Chapter 7
User-Defined Methods
Chapter Overview
In this chapter, students will learn more about predefined methods and using them in Java programs as well
as user-defined methods. Several methods can be put together to form larger programs.
Chapter Objectives
In this chapter, students will:
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Understand how methods are used in Java programming
Learn about standard (predefined) methods and discover how to use them in a program
Learn about user-defined methods
Examine value-returning methods, including actual and formal parameters
Explore how to construct and use a value-returning, user-defined method in a program
Learn how to construct and use user-defined void methods in a program
Explore variables as parameters
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Learn about the scope of an identifier
Become aware of method overloading
Introduction
In Java, there are predefined methods that are already written and provided and user-defined methods,
methods that you create. While working on one method, you can focus on just that part of the program and
construct it, debug it, and perfect it. Different people can work on different methods simultaneously. If a
method is needed in more than one place in a program, or in different programs, you can write it once and
use it many times. Using methods greatly enhances the program’s readability because it reduces the
complexity of the method main.
Predefined Methods
In Java, predefined methods are organized as a collection of classes, called class libraries. The class Math
contained in the package java.lang contains mathematical methods. The method type is the data type of the
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value returned by the method. Some predefined methods in the class Math are:
Math.abx(x)
Math.ceil(x)
Math.exp(x)
Math.floor(x)
Math.log(x)
Math.max(x, y)
Math.min(x, y)
Math.pow(x, y)
Math.round(x)
Math.sqrt(x)
Math.cos(x)
Math.sin(x)
Math.tan(x)
The class Character, also in the package java.lang contains the following methods:
Character.isDigit(ch)
Character.isLetter(ch)
Character.isLowerCase(ch)
Character.isUpperCase(ch)
Character.isSpaceChar(ch)
Character.isWhitespace(ch)
Character.toLowerCase(ch)
Character.toUpperCase(ch)
In general, to use predefined methods of a class in a program, you must import the class from the package
containing the class. By default Java automatically imports classes from the package java.lang. Therefore,
if you use a method from the class Math or Character you do not need to explicitly import these classes in
your program.
User-Defined Methods
There are two types of user-defined methods:
value-returning methods and void methods.
Value-returning methods, used in expressions, calculate and return a value. They methods are usually used
to save the value for further calculation, use the value in a calculation or to print the value. To use these
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methods you must know: the heading of the method (the name of the method, the number of parameters (if
any), the data type of each parameter, and the type of the method), and the body of the method (code required
to accomplish the task). In other words, you must know the definition of the method. For predefined
methods you only need to know the heading for ones you create, you must know the entire definition.
The variable declared in the heading, within the parentheses, of the method is called the formal parameter of
the method. The actual parameter is the variable or expression listed in a call to the method.
The syntax of a value-returning method is:
modifier(s) returnType methodName(formal parameter list)
{
statements
}
The modifier(s) indicates the visibility of the method; where in a program the method can be used (called).
Some of the modifiers are public, private, protected, static, abstract, and final. If you include more than one
modifier, they must be separated with spaces. You can select one modifier among public, protected, and
private. The modifier public specifies that the method can be called outside the class, and the modifier private
specifies that the method cannot be used outside the class. Similarly, you can choose one of the modifiers
static or abstract.
The returnType is the type of value that the method calculates and returns. This type is also called the type of
the value-returning method.
The methodName is a Java identifier, giving a name to the method, and statements enclosed between curly
braces form the body of the method.
The syntax of the formal parameter list is:
dataType identifier, dataType identifier, …
The syntax to a value-returning method call is:
methodName(actual parameter list)
If the parameter list is empty the parentheses remain empty. The syntax of the actual parameter list is:
Expression or variable, expression or variable, …
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In a method call, the number of actual parameters, together with their data types, must match the formal
parameters in the order given.
Once a value-returning method computes the value, the method returns this value via the return statement.
The syntax for the return statement uses reserved word return and is:
return expr;
where expr (a variable, constant, or expression) is evaluated and returned. The data type of the value that
expr computes must match the method type. When a return statement executes in a method, the method
immediately terminates and the control goes back to the caller.
Programming Example: Palindrome Number
An integer is a palindrome if it reads backwards and forwards the same way.
The input to this program is an integer (positive or negative). The output to this program is a message
indicating whether the integer is a palindrome.
The solution to this program consists of writing a method isPalindrome. The method returns a Boolean
value. Its only parameter is the integer string that needs to be checked. The body of the method gets the
length of the string and using a for loop compares the first half of the string to the last half one character at a
time. If the first and last character don’t match the method returns false. Otherwise, it increments the
position of the first character being checked and decrements the position of the last character checked and
compares them. This is done until the characters no longer match. If they match till the end of the loop
the method returns true.
Flow of Execution
When the program executes, the execution always begins with the first statement in the method main.
User-defined methods execute only when they are called. A call to a method transfers control from the
caller to the called method. In a method call statement, you specify only the actual parameters, not their
data type or the method type. When a method exits, the control goes back to the caller.
Programming Example: Largest Number
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The input to this program is a set of 10 numbers and the output is the largest of the numbers. The program
can be easily modified to accommodate any set of numbers.
The solution to this program consists of writing a method that compares two numbers and returns the larger
of the two. This method is then called in a for loop in the main method of the program. The first number
entered is stored as the maximum, and as each new number is entered it is compared to the current maximum.
The maximum is then replaced if the new number is larger than the current maximum.
Void Methods
Void methods do not have a data type. The syntax for the method without parameters is as follows:
modifier(s) void methodName()
{
statements
}
A call to a void method is a stand-alone statement. The method call for void methods, within the class has
the following syntax:
methodName();
void methods without parameters are often used for displaying information about the program or for printing
statements.
The syntax for a void method with parameters is as follows:
modifier(s) void methodName(formal parameter list)
{
statements
}
where the formal parameter list has the syntax:
dataType variable, dataType variable, …
The method call would have the syntax:
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methodName(actual parameter list);
Where the actual parameter list would have the syntax:
expression or variable, expression or variable, …
Quick Quiz
1. True or False: Void methods do not have a type.
Answer: True
2. A(n) ____ parameter is a variable declared in the method heading.
Answer: formal
3. True or False: To use the method Math.sqrt(x) you must import the package java.lang.
Answer: False
4.
True or False: Void methods are always stand-alone statements.
Answer: True
5. An integer or string is a(n) ____ if it reads the same forwards and backwards.
Answer: palindrome
Primitive Data Type Variables as Parameters
When a method is called, the value of the actual parameter is copied into the corresponding formal parameter.
If a formal parameter is a variable of a primitive data type, then after copying the value of the actual
parameter, there is no connection between the formal parameter and the actual parameter; a formal parameter
of the primitive type cannot pass any result back to the calling method. When the method executes, any
changes made to the formal parameters do not in any way affect the actual parameters. The actual parameter
has no knowledge of what is happening to the formal parameter. Thus, formal parameters of the primitive
data types cannot pass information outside the method. Formal parameters of the primitive data types provide
only a one-way link between actual parameters and formal parameters.
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Reference Variables as Parameters
A reference variable contains the address (memory location) of the actual data; both the formal and the value
parameters refer to the same object. Therefore, reference variables can pass one or more values from a
method and can change the value of the actual parameter.
Reference variables, as parameters are useful in three situations:
1. When you want to return more than one value from a method
2. When the value of the actual object needs to be changed
3. When passing the address would save memory space and time, relative to copying
a large amount of data
When a method is called, memory for its formal parameters and local variables is allocated in the method
data area. The value of the actual parameter is copied into the memory cell of its corresponding formal
parameter.
In the case of reference variables of the type String, we can use the assignment operator to allocate memory
space to store a string and assign the string to a String variable. If you pass a String variable (a reference
variable of the String type) as a parameter to a method, and within the method you use the assignment
operator to change the string, the string assigned to the actual parameter does not change; a new string is
assigned to the formal parameter.
If you want to pass strings as parameters to a method and want to change the actual parameter, you can use
the class StringBuffer, where strings assigned to StringBuffer variables can be altered using various methods
provided to manipulate strings (e.g. append, delete).
Scope of an Identifier Within a Class
The scope of an identifier refers to what other parts of the program can “see” an identifier; where it is
accessible (visible). A local identifier is an identifier declared within a method or block that can be accessed
only by code within that same method or block. Java does not allow the nesting of methods. That is, you
cannot include the definition of one method in the body of another method.
Within a class, an identifier is accessible throughout the class, with the exception that a static method cannot
access nonstatic identifiers. The identifier declared within a block is accessible:
1.
Only within the block from the point at which it is declared until the end of the block.
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2. By those blocks that are nested within that block if the nested block does not have an identifier with the
same name as that of the outside block (the block that encloses the nested block).
Within a method, an identifier used to name a variable in the outer block of the method cannot be used to
name any other variable in an inner block of the method. Within a class, any method can call any other
method, with the exception that a static method cannot call a nonstatic method. A variable can be declared in
the initialization statement of the for statement; the scope of the variable is limited only to the body of the
loop.
Method Overloading: An Introduction
Method overloading is creating several methods, within a class, with the same name. If several methods
within a class have the same name, every method must have a different number of formal parameters, or if
the number of formal parameters is the same, then the data type of the formal parameter, in the order listed,
must differ in at least one position. The types of parameters determine which method to execute.
Programming Example: Data Comparison
The input to this program is two different data files each with a course number followed by a series of scores.
The output is a file containing the course numbers, group numbers, and the course average for each group in
each course.
The solution to this program consists of reading data from two different files and printing the results to an
output file. It also contains the user-defined methods calculateAverage and printResult.
Quick Quiz
1. ____ variables are useful when you want to return more than one value from a method.
Answer: Reference
2. True or False: It is possible to create several methods with the same name in a single class.
Answer: True
3. A(n) ____ identifier is an identifier declared within a method or block that can be accessed only by code
within that same method or block.
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Answer: local
4. True or False: A method can be defined within the body of another method.
methods.
Answer: False
This is known as nesting
5. True or False: In the case of reference variables of the type String, we can use the assignment operator
to allocate memory space to store a string and assign the string to a String variable.
Answer: True
Discussion Questions
Some interesting topics of discussion in Chapter Seven include:
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Discuss examples of user-defined methods.
Discuss common errors when identifying the scope of identifiers.
Discuss how to overload methods correctly and when overloading is useful.
Projects to Assign
1. Assign odd Exercises.
2. Assign Programming Exercises 14 or 15.
material.
These are fairly extensive, but cover a lot of important
Key Terms
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Actual parameter: a variable or expression listed in a call to a method.
Ø Class libraries: organized collection of classes.
Ø Formal parameter: a variable declared in the method heading.
Ø Local identifier: an identifier declared within a method or block that can be accessed only by code
within that same method or block.
Ø Local variables: variables declared in the body of the method.
Ø Method type: data type of the value returned by the method.
Ø Modules: another name for methods.
Ø Palindrome: integer or string that reads the same forwards and backwards.
Ø Parameters: arguments of a method.
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Predefined methods: methods that are already written and provided by Java.
Scope: refers to what other parts of the program can “see” an identifier.
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User-defined methods: methods that the programmer creates.
Value-returning methods: methods that have a return type.
Void methods: methods that do not have a return type.
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