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Java Programming: From Problem Analysis to Program Design, Second Edition 1
Chapter 7: User-Defined Methods
Methods in Java programming.
Standard (predefined) methods.
User-defined methods.
Value-returning and void methods
Scope of an identifier.
Method overloading
Java Programming: From Problem Analysis to Program Design, Second Edition 2
Predefined Classes
Methods already written and provided by
Java.
Organized as a collection of classes (class
libraries).
To use, import package.
Method type: The data type of the value
returned by the method.
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Predefined Classes
Java Programming: From Problem Analysis to Program Design, Second Edition 4
Predefined Classes
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class Character (Package: java.lang)
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Syntax of Value-Returning Method
modifier(s) returnType methodName (formal parameter list) { statements }
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User-Defined Methods
Value-returning methods: Used in expressions. Calculate and return a value. Can save value for later calculation or print value. modifiers: public, private, protected, static, abstract, final. returnType: Type of the value that the method calculates and returns (using return statement). methodName: Java identifier; name of method.
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Syntax
Syntax of formal parameter list:
dataType identifier, dataType identifier,...
Syntax to call a value-returning method:
methodName(actual parameter list)
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Equivalent Method Definitions
public static double larger(double x, double y) { if (x >= y) return x; else return y; }
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Equivalent Method Definitions
public static double larger(double x, double y) { if (x >= y) return x;
return y; }
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Programming Example:
Palindrome Number
Palindrome: An integer or string that reads
the same forwards and backwards.
Input: Integer or string.
Output: Boolean message indicating whether
integer string is a palindrome.
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Solution: isPalindrome Method
public static boolean isPalindrome(String str) { int len = str.length(); int i, j; j = len - 1; for (i = 0; i <= (len - 1) / 2; i++) { if (str.charAt(i) != str.charAt(j)) return false; j--; } return true; }
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Flow of Execution
Execution always begins with the first statement in the method main. User-defined methods execute only when called. Call to method transfers control from caller to called method. In the method call statement, specify only actual parameters, not data type or method type. Control goes back to caller when method exits.
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Programming Example: Largest
Number
Input: Set of 10 numbers Output: Largest of 10 numbers Solution: Get numbers one at a time. Method largest number: Returns the larger of 2 numbers. For loop: Calls method largest number on each number received and compares to current largest number.
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Solution: Largest Number
static Scanner console = new Scanner(System.in); public static void main(String[] args) { double num; double max; int count; System.out.println("Enter 10 numbers."); num = console.nextDouble(); max = num; for (count = 1; count < 10; count++) { num = console.nextDouble(); max = larger(max, num); } System.out.println("The largest number is "
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Sample Run: Largest Number
Sample Run: Enter 10 numbers: 10.5 56.34 73.3 42 22 67 88.55 26 62 11 The largest number is 88.
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Void Methods with Parameters: Syntax
Method definition: The definition of a void method with parameters has the following syntax: modifier(s) void methodName (formal parameter list) { statements } Formal parameter list: The formal parameter list has the following syntax: dataType variable, dataType variable, ...
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Void Methods with Parameters:
Syntax
Method call: The method call has the following syntax: methodName(actual parameter list);
Actual parameter list: The actual parameter list has the following syntax: expression or variable, expression or variable, ...
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Primitive Data Type Variables as
Parameters
A formal parameter receives a copy of its
corresponding actual parameter.
If a formal parameter is a variable of a
primitive data type:
Value of actual parameter is directly stored. Cannot pass information outside the method. Provides only a one-way link between actual parameters and formal parameters.
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Reference Variables as Parameters
If a formal parameter is a reference variable:
Copies value of corresponding actual parameter. Value of actual parameter is address of the object where actual data is stored. Both formal and actual parameters refer to same object.
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Scope of an Identifier within a Class Local identifier: An identifier that is declared within a method or block and that is visible only within that 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 method or a block, an identifier must be declared before it can be used. Note that a block is a set of statements enclosed within braces. A method’s definition can contain several blocks. The body of a loop or an if statement also forms a block. Within a class, outside of every method definition (and block), an identifier can be declared anywhere.
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Scope of an Identifier within a Class
Within a method, an identifier that is 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. For example, in the following method definition, the second declaration of the variable x is illegal:
public static void illegalIdentifierDeclaration() { int x; //block { double x; //illegal declaration, //x is already declared ... } }
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Scope Rules
Scope rules of an identifier that is declared within a class and accessed within a method (block) of the class. An identifier, say X, that is declared within a method (block) is accessible: Only within the block from the point at which it is declared until the end of the block. By those blocks that are nested within that block. Suppose X is an identifier that is declared within a class and outside of every method’s definition (block). If X is declared without the reserved word static (such as a named constant or a method name), then it cannot be accessed in a static method. If X is declared with the reserved word static (such as a named constant or a method name), then it can be accessed within a method (block) provided the method (block) does not have any other identifier named X.
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Example 7- public class ScopeRules { static final double rate = 10.50; static int z; static double t; public static void main(String[] args) { int num; double x, z; char ch; //... } public static void one(int x, char y) { //... }
Scope Rules
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Method Overloading: An
Introduction
Method overloading: More than one method can have the same name. Two methods are said to have different formal parameter lists : If both methods have a different number of formal parameters. If the number of formal parameters is the same in both methods, the data type of the formal parameters in the order you list must differ in at least one position.
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Method Overloading
public void methodOne(int x) public void methodTwo(int x, double y) public void methodThree(double y, int x) public int methodFour(char ch, int x, double y) public int methodFive(char ch, int x, String name)
These methods all have different formal parameter lists.
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Method Overloading
public void methodSix(int x, double y, char ch) public void methodSeven(int one, double u, char firstCh)
The methods methodSix and methodSeven both have three formal parameters, and the data type of the corresponding parameters is the same.
These methods all have the same formal parameter lists.
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Method Overloading
Method overloading: Creating several methods within a class with the same name.
The signature of a method consists of the method name and its formal parameter list. Two methods have different signatures if they have either different names or different formal parameter lists. (Note that the signature of a method does not include the return type of the method.)
