JFo_4_1.pdf

Java Foundations 4-1 What Is a Method? Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered trademarks of Oracle and/or its affiliates. Other names may be trademarks of their respective owners. Objective...

Java Foundations 4-1 What Is a Method? Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered trademarks of Oracle and/or its affiliates. Other names may be trademarks of their respective owners. Objectives This lesson covers the following objectives: −Structure code within a class −Instantiate an object −Understand the benefits of methods −Use the dot operator (.) to access an object’s fields and methods −Supply arguments to a method −Return values from a method JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 3 respective owners. Classes You’ll Encounter In Java development, you’ll encounter many classes for many different object types, including … −Classes you’ll write yourself −Classes written by someone else −Classes belonging to Java JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 4 respective owners. Classes You’ll Encounter These classes outline objects’ … −Properties (fields) −Behaviors (methods) The goal of this lesson is to give you an understanding of how to work with any class, its fields, and its methods The remaining lessons of this section explore important classes provided by Java We’ll start by exploring classes and methods a little deeper JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 5 respective owners. Exercise 1, Part 1 Let’s look at a scenario and see how we can model the components involved: −It’s Alex’s birthday! You’ve arranged a group of eight friends to celebrate at a local restaurant. When your party receives their bill, nobody is quite sure what they owe. You only know everyone’s total before tax (5%) and tip (15%). But lucky you! You brought your laptop and are asked to write a program that calculates everybody’s total JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 6 respective owners. Exercise 1, Part 2 Create a new project and add the Tip01.java file to the project This is what everyone owes before tax (5%) and tip (15%): Person 1: $10 Person 5: $7 Person 2: $12 Person 6: $15 (Alex) Person 3: $9 Person 7: $11 Person 4: $8 Person 8: $30 JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 7 respective owners. Exercise 1, Part 3 Your program should produce the following output: person1: $12.0 person2: $14.4 person3: $10.8 person4: $9.6 person5: $8.4 person6: $18.0 person7: $13.2 person8: $36.0 JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 8 respective owners. Modeling Objects You may have been tempted to model each person’s total by writing this: public class Tip01{ public static void main(String args[]){ double person1 = 10; double total1 = person1*(1 +.05 +.15); System.out.println(total1); }//end method main } //end class Tip01 JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 9 respective owners. Modeling More Objects When you needed to model two dinner guests, you may have been tempted to copy, paste, and rename: public class Tip01{ public static void main(String args[]){ double person1 = 10; double total1 = person1*(1 +.05 +.15); System.out.println(total1); double person2 = 12; double total2 = person2*(1 +.05 +.15); System.out.println(total2); }//end method main }//end class Tip01 JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 10 respective owners. Modeling Many Objects What if you needed to figure 1,000 guests? //You might think … //Do I really have to copy, paste, and rename 1,000 //times? What if one of your friends forgets their wallet? What if you made a mistake with your formula? //You might think … //Do I need to make 1,000 edits?! //There has to be a better way!!! JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 11 respective owners. Variables Offer Flexibility If the tax rate or tip percentage needs to change … −We don’t need to make 1,000 edits −We simply edit each variable once double tax = 0.05; double tip = 0.15; double person1 = 10; double total1 = person1*(1 +tax +tip); System.out.println(total1); double person2 = 12; double total2 = person2*(1 +tax +tip); System.out.println(total2); JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 12 respective owners. Methods Offer Similar Flexibility The same math and printing behaviors are repeated Instead, this logic can be written once in a method double tax = 0.05; double tip = 0.15; double person1 = 10; double total1 = person1*(1 +tax +tip); System.out.println(total1); double person2 = 12; double total2 = person2*(1 +tax +tip); System.out.println(total2); JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 13 respective owners. When to Use Methods It’s a good idea to write a method if you … Find yourself repeating very similar lines of code, including calculations Need to describe an object’s behavior JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 14 respective owners. How to Use a Main Method The main method is known as a driver −Use it to drive the events of a program −Use it to access fields and methods or other classes The main method doesn’t describe the behavior of any particular object −Keep it separate from your object classes −Use only one main method for each application JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 15 respective owners. What Do Object Classes Look Like? We will change the code to fit the following format Let’s see how we can get our code to look like this: 1 public class Calculator{ 2 3 4 5 Properties 6 7 8 9 Behaviours 10 11 } JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 16 respective owners. Step 1) Move Fields from the Main Method public class Calculator{ //Fields Local variables public double tax = 0.05; become fields public double tip = 0.15; public double originalPrice = 10; public static void main(String args[]){ //double tax = 0.05; //double tip = 0.15; //double person1 = 10; double total1 = person1*(1 + tax + tip); System.out.println(total1); }//end method main }//end class Calculator JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 17 respective owners. Step 2) Move Repeated Behaviors from the Main Method public class Calculator{ //Fields public double tax = 0.05; public double tip = 0.15; public double originalPrice = 10; //Methods public void findTotal(){ //Calculate total after tax and tip //Print this value }//end method findTotal public static void main(String args[]){ //double total1 = person1*(1 + tax + tip); //System.out.println(total1); } //end method main } //end class Calculator JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 18 respective owners. Step 3) Remove the Main Method public class Calculator{ //Fields public double tax = 0.05; public double tip = 0.15; public double originalPrice = 10; //Methods public void findTotal(){ //Calculate total after tax and tip //Print this value }//end method findTotal //public static void main(String args[]){ //double total1 = person1*(1 + tax + tip); //System.out.println(total1); //}//end method main } //end class Calculator JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 19 respective owners. Success! public class Calculator{ //Fields public double tax = 0.05; public double tip = 0.15; public double originalPrice = 10; //Methods public void findTotal(){ //Calculate total after tax and tip //Print this value }//end method findTotal } //end class Calculator JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 20 respective owners. Where Do I Put the Main Method? public class CalculatorTest { public static void main(String args[]){ //Create Calculator object instance Calculator calc = new Calculator(); calc.tip = 0.10; //Altering a field calc.findTotal(); //Calling a method }//end method main }//end class CalculatorTest Put the main method in another class, such as a test class The main method drives the action of the program: − It creates instances of objects − It calls an instance’s fields and methods by using the dot operator (.) JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 21 respective owners. Variables for Objects int age = 22; String str = "Happy Birthday!"; Scanner sc = new Scanner(); Calculator calc = new Calculator(); Objects, like primitives, are represented by variables Most objects require the new keyword when they’re initialized to create new instances −This is called instantiating an object −There are some exceptions, like String objects, that don’t require the new keyword JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 22 respective owners. Using the Dot Operator Place the dot operator (.) after a variable’s name to access its fields or methods public class CalculatorTest { public static void main(String args[]){ Calculator calc = new Calculator(); calc.printTip(); //prints 0.15 calc.tip = 0.10; calc.printTip(); //prints 0.10 }//end method main }//end class CalculatorTest public class Calculator{ public double tip = 0.15; //initialized value 0.15 public void printTip(){ System.out.println(tip); }//end method printTip }//end class Calculator JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 23 respective owners. Exercise 2, Part 1 Create a new project and add the CalculatorTest2.java and Calculator2.java files to the project Complete the findTotal() method, which should: −Calculate a total based on the tax, tip, and originalPrice fields −Print a person’s total JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 24 respective owners. Exercise 2, Part 2 From the main method: −Instantiate a Calculator object named calc −Observe your IDE after typing "calc" −Access this object’s fields and methods to print the total for each person at the birthday party Change tip and tax if you prefer different values JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 25 respective owners. What You May Have Written You may have written your program like this: −Two lines are required for each person −And more if you decide to print names or change tax/tip values public class CalculatorTest{ public static void main(String args[]){ Calculator calc = new Calculator(); calc.originalPrice = 10; calc.findTotal(); calc.originalPrice = 12; calc.findTotal(); }//end method main }//end class CalculatorTest JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 26 respective owners. Becoming More Flexible But it’s possible to do the same work in a single line It’s also dangerous to write programs that access fields directly −You’ll learn about this later −The goal of this lesson is just to prepare you to work with important Java-provided classes calc.originalPrice = 10; //Dangerous calc.findTotal(); JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 27 respective owners. Remember the JOptionPane When we add the String literal "type something:" to the method call, we’re supplying arguments to the method This argument alters the resulting JOptionPane JOptionPane.showInputDialog("Type something:"); JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 28 respective owners. When Can Methods Accept Arguments? You’ll find that many methods are affected by arguments −But methods must be written in a way to accept arguments −Otherwise, the compiler complains −The calculate method is written to accept no arguments Calculator calc = new Calculator(); calc.calculate(); //Good calc.calculate(3, 2.0); //Fail public void calculate(){ //How do I calculate? }//end method calculate JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 29 respective owners. Method Argument But this calculate method is written to accept two arguments: −The first argument must be an int −The second argument must be a double Calculator calc = new Calculator(); calc.calculate(3, 2.0); public void calculate(int x, double y){ System.out.println(x/y); //prints 1.5 }//end method calculate The variable int x is assigned a value of 3 The variable double y is assigned a value of 2.0 JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 30 respective owners. Argument Order Matters What happens if we reverse the order of our arguments? Calculator calc = new Calculator(); calc.calculate(2.0, 3); We get a compiler error: −int x cannot be assigned a double value −The first argument must be an int public void calculate(int x, double y){ System.out.println(x/y); }//end method calculate 2,0 3 JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 31 respective owners. Exercise 3, Part 1 Create a new project and add the CalculatorTest3.java and Calculator3.java files to the project From the main method: −Use a Calculator object instance and supply arguments to findTotal() to print the total for each person −Hint: Observe the findTotal() method in the Calculator class to figure out how many arguments this method accepts JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 32 respective owners. Exercise 3, Part 2 Who does each total belong to? Modify the findTotal() method to accept an additional String name argument Concatenate the print statement to include name Observe your IDE’s complaint in the main method and revise your findTotal() method calls JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 33 respective owners. Method Arguments and Parameters An argument is a value that’s passed during a method call: Calculator calc = new Calculator(); calc.calculate(3, 2.0); //should print 1.5 Arguments A parameter is a variable that’s defined in the method declaration: 3 2.0 public void calculate(int x, double y){ System.out.println(x/y); }//end method calculate Parameters JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 34 respective owners. Method Parameters: Examples Methods may have any number or type of parameters: public void calculate0(){ System.out.println("No parameters"); }//end method calculate0 public void calculate1(int x){ System.out.println(x/2.0); }//end method calculate1 public void calculate2(int x, double y){ System.out.println(x/y); }//end method calculate2 public void calculate3(int x, double y, int z){ System.out.println(x/y +z); }//end method calculate3 JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 35 respective owners. The Scope of Parameters Methods need to be told what to do with the arguments they receive You do that by using method parameters −Method parameters are variables that exist within the entire scope of a method −They’re created within the method declaration −Scope refers to the {block of code} belonging to a method following its declaration public void calculate(int x, double y){ System.out.println(x/y); }//end method calculate JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 36 respective owners. Referencing Method Parameters A variable may be referenced anywhere within its current block after being declared A variable can’t be referenced outside the block where it was declared or before being declared public void calculate(int x, double y){ System.out.println(x/y); Scope x } //end method calculate public void calculate2(){ Not in Scope x System.out.println(2*x); //What is x? } //end method calculate2 JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 37 respective owners. Finding the Grand Total: Scenario Your friends are impressed with how much you’re learning from the Java Foundations course! Alex asks, “What should the entire table’s total be?” Knowing the answer to this question would help make sure everyone has contributed and that the server has received the correct amount How can this be included in your code? JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 38 respective owners. Adding Totals Another way to think about this: −I’ve calculated a value within a method … −But it’s stored as a variable that can’t exist outside the scope of its method block … −How do I get this value out of there? public void findTotal(double price, String name){ double total = price * (1 + tax + tip); System.out.println(name + ": $ " + total); } //end method findTotal JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 39 respective owners. Adding Totals public class CalculatorTest public class Calculator public static void public void findTotal() main(String[] args) double total Ha ha! Try to get me! JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 40 respective owners. Adding Totals If you thought about writing your program like this: public class CalculatorTest{ public static void main(String args[]){ Calculator calc = new Calculator(); calc.findTotal(10); calc.findTotal(12); System.out.println(calc.findTotal(10) + calc.findTotal(12)); }//end method main }//end class CalculatorTest You’re half correct But your IDE gives you the following error: −'void' type not allowed here JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 41 respective owners. What Is a Void Type? showInputDialog() is a String type method −It returns a value that can be stored as a String String input = JOptionPane.showInputDialog("Type something:"); Void type methods don’t return any values −There are no values to store after a void method is called System.out.println("println is a void type method"); JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 42 respective owners. Method Return Types Variables can have values of many different types: long Calculator Method calls also return values of many different types: long Calculator How to make a method return a value: −Declare the method to be a nonvoid return type −Use the keyword return within a method, followed by a value JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 43 respective owners. Method Return Types: Examples Methods must return data that matches their return type: public void printString(){ System.out.println("Hello"); }//end method printString public String returnString(){ return("Hello"); }//end method returnString public int sum(int x, int y){ return(x + y); }//end method sum public boolean isGreater(int x, int y){ return(x > y); }//end method isGreater JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 44 respective owners. Method Return The following code examples produce equivalent results: public static void main(String[] args){ int num1 = 1, num2 = 2; int result = num1 + num2; System.out.println(result); }//end method main public static void main(String[] args){ int num1 = 1, num2 = 2; int result = sum(num1, num2); System.out.println(result); }//end method main public static int sum(int x, int y){ return(x + y); }//end method sum JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 45 respective owners. Passing Arguments and Returning Values 1 2 Value passed from Value received by caller method to worker method worker method Object Method 1 2 3 1 4 2 5 3 6 4 7 3 Value returned to caller method JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 46 respective owners. Exercise 4, Part 1 Edit your Exercise3 solution −Or create a new project and add the CalculatorTest4.java and Calculator4.java files to the project Find and print the entire table’s total, including tax and tip −You'll need to edit findTotal() so that it returns its calculated value JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 47 respective owners. Exercise 4, Part 2 Person8 forgot their wallet And Alex’s meal was meant to be a birthday present Modify findTotal() so that the cost of their meals are shared equally with the rest of the party Recalculate the entire table’s total This number shouldn’t have changed JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 48 respective owners. Summary About Method Syntax Method name Parameters Method return type public double calculate(int x, double y){ double quotient = x/y; return quotient; Implementation }//end method calculate JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 49 respective owners. Summary In this lesson, you should have learned how to: −Structure code within a class −Instantiate an object −Understand the benefits of methods −Use the dot operator to access an object’s fields and methods −Supply arguments to a method −Return values from a method JFo 4-1 Copyright © 2022, Oracle and/or its affiliates. Oracle, Java, and MySQL are registered What is a Method? trademarks of Oracle and/or its affiliates. Other names may be trademarks of their 50 respective owners.