Transcript Methods

Introduction to Method
Example
Java Method （1）
The 2 types (kinds) of methods in Java
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Class methods
Instance methods
Methods can do more work than statements:
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A statement can only accomplish a very small amount of
work.
A method contains multiple statements.
Java Method （2）
• Methods allows us to build up a library of useful tools
You define (describe) a method once
Afterwards, you can executed (invoked) the method as many
times as you want.
• Therefore, we can build up a library of problem solving methods
E.g., Math.sin(), Math.sqrt(), Scanner methods for
input, etc.
Java Method （3）
• Methods allows us to solves a complex problem using the
divide and conquer methodology
• In the divide and conquer problem solving technique
1. we break a complex problem/task into a number of
smaller problems/tasks
2. Each of the smaller problem/task is then solved using one
method.
Steps in using methods in a Java program （1）
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First, you must define the method.
How to define a method:
Write down the steps (= statements) contained in the method.
Attach a name to the steps (= statements)
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Notes:
You only need to define a method once
(Remember that in Java, you must define the method inside some class.)
Steps in using methods in a Java program （2）
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After defining the method, you can then call a method using the name of
the method
When a method is called, the statements inside the corresponding method
are executed
When all statements in the method has been executed, the execution will
resume at the program location of the method call
This mechanism is called method invocation (an older term is procedure
call)
You can invoke a method as many times as you wish
Example Code(1)
We have previously seen an algorithm to find the smaller of 2 values x and y
Example Code(2)
Example Code(3)
Example Code(4)
• A non-savvy user that wants to use the ToolBox.min method does not need to
know the statements contained inside the method ToolBox.min !!!
• A non-savvy user will only need to know the following in order to use the
method:
1. The (complete) name of the method (i.e.: ToolBox.min)
2. What information the method needs to do the task (i.e.: 2 numbers)
3. What information the method returns to the user (i.e.: 1 number)
Effect of a return statement:
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The return statement is used to terminate the execution of a method.
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When the program executes a return statement, the method terminates
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The execution will continue at the location where the method was called
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If a return statement returns an EXPRESSION, then the value (= result) of
the EXPRESSION will be used to replace the method call at the point of
call.
What happens in a method invocation: (1)
What happens in a method invocation: (2)
What happens in a method invocation: (3)
What happens in a method invocation: (4)
Defining a (class) method (1)
Defining a (class) method (2)
Defining a (class) method (3)
Multiple methods with
the same name in the same class
(1)
When you use/invoke a method in your Java program, the Java compiler
will use the following information to identify which method you want to use:
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The method name (i.e., ClassName.MethodName)
The number and the type of the parameters that you specify in the method
invocation.
Multiple methods with
the same name in the same class
(2)
Defining the min method inside
the same class as the main method
A short hand for invoking a method
defined inside the same class