Syntax to invoke a
method:
- Syntax to invoke a
method that does not return
any value:
MethodName( parameters ) ; // Runs the methodYou can recognize such methods by the return type void.
- Syntax to invoke a
method that returns a value:
var = MethodName( parameters ) ; // Runs the method and when the method completes, // stores the value returned by the method // in the variable "var" // Note: the data types of the variable and // the returned value must matched !!You can recognize such methods by their return type which is not equal to void.
- Syntax to invoke a
method that does not return
any value:
- Consider a Mathematical function:
f(x, y, z) = x2 + y3 + z4 Examples: f(1, 1, 1) = 3 f(2, 2, 1) = 13Notes:
- A Mathematical function has a
unique name
- A Mathematical function has a number of
function arguments
(x,
y and
z)
The Mathematical function uses the values of the arguments to compute the answer
- A Mathematical function will produce
an answer
The answer is the output value of the Mathematical function
- A Mathematical function has a
unique name
- A method resembles a
Mathematical function:
- A method has a
unique "signature"
(will learn more about "signatures"
later in the course)
- A method has a number
of input parameters
- A method may
return
one output value
(some methods do not return any output values)
- When a method returns a value, you must store the return value in some variable (using an assignment statement) or else, the returned value will be lost !
- A method has a
unique "signature"
(will learn more about "signatures"
later in the course)
- Facts:
- Mathematical functions
are implemented as
Java methods
Examples:
- The sine function is implemented as the Math.sin method in the Java standard library.
- The cosine function is implemented as the Math.cos method in the Java standard library.
- The square root function is implemented as the Math.sqrt method in the Java standard library.
- Methods can be used for
more than just
write Mathematical functions
After learning Java, you can write methods to solve problems
Examples:
- You can write a method to find the Greatest Common Divisor of 2 numbers
- You can write a method to find the best way to pack boxes of different sizes into a crate
- And so on...
- Mathematical functions
are implemented as
Java methods
- You need the look at the
documentation of the
method in the Java library to
find out
what input parameters a method needs and
whether the method will
return a value
- Website of the Java's documentation: click here
- Suppose we want to learn
how to use
the Math.sqrt function
Finding the documentation for the Math.sqrt method:
- The name
Math.sqrt means:
the sqrt method
inside the Math class
- We must first
find the Math class:
- Scroll down the lower left panel
to the class Math
and click on the link:
- The main window will show the
documentation for
the Math class:
- Scroll down the lower left panel
to the class Math
and click on the link:
- Next, we find the
sqrt method inside
the documentation page for
the Math class:
Meaning of the method header:
- The name
Math.sqrt means:
the sqrt method
inside the Math class
- Example using the Math.sqrt method:
import java.lang.Math; // Not needed... already done by Java compiler public class Sqrt { public static void main(String[] args) { double a, b, c; a = 2.0; b = Math.sqrt(a); c = 2.0*Math.sqrt(a) + 1.0; System.out.print("a = "); System.out.println(a); System.out.print("Sqrt of a = "); System.out.println(b); System.out.print("2*sqrt(a) + 1 = "); System.out.println(c); } }
- Parameters:
- You must pass the
correct number of
parameter of the
correct data type
to a method
Look at the parameter definitions in the method header to find this out.
- You must pass the
correct number of
parameter of the
correct data type
to a method
- Return value:
- If the method returns a value, you must assign it to a variable of the correct type
- Let us examine a portion of
the previous example program:
double a, b; a = 2.0; b = Math.sqrt(a);
- The following figure shows the
situation
just before the program
executes
b = Math.sqrt(a);:
- When the method call
Math.sqrt(a) is
executed, the computer
will do the following:
- Pass
the value of the
variable a
as parameter
to the sqrt method:
The result is: the sqrt method will use the value 2.0 in the computations perform by the statements in its method body
- Execute the statements
in the method body of sqrt:
- When it completes,
the execution returns to
the location of the method call
The return value replaces the method call in the expression:
- After the assigning the value
to variable b, we have:
The statement b = Math.sqrt(a); have computed the value √(a) and stored it inside the variable b.
- Pass
the value of the
variable a
as parameter
to the sqrt method: