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    9781111530532 ppt ch07 9781111530532 ppt ch07 Presentation Transcript

    • Java Programming: From Problem Analysis to Program Design, 5e Chapter 7 User-Defined Methods
    • Chapter Objectives
      • 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
      Java Programming: From Problem Analysis to Program Design, 5e
    • Chapter Objectives (continued)
      • 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
      • Learn about the scope of an identifier
      • Become aware of method overloading
      Java Programming: From Problem Analysis to Program Design, 5e
    • Predefined Classes
      • Methods already written and provided by Java
      • Organized as a collection of classes (class libraries)
      • To use: import package
      • Method type: data type of value returned by method
      Java Programming: From Problem Analysis to Program Design, 5e
    • Predefined Classes (continued) Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e Predefined Classes (continued)
    • Java Programming: From Problem Analysis to Program Design, 5e Predefined Classes (continued)
    • Java Programming: From Problem Analysis to Program Design, 5e Predefined Classes (continued)
    • class Character (Package: java.lang ) Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e class Character (Package: java.lang ) (continued)
    • Java Programming: From Problem Analysis to Program Design, 5e class Character (Package: java.lang ) (continued)
    • Java Programming: From Problem Analysis to Program Design, 5e To simplify the use of (public) static methods of a class, Java 5.0 introduces the following import statements: These are called static import statements. After including such statements in your program, when you use a (public) static method (or any other public static member) of a class, you can omit the name of the class and the dot operator.
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Syntax: Value-Returning Method Java Programming: From Problem Analysis to Program Design, 5e
    • 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
      Java Programming: From Problem Analysis to Program Design, 5e
    • Syntax
      • Syntax: formal parameter list
        • -The syntax of the formal parameter list is:
      • Method call
        • -The syntax to call a value-returning method is:
      Java Programming: From Problem Analysis to Program Design, 5e
    • Syntax (continued)
      • Syntax: actual parameter list
        • -The syntax of the actual parameter list is:
      Java Programming: From Problem Analysis to Program Design, 5e
      • Syntax: return statement
        • -The return statement has the following syntax:
      • return expr;
    • Equivalent Method Definitions Java Programming: From Problem Analysis to Program Design, 5e public static double larger( double x, double y) { double max; if (x >= y) max = x; else max = y; return max; }
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Equivalent Method Definitions (continued)
      • public static double larger( double x, double y)
      • {
      • if (x >= y)
      • return x;
      • else
      • return y;
      • }
      Java Programming: From Problem Analysis to Program Design, 5e
    • Equivalent Method Definitions (continued)
      • public static double larger( double x, double y)
      • {
      • if (x >= y)
      • return x;
      • return y;
      • }
      Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e The int variable num contains the desired sum to be rolled
    • Palindrome Number
      • Palindrome: integer or string that reads the same forward and backward
      • The method isPalindrome takes a string as a parameter and returns true if the string is a palindrome, false otherwise
      Java Programming: From Problem Analysis to Program Design, 5e
    • Solution: isPalindrome Method Java Programming: From Problem Analysis to Program Design, 5e 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 ; }
    • 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 method call statement, specify only actual parameters, not data type or method type
      • Control goes back to caller when method exits
      Java Programming: From Problem Analysis to Program Design, 5e
    • 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 two numbers
        • For loop: calls method largest number on each number received and compares to current largest number
      Java Programming: From Problem Analysis to Program Design, 5e
    • Solution: Largest Number Java Programming: From Problem Analysis to Program Design, 5e static Scanner console = new Scanner(System.in); public static void main(String[] args) { double num; double max; int count; System.out.println(&quot;Enter 10 numbers.&quot;); num = console.nextDouble(); max = num; for (count = 1; count < 10; count++) { num = console.nextDouble(); max = larger(max, num); } System.out.println(&quot;The largest number is &quot; + max); }
    • Sample Run: Largest Number Java Programming: From Problem Analysis to Program Design, 5e
      • Sample Run
      • Enter 10 numbers:
      • 10.5 56.34 73.3 42 22 67 88.55 26 62 11
      • The largest number is 88.55
    • Void Methods
      • Similar in structure to value-returning methods
      • Call to method is always stand-alone statement
      • Can use return statement to exit method early
      Java Programming: From Problem Analysis to Program Design, 5e
    • Void Methods with Parameters: Syntax Java Programming: From Problem Analysis to Program Design, 5e
    • Void Methods with Parameters: Syntax (continued) Java Programming: From Problem Analysis to Program Design, 5e
    • 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
      Java Programming: From Problem Analysis to Program Design, 5e
    • 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 parameter refer to same object
      Java Programming: From Problem Analysis to Program Design, 5e
    • Uses of Reference Variables as Parameters
      • Can return more than one value from a method
      • Can change the value of the actual object
      • When passing address, would save memory space and time, relative to copying large amount of data
      Java Programming: From Problem Analysis to Program Design, 5e
    • Reference Variables as Parameters: type String Java Programming: From Problem Analysis to Program Design, 5e
    • Reference Variables as Parameters: type String (continued) Java Programming: From Problem Analysis to Program Design, 5e
    • Reference Variables as Parameters: type String (continued) Java Programming: From Problem Analysis to Program Design, 5e
    • Reference Variables as Parameters: type String (continued) Java Programming: From Problem Analysis to Program Design, 5e
    • Reference Variables as Parameters: type String (continued) Java Programming: From Problem Analysis to Program Design, 5e String str = &quot;Hello&quot;; //Line 5
    • Reference Variables as Parameters: type String (continued) Java Programming: From Problem Analysis to Program Design, 5e stringParameter(str); //Line 7
    • Reference Variables as Parameters: type String (continued) Java Programming: From Problem Analysis to Program Design, 5e pStr = &quot;Sunny Day&quot;; //Line 14
    • Reference Variables as Parameters: type String (continued) Java Programming: From Problem Analysis to Program Design, 5e Variables before the statement in Line 8 executes
    • Java Programming: From Problem Analysis to Program Design, 5e
      • The class StringBuffer contains the method append , which allows you to append a string to an existing string, and the method delete , which allows you to delete all the characters of the string
      • The assignment operator cannot be used with StringBuffer variables; you must use the operator new ( initially ) to allocate memory space for a string
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e
      • Primitive Type Wrapper Classes as Parameters
      • If a formal parameter is of the primitive data type and the corresponding actual parameter is a variable, then the formal parameter cannot change the value of the actual parameter
      • Only reference variables can pass values outside the method (except, of course, for the return value)
      • Corresponding to each primitive data type, Java provides a class so that the values of primitive data types can be wrapped in objects
      • The class Integer does not provide a method to change the value of an existing Integer object
      • The same is true of other wrapper classes
    • Java Programming: From Problem Analysis to Program Design, 5e
      • Primitive Type Wrapper Classes as Parameters (continued)
      • If we want to pass a String object as a parameter and also change that object, we can use the class StringBuffer
      • Java does not provide any class that wraps primitive type values in objects and when passed as parameters changes their values
      • If a method returns only one value of a primitive type, then you can write a value-returning method
      • If you encounter a situation that requires you to write a method that needs to pass more than one value of a primitive type, then you should design your own classes
      • Appendix D provides the definitions of such classes and shows how to use them in a program
    • Scope of an Identifier within a Class
      • Local identifier: identifier declared within a method or block, which is visible only within that method or block
      • Java does not allow the nesting of methods; 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; a block is a set of statements enclosed within braces
      Java Programming: From Problem Analysis to Program Design, 5e
    • Scope of an Identifier within a Class (continued)
      • A method’s definition can contain several blocks
        • The body of a loop or an if statement also form a block
      • Within a class, outside of every method definition (and every block), an identifier can be declared anywhere
      Java Programming: From Problem Analysis to Program Design, 5e
    • Scope of an Identifier within a Class (continued)
      • 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
      • For example, in the method definition on the next slide, the second declaration of the variable x is illegal
      Java Programming: From Problem Analysis to Program Design, 5e
    • Scope of an Identifier within a Class (continued)
      • public static void illegalIdentifierDeclaration()
      • {
      • int x;
      • //block
      • {
      • double x; //illegal declaration,
      • //x is already declared
      • ...
      • }
      • }
      Java Programming: From Problem Analysis to Program Design, 5e
    • Scope Rules
      • Scope rules of an identifier declared within a class and accessed within a method (block) of the class
      • An identifier, say x , 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
      Java Programming: From Problem Analysis to Program Design, 5e
    • Scope Rules (continued)
      • Suppose x is an identifier 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
      Java Programming: From Problem Analysis to Program Design, 5e
      • Example 7-11 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 (continued) Java Programming: From Problem Analysis to Program Design, 5e
      • public static int w;
      • public static void two( int one, int z)
      • {
      • char ch;
      • int a;
      • //block three
      • {
      • int x = 12;
      • //...
      • } //end block three
      • //...
      • }
      • }
      Scope Rules (continued) Java Programming: From Problem Analysis to Program Design, 5e
    • Scope Rules: Demonstrated Java Programming: From Problem Analysis to Program Design, 5e
    • Scope Rules: Demonstrated (continued) Java Programming: From Problem Analysis to Program Design, 5e
    • 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, or
        • If the number of formal parameters is the same, then the data type of the formal parameters, in the order you list, must differ in at least one position
      Java Programming: From Problem Analysis to Program Design, 5e
    • 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
      Java Programming: From Problem Analysis to Program Design, 5e
    • Method Overloading (continued)
      • 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
      Java Programming: From Problem Analysis to Program Design, 5e
    • Method Overloading (continued)
      • 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
      Java Programming: From Problem Analysis to Program Design, 5e
    • Method Overloading (continued)
      • The following method headings correctly overload the method methodXYZ :
      • public void methodXYZ()
      • public void methodXYZ( int x, double y)
      • public void methodXYZ( double one, int y)
      • public void methodXYZ( int x, double y,
      • char ch)
      Java Programming: From Problem Analysis to Program Design, 5e
    • Method Overloading (continued)
      • public void methodABC( in t x, double y)
      • public int methodABC( int x, double y)
      • Both these method headings have the same name and same formal parameter list
      • These method headings to overload the method methodABC are incorrect
      • In this case, the compiler will generate a syntax error
        • Notice that the return types of these method headings are different
      Java Programming: From Problem Analysis to Program Design, 5e
    • Programming Example: Data Comparison
      • Input: data from two different files
      • Data format: course number followed by scores
      • Output: course number, group number, course average
      • Solution
        • Read from more than one file, write output to file
        • Generate bar graphs
        • User-defined methods and re-use ( calculateAverage and printResult )
        • Parameter passing
      Java Programming: From Problem Analysis to Program Design, 5e
      • Sample Output
      • Course No Group No Course Average
      • CSC 1 83.71
      • 2 80.82
      • ENG 1 82.00
      • 2 78.20
      • HIS 1 77.69
      • 2 84.15
      • MTH 1 83.57
      • 2 84.29
      • PHY 1 83.22
      • 2 82.60
      • Avg for group 1: 82.04
      • Avg for group 2: 82.01
      Programming Example: Data Comparison (continued) Java Programming: From Problem Analysis to Program Design, 5e
    • Java Programming: From Problem Analysis to Program Design, 5e Programming Example: Data Comparison (continued)
    • Java Programming: From Problem Analysis to Program Design, 5e
      • A program may contain a number of methods. In a complex program, usually, when a method is written, it is tested and debugged alone.
      • You can write a separate program to test the method. The program that tests a method is called a driver program .
      • Before writing the complete program, you could write separate driver programs to make sure that each method is working properly.
    • Java Programming: From Problem Analysis to Program Design, 5e
      • Sometimes the results calculated by one method are needed in another method.
      • In that case, the method that depends on another method cannot be tested alone.
      • A method stub is a method that is not fully coded.
      • For a void method, a method stub might consist of only a method header and a set of empty braces, {}.
      • For a value-returning method it might contain only a return statement with a plausible return value.
    • Java Programming: From Problem Analysis to Program Design, 5e
      • If the problem is large and complex, it must be broken into subproblems, and if a subproblem is still complex, it must further be divided into subproblems.
      • The subdivision of a problem should continue to the point where the solution is clear and obvious.
      • Once a subproblem is solved, we can continue with the solution of another subproblem and if all the subproblems of a problem are solved, we can continue with the next level.
      • Eventually, the overall solution of the problem must be assembled and tested to ensure that the programming code accomplishes the required task.
    • Java Programming: From Problem Analysis to Program Design, 5e
      • A Java program is a collection of classes, and a class is a collection of data members and methods.
      • Each class and each method must work properly.
      • To accomplish this, as explained in the previous section, once a method is written, it can be tested using stubs and drivers.
      • Since a method can be tested in isolation, it is not necessary to code all the methods in order.
      • Once all the methods are written, the overall program must be tested.
    • Java Programming: From Problem Analysis to Program Design, 5e
      • The technique to solve a problem by subdividing into smaller problems is known as divide and conquer and top-down design approach.
      • These techniques are suitable and work for many kinds of problems, including most of the problems given in this book and the problems you will encounter as a beginning programmer.
      • To simplify the overall solution of a problem that consists of many subproblems, we write and test the code one piece at a time.
      • Typically, once a subproblem is solved and the code is tested, it is saved as the first version or a version of the program.
      • We continue to add and save the program one piece at a time. Keep in mind that a working program with fewer features is better than a nonworking one with many features.
    • Chapter Summary
      • Predefined methods
      • User-defined methods
        • Value-returning methods
        • Void methods
        • Formal parameters
        • Actual parameters
      • Flow of execution
      Java Programming: From Problem Analysis to Program Design, 5e
    • Chapter Summary (continued)
      • Primitive data type variables as parameters
        • One-way link between actual parameters and formal parameters (limitations caused)
      • Reference variables as parameters
        • Can pass one or more variables from a method
        • Can change value of actual parameter
      • Scope of an identifier within a class
      • Method overloading
      Java Programming: From Problem Analysis to Program Design, 5e
    • Chapter Summary (continued) Java Programming: From Problem Analysis to Program Design, 5e
      • Debugging: using drivers and stubs
      • Avoiding bugs: one-piece-at-a-time coding