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An Introduction to Java Programming and Object-Oriented Application Development

An Introduction to Java Programming and Object-Oriented Application Development. Chapter 5 Methods and Classes. Objectives. In this chapter you will: Learn about the two types of methods in a Java program: predefined methods and programmer-defined methods

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An Introduction to Java Programming and Object-Oriented Application Development

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  1. An Introduction to Java Programming and Object-Oriented Application Development Chapter 5 Methods and Classes

  2. Objectives In this chapter you will: • Learn about the two types of methods in a Java program: predefined methods and programmer-defined methods • Explore how to use a wide variety of predefined Java methods in the Math, String, NumberFormat, and GregorianCalendar classes • Learn how to create and apply programmer-defined methods within programs An Introduction to Java Programming and Object-Oriented Application Development

  3. Objectives (continued) • Explore the differences in static and instance class members • Learn how to pass variables to methods by value and by reference • Understand the scope of variables declared within a Java program An Introduction to Java Programming and Object-Oriented Application Development

  4. Methods and Classes • There are two basic kinds of methods in Java • Predefined methods • Programmer-defined methods • An argument is a literal variable or expression that is passed or sent when a method is called • If several arguments are included, they form an argument list • Example: JOptionPane.showInputDialog(menu) An Introduction to Java Programming and Object-Oriented Application Development

  5. Methods and Classes (continued) An Introduction to Java Programming and Object-Oriented Application Development

  6. Predefined Java Methods • Java contains predefined classes organized in packages • More than 160 packages and 3000 classes • The Java API http://java.sun.com/j2se/1.5.0/docs/api/ • Classes have two types of members • Method members • Data members An Introduction to Java Programming and Object-Oriented Application Development

  7. Working with Mathematical Functions • The class Math is in java.lang • The java.lang package does not need an import statement • To call a method in Math use this syntax: Math.<method name>() • The class name followed by the member access operator followed by parentheses • Parentheses may be empty or contain arguments • The member access operator is the dot operator An Introduction to Java Programming and Object-Oriented Application Development

  8. Working with Mathematical Functions (continued) • Example: pval = Math.pow(num1,num2); • JVM looks in the class Math of the method pow • The arguments are num1 and num2 • pow raises num1 to the power of num2 • The result is returned and stored in pval • Math has two fields • E, the natural logarithm base: 2.71828183 • PI, with value 3.14159265 • Access fields with the dot operator: Math.PI An Introduction to Java Programming and Object-Oriented Application Development

  9. Working with Mathematical Functions (continued) • A static method is a general method of a class • A static field is a general field of a class • Can be used without creating an object • An instance variable is owned by an individual object • An instance method can only be called by an object, not by a class An Introduction to Java Programming and Object-Oriented Application Development

  10. Working with Strings • The class String is in the package java.lang • The String class contains constructors • A constructor is a special method to create an object from a class • Math has no constructors; all methods are static • Most methods in String are instance methods An Introduction to Java Programming and Object-Oriented Application Development

  11. Working with Strings (continued) • Stringstr1=“Hello”; is a shortcut for Stringstr1=newString ({‘H’,‘e’,‘l’,‘l’,‘o’}); • Since str1 is an object of the String class, all methods in String can be called by it • Example: str1.substring(3); • Notice that the String object is calling the method (str1 calls substring) An Introduction to Java Programming and Object-Oriented Application Development

  12. Working with Strings (continued) • Static methods are called by the class: StringmyPi=String.valueOf(Math.PI); • Instance methods are called by an object of the class: String myL = “Hello”; int len = myL.length(); • The first character in a string has index 0 • myL.indexOf(‘e’) returns 1 An Introduction to Java Programming and Object-Oriented Application Development

  13. Formatting Numbers • The class NumberFormat has methods to format numbers for output • Complicated operations like rounding or converting a number to a percentage can be done using predefined methods • Using predefined methods makes code more accurate and reliable An Introduction to Java Programming and Object-Oriented Application Development

  14. Formatting Numbers (continued) • Create a NumberFormat object: NumberFormatmyNum=NumberFormat.getNumberInstance(); • Use the object to call additional (nonstatic) methods required for formatting: myNum.setMaximumFractionDigits(2); • Call the method format, with the literal or variable to be formatted: myNum.format(aDecimal); An Introduction to Java Programming and Object-Oriented Application Development

  15. Working with Times and Dates • The classes Calendar, GregorianCalendar and Date create time and date objects to use in processing • Calendar objects are created using the static method getInstance: Calendartoday=Calendar.getInstance(); • get accesses the MONTH, DAY, and YEAR fields in Calendar: int day = today.get(Calendar.MONTH); An Introduction to Java Programming and Object-Oriented Application Development

  16. Working with Times and Dates (continued) • Calendar is useful for processing information about specific dates and times • GregorianCalendar inherits all members of Calendar • public members of Calendar are accessible to GregorianCalendar • Create a GregorianCalendar object: GregorianCalendar today = new GregorianCalendar(); An Introduction to Java Programming and Object-Oriented Application Development

  17. Working with Times and Dates (continued) • The keyword new is the most common way of creating Java objects in memory • The class DateFormat formats date for output • DateFormat is in the package java.text • The output of Example 5.1 is 5 An Introduction to Java Programming and Object-Oriented Application Development

  18. Working with Times and Dates (continued) An Introduction to Java Programming and Object-Oriented Application Development

  19. Working with Times and Dates (continued) An Introduction to Java Programming and Object-Oriented Application Development

  20. Apply the Concept • Develop an application to calculate the value of an investment at retirement • Extract the date information from the input string • Use GregorianCalendar to convert date strings to date values for calculating interest • Interest is calculated using a financial formula • Output is formatted for display An Introduction to Java Programming and Object-Oriented Application Development

  21. Apply the Concept (continued) • Trim leading and trailing white space from the input name = name.trim(); • Extract information by looking for white space lastName = name.substring (i+1,name.length()); • Convert strings to integers month=Integer.parseInt(monthStr)–1; An Introduction to Java Programming and Object-Oriented Application Development

  22. Apply the Concept (continued) • The variable month is decremented because Java months start at 0 • GregorianCalendar objects convert dates to milliseconds and determine the number of years between to time values • Convert milliseconds to years with multiple divisions, ignoring leap years • Retirement amount is calculated by a standard formula An Introduction to Java Programming and Object-Oriented Application Development

  23. Apply the Concept (continued) • Retirement amount formatted as currency • Retirement date formatted as a long date • Date entered by user as 6/1/2018 displayed as “June 1, 2018” • Retirement amount displayed with $ and two decimal places An Introduction to Java Programming and Object-Oriented Application Development

  24. Programmer-Defined Methods • Previous section covered data members and method members of Java classes • Learned how to use predefined classes • Introduced inheritance (covered later) • Next: Programmer-defined methods An Introduction to Java Programming and Object-Oriented Application Development

  25. Overview of Programmer-Defined Methods • Methods make code more readable and allow the programmer to focus on smaller tasks • Complex projects can be delegated to many programmers • Code that will be executed multiple times can be modularized, making it less error-prone • Methods make code easier to maintain and reuse • Modularity is central to OO development An Introduction to Java Programming and Object-Oriented Application Development

  26. Anatomy of Programmer-Defined Methods and Method Calls • The code for predefined methods is not available to the programmer • Programmer-defined methods use a syntax like main • First line of code is the method heading • Access modifier: public • If the method is a class method: static • Return type: void (or String, int, Calendar…) • Method name: main • Parameter list: (String args[]) An Introduction to Java Programming and Object-Oriented Application Development

  27. Anatomy of Programmer-Defined Methods and Method Calls (continued) • The method name and parameter list are the method signature • The method body follows the method heading, enclosed in curly braces • The method body is a code block • If the method is never called, the method body never executes • Order of methods in the class does not matter An Introduction to Java Programming and Object-Oriented Application Development

  28. Passing Data to Methods: Primitive Variables and Reference Variables • Can pass data to methods using literals, such as findMax(2, 3) • Can pass data to methods using variables, as in findMax(x, y) • When variables are passed to methods, they are passed by value • Actual values of the variables are passed An Introduction to Java Programming and Object-Oriented Application Development

  29. Passing Data to Methods: Primitive Variables and Reference Variables (continued) • Variables in method headings are different memory locations than variables in method calls • A nonprimitive variable is a reference variable because it refers to an object • A primitive variable holds a value • When a method call passes a reference variable, the variable is passed by reference • The memory location of the variable is sent An Introduction to Java Programming and Object-Oriented Application Development

  30. The Scope of Variables • A Java file contains one or more classes and a class contains one or more methods • A method contains blocks of code enclosed in curly braces • A variable declared in the class outside of a method is available throughout the class • A variable declared in a code block or parameter list has scope only within the block or method • When program flow leaves a block, variables with local scope are destroyed An Introduction to Java Programming and Object-Oriented Application Development

  31. Apply the Concept • Modify code from Chapter 4 (Figure 4.24) to incorporate programmer-defined methods • The only difference is that some of the processing logic is located in separate methods • The method calculateAndDisplayAvg is called with arguments totalIncome, numberOfEntries, and currencyFormatter • The parameters in the method heading have slightly different names to avoid confusion An Introduction to Java Programming and Object-Oriented Application Development

  32. Apply the Concept (continued) An Introduction to Java Programming and Object-Oriented Application Development

  33. Apply the Concept (continued) An Introduction to Java Programming and Object-Oriented Application Development

  34. Apply the Concept (continued) • Using currencyFormatter in calculateAndDisplayAvg without passing it, as an argument generates a compiler error An Introduction to Java Programming and Object-Oriented Application Development

  35. Apply the Concept (continued) An Introduction to Java Programming and Object-Oriented Application Development

  36. Case Study: MusicWorld • MusicWorldApp4 allowed a user to enter data for multiple-item CD order • Displayed total with quantity discount and tax • MusicWorldApp5.java will modularize the program • Currency and Percent formatting will improve the display An Introduction to Java Programming and Object-Oriented Application Development

  37. The Flowchart of MusicWorldApp5 • The flowchart is virtually identical to MusicWorldApp4 • Two methods accomplish the same functionality but make the code more readable An Introduction to Java Programming and Object-Oriented Application Development

  38. The Flowchart of MusicWorldApp5 (continued) An Introduction to Java Programming and Object-Oriented Application Development

  39. Program Code for MusicWorldApp5 • Create a DecimalFormat object by casting NumberFormat • DecimalFormat dF = (DecimalFormat NumberFormat.getPercentInstance(); • The method applyPattern in the class DecimalFormat • # represents integers, 0 represents decimal places and % represents percentage • dF.applyPattern(“###.000%”); An Introduction to Java Programming and Object-Oriented Application Development

  40. Program Code for MusicWorldApp5 (continued) • Method calls to displayItemOutput • Different variable names in the method call and the method header make the code more readable • Return statement sends back a reference to a String to main • Method displayItemOutput calculates the order total • Output is virtually the same, but the currency is formatted in a more acceptable way An Introduction to Java Programming and Object-Oriented Application Development

  41. Summary • Two kinds of methods in Java • Predefined methods in the Java API • Programmer-defined methods • Methods are executed when they are called • An argument is passed to a method when it is called • Static members are accessed by the class • Instance members are accessed by objects in the class An Introduction to Java Programming and Object-Oriented Application Development

  42. Summary (continued) • NumberFormat is in java.text and has methods to format currency, percentages, and decimals • Calendar and GregorianCalendar are in java.util and have methods to format times and dates • A method definition consists of a method heading followed by program code within {} • The parameter list in a method call must agree in number and data type and order with the method definition An Introduction to Java Programming and Object-Oriented Application Development

  43. Summary (continued) • Primitive variables represent primitive data types and are passed by value • Reference variables represent objects and are passed by reference • A variable that is defined in a method or block only has scope within that block • Variables declared within a class, outside of a method, are accessible everywhere in the class An Introduction to Java Programming and Object-Oriented Application Development

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