Introduction To Scientific Programming

Introduction To Scientific Programming Chapter 5 – More About Objects and Methods

Overview – More, More, More ! • More on Methods • Referencing • Static Methods (and Variables) • Overloading • More on Classes • Wrapper Classes • Constructors • Groups of Classes - Packages • More on Software Design • Top-Down Design • Using Methods: Main/Helper/Class Interface

I. More On Methods - Referencing • What are the possibilities? • Reference a method outside of its’ class • Reference a method from inside its’ class • Also must consider lifetime: • Reference a method before a class object has been created • Reference a method after a class object has been created

When an Object Name Is Required • Public methods called outside and after an object definition has occurred simply require the object name to precede the method name. • For example: //Define object myOracle Oracle myOracle = new Oracle(); ... //dialog is a method defined in Oracle class myOracle.dialog(); ...

When An Object Name Is Not Required • What if you want to use another method within the same class? • You would need objectName, but it doesn’t exist yet. • Answer: this. refers to the object that contains the current reference. Essentially, this. stands in for the object name. • You may either use this., or omit it, since it is presumed – i.e. methods called within an class definition file do not need to reference itself.

Class Method Reference Example public class AddressBook { private final int MAX_ENTRIES = 100; //Fixed size limit of 100 private String BookName; private int numberOfEntries; … //--------------------------------------------------------------------------- // Method to find and display an entry in address book. //--------------------------------------------------------------------------- public boolean emptyBook() { if (this.numberOfEntries == 0) then //this. is optional return true; else return false; } //--------------------------------------------------------------------------- // Method that copies contents of AddressBook object //--------------------------------------------------------------------------- public void makeEqual(AddressBook otherObject) { otherObject.BookName = this.BookName; otherObject.numberOfEntries = this.numberOfEntries; otherObject.updateDate = this.updateDate; for (int i=1; i<this.numberOfEntries; i++) { If !(emptybook) then //this. not used, o.k.! … } } }

I.-B. Static Methods • You’ve thought/seen that some methods don't need an object to do their job: • Ex. a method to calculate the area of a circle of radius r • You should just pass the required parameter and return the area! • This is accomplished by using a static method. These methods are also called class methods. • Use the className that contains the method instead of an objectName to invoke it. Ex. Math.pow(x,3) • Declare static methods with the static modifier. Ex: public static double area(double radius) ...

Uses for Static Methods • Static methods are commonly used to provide libraries of useful and related methods. • It is good practice to group static methods together, when possible. • Examples: • the Math class provided with Java • methods include pow, sqrt, max, min, etc. • the SavitchIn class for console Input • not automatically provided with Java • methods include readLineInt, readLineDouble, etc.

The Matrix – Method Invocation

Reconciling Static Methods and Method References • Watch out! A static method cannot reference an instance variable from it’s class • Likewise, a static method cannot automatically reference a non-static method from its or any other class. • The only exception to above is if the static method creates an object of the class to use as a calling object.

Static Variables • By default, variables are dynamic. However, you can also create static variables with the reserved word static: static int numberOfInvocations = 0; • May be public or private but are usually private for the same reasons instance variables are (encapsulation). private static int numberOfInvocations = 0; • There is only one copy of a static variable and it can be accessed by any object of the class. • Can be used to let objects of the same class coordinate.

Static Variables - II • Static variables defined at the class level are called Class variables. • If the variable is public, it can be referenced outside of the class by ClassName.variableName • Ex. Math.PI • A public static variable at the program class level has visibility throughout the program – i.e. a global variable.

Definition: Privacy Leaks • Anytime you return an object from a method, you are giving back an address, not a value • the object is "unprotected" (usually undesirable) • The object looses it’s privacy – privacy leak • This applies to any class object (custom, wrapper, etc..) • One solution is to stick to returning primitive types (int, char, double, boolean, etc. • Best solution is called cloning … temp = passedObject.clone(); // method that clones (copies) object return temp; }

I.-C. Overloading • You can have the same method name with more than one definitionwithin the same class! • Why do this? • Same calculations, different data types • Same object, different initialization conditions • Each definition must have a different “signature” • This means the same name but different argument types, or a different number of arguments, or a different ordering of argument types. • The return type is not part of the signature and cannot be used to distinguish between two methods.

Signature • A method signature is the combination of method name and number and types of arguments, in order: • .equals(int) has a different signature than .equals(String) (same method name, different argument types) • myMethod(1)has a different signature than myMethod(1, 2) same method name, different number of arguments • myMethod(1,1.2)different signature than myMethod(1.2,1) (same method name and number of arguments, but different order of argument types.)

Method Overloading Example • static double max(double a, double b) Returns the greater of two double values. • static float max(float a, float b) Returns the greater of two float values. • static int max(int a, int b) Returns the greater of two int values. • static long max(long a, long b) Returns the greater of two long values.

It’s a Complicated World – Potential Issues with Overloading • If you accidentally use the wrong data type as an argument, you can invoke a different method. • If Java does not find a signature match, it attempts some automatic type conversions, e.g. int to double. Hence, an unwanted version of the method may execute. • Finally, you cannot have two methods that only differ in return type. This will produce an error: public double getWeight(); public float getWeight();

II. More On Classes - Wrapper Classes • Used to wrap primitive types in a class structure • All primitive types have an equivalent class • The class includes useful constants and static methods (including conversion back to primitive type)

Wrapper class example - Integer • Declare an Integer class variable: Integer n = new Integer(); • Convert the value of an Integer variable to its primitive type, int: int i = n.intValue(); //intValue returns an int • Some useful Integer methods/constants: • Integer.MAX_VALUE,Integer.MIN_VALUE - maximum and minimum integer value the computer can represent • Integer.valueOf("123"),Integer.toString(123) to convert a string to an integer and visa versa

Wrapper Class variables contain address of value variable declaration example:Integer n; variable declaration & init: Integer n = new Integer(0); assignment: n = new Integer(5); Primitive Type variables contain the value variable declaration example:int n; variable declaration & init.:int n = 0; assignment: n = 99; Usage of Wrapper Classes There are some important differences in the code using wrapper classes versus primitive types

II.-B. Constructors • A constructor is a special method designed to initialize instance variables • Automatically called when an object is created using new: Integer n = new Integer(0); • Often overloaded - more than one per class! • You may have different versions to initialize all, some, or none of the instance variables • Each constructor has a different signature (a different number or sequence of argument types)

Defining Constructors • Constructor headings have the same name as the class and do not include any reference to a return type or void. • A constructor with no parameters is called a default constructor. • If no constructor is provided, Java automatically creates a default constructor. • If any constructor is provided, then no constructors are automatically created by the compiler

Constructor Programming Suggestions • In your custom Classes, include a constructor that initializes all instance variables. • Also, include a constructor that has no parameters (default constructor) • This overrides any automatic action by the compiler • Include overloaded versions of constructor to handle different initializations • Important note: you cannot call a constructor for an object after it is created • If you want to change values of instance variables after you have created an object, you should create a set method!

Constructor Example - PetRecord public class PetRecord { private String name; private int age; //in years private double weight; //in pounds . . . public PetRecord(String initialName) { name = initialName; age = 0; weight = 0; } Initializes three instance variables: name from the parameter and age and weight with default initial values. Sample use: PetRecord pet1 = new Pet("Eric");

You Can Use Other Methods in a Constructor … public PetRecord(String initialName, int initialAge, double initialWeight) { set(initialName, initialAge, initialWeight); } … … private void set(String initialName, int initialAge, double initialWeight) { name = initialName; if ((initialAge < 0) || (initialWeight < 0)) { System.out.println(“Error: …”); } else { age = initialAge; weight = initialWeight; } }

II.-C. Packages • A package is a way to group a collection of related classes • Think of a package as a library of classes • They do not have to be in the same directory as your program • The first line of each class in the package must bethe keyword package followed by the name of the package: package mystuff.utilities; • To use classes from a package in a program put an import statement at the start of the file: import mystuff.utilities.*; //all classes in directory

A Package Name myjava is the base path myjava myjava.general.utilities is a specific package name general Classes in the package utilities AClass.java AnotherClass.java

Package Naming Conventions • Use lowercase • The package name is the pathname with subdirectory separators ("/") replaced by dots • For example, if the package is in a directory named "utilities" in directory "mystuff", the package name is: imports myjava.general.utilities; imports myjava.general.*; imports myjava.*;

Adding A Custom Package Path To JCreator

III. Software Design – “Top-Down” Program Design • Step 1: Define potential objects and formulas needed • Step 2: Write a list of tasks, in pseudocode, that the program must do. • Step 3: Create a flow diagram • Step 4: For each task in the program, write a list of subtasks, in pseudocode, that the method must do. • Step 5: If you can easily write Java statements for any subtask, you are finished with that subtask. • Step 6: Repeat Steps 4. & 5. until done.

Program Design Example – “Lab 3 Investment Account” • Step 1: Define Class InvestmentAccount w/ attributes of a savings account: balance, interest rate, deposit, contributions… Methods include open, deposit, add interest, close, … • Step 2: Program Greeting Open investment account #1 w/ deposit Open investment account #2 w/ deposit Run investment strategy #1 & compute rate of return Run investment strategy #1 & compute rate of return Compute difference and declare the superior strategy

Lab 3 Investment Account – Step 3

Lab 3 Investment Account – Step 4 • Step 4: … Open Investment Account #1 by instantiating an InvestmentAccount object with an account name, account number, initial deposit, … … Open Investment Account #2 by instantiating an InvestmentAccount object with an account name, account number, initial deposit, … … For investment strategy #1: for each of the 12 cycles, add $500 to account and compute new balance with interest …

Programming Tip:Put A main In All Classes! • A main method can be used to test all members of any class. • It serves as test or “driver” code that can be left in a class and added to as you go. • If you change the name of your project/program to the name of just your custom class, it will execute the classes’ main method. • Otherwise, when the class is used to create objects, the main method is ignored.

Diagnostic main • Main methods in Classes are referred to as diagnostic main methods. • Remember: Because main must be static, you can't automatically invoke non-static methods of the class. • You can however, invoke a non-static method in main if you create an object of the class.

Summary – Further Additions To A Class • Diagnostic main methods. • set… method to change an instance variable (complement of get… method) • equals method to define equivalent objects • clone method to copy your object to a new object • Also, don’t forget writeOutput or toString to display contents of an object.

Introduction To Scientific Programming