Methods.ppt
- 1. Chapter 4: Methods Asserting Java ©Rick Mercer Methods
- 2. Methods We have used some existing methods without fully understanding their implementation – System.out's print, println – String's length, charAt, indexOf, toUpperCase – Scanner's nextDouble, nextInt – BankAccount's withdraw, deposit Java has thousands of methods – We often need to create our own
- 3. Methods There are two major components to a method – the method heading with access mode, return type, name, parameters – the block a pair of curly braces containing code that fulfills the method's responsibility Method headings specify the number and types of arguments required to use the method
- 4. Method Heading with documentation /* * Return a new string that is a substring of this string. * The substring begins at the specified beginIndex and * extends to the character at index endIndex-1. * Thus the length of the substring is endIndex-beginIndex. * * Examples: * "hamburger".substring(4, 8) returns "urge" * "smiles".substring(1, 5) returns "mile" * * Parameters: * beginIndex - the beginning index, inclusive. * endIndex - the ending index, exclusive. * * Returns: the specified substring. */ public String substring(int beginIndex, int endIndex)
- 5. Using JUnit to demo substring
- 6. What method headings tell us Method headings provide the information needed to use it they show us how to send messages public String substring(int beginIndex, int endIndex) 1 2 3 5 4 5 4 1 Where is the method accessible 2 What does the method evaluate to? 3 What is the method name? 4 What type arguments are required? 5 How many arguments are required?
- 7. Arguments are assigned to parameters The substring method requires two arguments in order to specify the portion of the string to return When the message is sent – the 1st argument 0 is assigned to parameter beginIndex – the 2nd argument 6 is assigned to parameter endIndex fullName.substring(0, 6); public String substring(int beginIndex, int endIndex) Implementation of the method is not shown here
- 8. Arguments Parameters When a message is sent – the first argument is assigned to the first parameter, – second argument gets assigned to the second parameter,... If you do not supply the correct number and type of arguments, you get compiletime errors fullName.substring("wrong type"); fullName.substring(0, 6, fullName.length()); fullName.substring(); fullName.substring(0.0, 6.0); BTW: This returns the string form index to the end fullName.substring(2); // sometimes convenient
- 9. Method Heading: General Form General form of a Java method heading public return-type method-name ( parameter-1, parameter-2, parameter-n, ... ) – public says a method is known where objects are constructed – return-type may be any primitive type, any class, or void – A void method returns nothing, therefore, – a void method can not be assigned to anything – a void method can not be printed with println
- 10. Method Headings Example method headings think of class as type public char charAt(int index) // class String public int indexOf(String sub) // class String public void withdraw(double amt) // class BankAccount public String getText() // class Jbutton public String setText(String str) // class Jbutton public void setSize(int x, int y) // class JFrame public int nextInt() // class Scanner public int nextDouble() // class Scanner public int next() // class Scanner public int nextLine() // class Scanner
- 11. Parameters Parameters, which are optional, specify the number and type of arguments required in the message – Sometimes methods need extra information – How much to deposit? – substring need to know begin- and end-indexes? General form of a parameter between ( and ) in method headings class-name identifier -or- primitive-type identifier
- 12. The Block The method body is Java code enclosed within a block { } Curly braces have the same things we've seen in main methods – variable declarations and initializations int creditOne = 0; – objects String str = "local"; – messages boolean less = str.compareTo("m") < 0; Method bodies have access to parameters – Hence, methods are general enough to be reused with many different arguments
- 13. The return statement All non-void methods must return a value – The type of the value is defined in the method heading Use Java's return statement return expression ; Example in the context of a method's block public double f(double x) { return 2.0 * x - 1.0; }
- 14. Code Demo Given the following documented method heading, – Write a test method in ControlFunTest.java The assertions are expected to fail – Write the actual method in ControlFun.java /* * Return largest of 3 integer arguments * max(1, 2, 3) returns 3 * max(1, 3, 2) returns 3 * max(-1, -2, -3) returns -1 */ public int max(int a, int b, int c) { return 0; }
- 15. Methods: A Summary Method headings provide this information on usage: – is the method is available from other places in code? public methods are known in the block where constructed – return-type the kind of value a message evaluates to – method-name that begins a valid method call – parameter-list the number and type of needed arguments – documentation to describe what the method does The block is where your algorithm gets implemented The parameters are accessible within the block Methods usually return a value of the correct type Methods must be fully tested (at least in this course)