1

Methods

Chapter 5Spring 2007CS 101Aaron Bloomfield

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Preparation Scene so far has been background material and experience

Computing systems and problem solving Variables Types Input and output Expressions Assignments Using objects Standard classes and methods Decisions (if, switch) Loops (while, for, do-while)

Next: Experience what Java is really about Design and implement objects representing information

and physical world objects

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Object-oriented programming Basis

Create and manipulate objects with attributes and behaviors that the programmer can specify

Mechanism Classes

Benefits An information type is design and implemented once

Reused as needed No need reanalysis and re-justification of the

representation

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Known Classes Classes we’ve seen

BigInteger String Rectangle Vector Scanner System

Classes we’ll be seeing soon BigDecimal

But the first step is on creating methods…

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MethodsMethods

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Methods we’ve seen We’ve seen methods (functions) before

angleSin = Math.sin (90 * PI/180.0); System.out.println (“Hello world”); value = card.getBlackjackValue();

We are going to start defining them

Note that many of these “return” a value Math.sin() and card.getBlackjack()

The way to name methods is the same as variables allTheWordsTogether With the first letter of each word capitalized Except the very first letter is lower case

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Our first class with methodspublic class Methods1 {

public static void main (String args[]) {Scanner stdin = new Scanner (System.in);System.out.println ("Enter a valid int value");int value = stdin.nextInt();if ( value == 1 ) validValue();else if ( value == 2 ) validValue();else if ( value == 3 ) invalidValue();else if ( value == 4 ) invalidValue();else validValue();

}

}

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Our first class with methods, continued

public static void invalidValue() { System.out.println ("You have entered an invalid value."); System.out.println ("The program will now exit."); System.exit (0);

}

public static void validValue() { System.out.println ("You have entered an valid value."); System.out.println ("Congratulations!"); System.out.println ("The program will now exit."); System.exit (0);

}

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Program DemoProgram Demo

Methods1.javaMethods1.java

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public static void validValue() { System.out.println ("You have entered an valid value."); System.out.println ("Congratulations!"); System.out.println ("The program will now exit."); System.exit (0);

}

public static void main (String args[]) { Scanner stdin = new Scanner (System.in); System.out.println ("Enter a valid int value"); int value = stdin.nextInt(); if ( value == 1 ) validValue();

// ... }

What’s happening there

Scanner

stdin

value 1

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Notes on these methods At this point, all methods in the class are static

We will be discussing what static means later in this slide set

Until then, I’ll be ignoring it, and just telling you when things should and should not be static Sorry!

None of those two methods return a value Notice the “void” before the method name

And none take in any parameters Notice the empty parameters after the method name

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A previous HW J3A previous HW J3

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Revamping last semester’s HW J3

Start over

Publicdanger

Defensecontractor

Promoted

Cynicalguru

Becomea coder

Done tech support?

Been asysadmin?

Workedwith NT?

Want toplay

w/nukes?

Hatepeople?

Bitteryet?

Successfulmanaging?

Starthere

Start over

A revised HW 3 flowchart

Green paths are “yes” paths, red are “no” paths

We’ll make a slight modification to the diagram:

Note that this part is repeated twice!

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HW J3 Code The yellow

boxed part is what was repeated from the previous slide

if ( extractor.askUser(Q_TECH_SUPPORT) ){ if ( extractor.askUser(Q_BITTER_YET) )

if ( extractor.askUser(Q_MANAGEMENT) ) System.out.println (A_CYNICAL);else System.out.println (A_PROMOTED);

elseSystem.out.println (A_START_OVER);

} else if ( extractor.askUser(Q_BEEN_SYSADMIN) ){ if ( extractor.askUser(Q_HATE_PEOPLE) ) {

System.out.println (A_CODER); } else if ( extractor.askUser(Q_WINNT) ){

if ( extractor.askUser(Q_NUCLEAR_WEAPONS) ) System.out.println (A_DEFENSE_CONTRACTOR);else System.out.println (A_PUBLIC_DANGER);

} elseSystem.out.println (A_START_OVER);

} else if ( extractor.askUser(Q_WINNT) ){ if ( extractor.askUser(Q_NUCLEAR_WEAPONS) )

System.out.println (A_DEFENSE_CONTRACTOR); else

System.out.println (A_PUBLIC_DANGER);} else System.out.println (A_START_OVER);

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HW J3 Code with methods The yellow

boxed part is what was repeated from the previous slide

if ( extractor.askUser(Q_TECH_SUPPORT) ){ if ( extractor.askUser(Q_BITTER_YET) )

if ( extractor.askUser(Q_MANAGEMENT) ) System.out.println (A_CYNICAL);else System.out.println (A_PROMOTED);

elseSystem.out.println (A_START_OVER);

} else if ( extractor.askUser(Q_BEEN_SYSADMIN) ){ if ( extractor.askUser(Q_HATE_PEOPLE) ) {

System.out.println (A_CODER); } else

doBottomPartOfFlowchart();} else doBottomPartOfFlowchart();

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HW J3 Code with methods The doBottomPartOfFlowchart method:

public static void doBottomPartOfFlowchart() { if ( extractor.askUser(Q_WINNT) ){

if ( extractor.askUser(Q_NUCLEAR_WEAPONS) ) System.out.println (A_DEFENSE_CONTRACTOR);else System.out.println (A_PUBLIC_DANGER);

} elseSystem.out.println (A_START_OVER);

}

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What happened here We took a common set of code

Wrote it once But used it multiple times (twice in this case)

Granted, the code was a small segment (7 lines) But, in other programs, could be very large

This is called Refactoring It is an essential principle of software engineering Has other names: factoring (notice there is no ‘re’ at the

beginning), extracting a method, etc.

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Pros of Refactoring Benefits of Refactoring

Reduce length of code As you don’t have to repeat that section of code

multiple times Make code easier to read

The main if-else-if statement is shorter, thus easier to understand what’s going on

Changes are easier to make If we want to modify that part of the flowchart, we only

have to do it once Rather than searching for each of the repeated code

segments in a program

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Cons of Refactoring Drawbacks of Refactoring

Because you are calling another method, it will be slightly slower On the order of a few nanoseconds Modern compilers can sometimes eliminate this

penalty

The general consensus is that the benefits of Refactoring far outweigh the drawback(s)

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Return ValuesReturn Values

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The return keyword The return keyword immediately stops execution of a method

And jumps back to whatever called that method And possibly returns a value (we’ll see this next)

Consider the following method

public static void foo (int x) { if ( x == 1 ) return; System.out.println (“x is not 1”);}

This method will only print the String if x is not 1

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Return values At some point in those methods, Java must be told to take a value

and “pass” it back

Consider angleSin = Math.sin (90 * PI/180.0); At some point in the Math.sin() method, the sin has been

computed And that value must be “passed back” to be stored in angle

Consider value = card.getBlackjackValue(); At some point in the card.getBlackjackValue() method, the value

has been computed And that value must be “passed back” to be stored in value

This is called “returning” a value from a method

Note that some methods don’t return a value System.out.println(), for example

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Return values (aka return types)public class Methods2 {

public static int returnsAValue () { return 1;

}

public static double alsoReturnsAValue() { return 1.0;

}

public static void main (String args[]) { int value1 = returnsAValue(); System.out.println (value1);

double value2 = alsoReturnsAValue(); System.out.println (value2);

// The following line requires a cast int value3 = (int) alsoReturnsAValue(); System.out.println (value3);

}

}

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Program DemoProgram Demo

Methods2.javaMethods2.java

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Return types All a return statement does is take the value

Which could be a number Or a value in a variable Or an expression (such as x+1)

And “pass” it back to whatever called the method

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How well do you feel you How well do you feel you understand return values?understand return values?

a)a) Very well! This stuff is so easy.Very well! This stuff is so easy.

b)b) With a little review, I’ll be good.With a little review, I’ll be good.

c)c) Not very well at all.Not very well at all.

d)d) I’m so lost. What’s a return type I’m so lost. What’s a return type again?again?

e)e) I’d rather not answer this question, I’d rather not answer this question, thanks.thanks.

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Parameters Sometimes you need to pass in parameters to tell a method

how to perform Consider Math.sin() – it needs to know the angle

The parameters are listed between the parenthesis after the method name

public static void main (String args[])

The methods we will study next compute (and return) x2, x3, and x4

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The methods

public static int square (int x) {int theSquare = x * x;return theSquare;

}

public static int cube (int x) {return x * x * x;

}

public static int fourthPower (int x) {return square(x) * square(x);

}

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A method with multiple parameters

public static int squareOrCube (int which, int value) {if ( which == 1 ) return value * value;else if ( which == 2 ) { int cube = value * value * value; return cube;} else return 0;

}

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The main() methodimport java.util.*;

public class Methods3 {

// the previous methods go here

public static void main (String args[]) { Scanner stdin = new Scanner (System.in); System.out.println ("Enter an int value"); int value = stdin.nextInt(); int theSquare = square(value); System.out.println ("Square is " + theSquare); System.out.println ("Cube is " + cube (value)); System.out.println ("Square is " + squareOrCube (1, value)); System.out.println ("Cube is " + squareOrCube (2,value)); System.out.println ("Fourth power is " + fourthPower (value));

}

}

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Program DemoProgram Demo

Methods3.javaMethods3.java

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Returning objects We can also return objects from methods

What gets returned is the reference to the object

public class Methods4 {

public static String getCourse () {String name = "CS 101";return name;

}

public static void main (String args[]) {String courseName = getCourse();System.out.println (courseName);

}

}

String

“CS 101”

courseName

name

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Program DemoProgram Demo

Methods4.javaMethods4.java

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Modifying parameters Consider the following code

public class Methods5 {

public static void changeValue (int x) { x = 7;

}

public static void main (String args[]) { int y = 5; changeValue(y); System.out.println (y);

}

}

What gets printed?

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Program DemoProgram Demo

Methods5.javaMethods5.java

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Pass by value Java is a pass-by-value language

This means that a COPY of the parameter’s value is passed into the method If a method changes that value, only the COPY is changed Once the method returns, the copy is forgotten And thus the change is not visible outside the method

There are other manners of returning values that are used in other languages Pass by reference Pass by name (nobody uses this anymore)

We will see about trying to change object parameters later in this slide set

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How well do you feel you How well do you feel you understand parameters?understand parameters?

20% 20% 20%20%20%1.1. Very well! This Very well! This stuff is easy!stuff is easy!

2.2. Fairly well – with Fairly well – with a little review, I’ll a little review, I’ll be goodbe good

3.3. Okay. It’s not Okay. It’s not great, but it’s not great, but it’s not horrible, eitherhorrible, either

4.4. Not well. I’m Not well. I’m kinda confusedkinda confused

5.5. Not at all. I’m Not at all. I’m soooooo lostsoooooo lost

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Variable scoping A variable is visible within the block it is declared

Called the “scope” of the variable

public class Scoping {static int z

public static void foo (int x) {// ...

}

public static void bar () {// ...

}

public static void main (String[] args) {int y;// ...

}}

This local variable is visible until the end of the main() method

This variable is visible anywhere in the Scoping class

This parameter is visible only in the foo() method

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How well do you feel you How well do you feel you understand variable understand variable

scoping?scoping?20% 20% 20%20%20%1.1. Very well! This Very well! This

stuff is easy!stuff is easy!2.2. Fairly well – with Fairly well – with

a little review, I’ll a little review, I’ll be goodbe good

3.3. Okay. It’s not Okay. It’s not great, but it’s not great, but it’s not horrible, eitherhorrible, either

4.4. Not well. I’m Not well. I’m kinda confusedkinda confused

5.5. Not at all. I’m Not at all. I’m soooooo lostsoooooo lost

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Method notes summary You can put the methods in a class in any order

Java doesn’t care which one is listed first Thus, you can call a method listed later in the method

This is different than C/C++

All methods must specify a return type If it’s void, then no value is returned

Parameters can’t be changed within a method Although the objects that the parameters point to can be

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Agricultural Agricultural historyhistory

PhysicsPhysics MedicineMedicine LiteratureLiterature PeacePeace EconomicsEconomics

ChemistryChemistry BiologyBiology NutritionNutrition

Fluid dynamicsFluid dynamics

The 2005 Ig Nobel PrizesThe 2005 Ig Nobel Prizes““The Significance of Mr. Richard Buckley’s Exploding The Significance of Mr. Richard Buckley’s Exploding

Trousers”Trousers”

The pitch drop experiment, started in 1927The pitch drop experiment, started in 1927

Neuticles – artificial replacement testicles for dogsNeuticles – artificial replacement testicles for dogs

The 409 scams of Nigeria for a “cast of rich characters”The 409 scams of Nigeria for a “cast of rich characters”

Locust brain scans while they were watching Star WarsLocust brain scans while they were watching Star Wars

For an alarm clock that runs away, thus making people For an alarm clock that runs away, thus making people more productivemore productive

““Will Humans Swim Faster or Slower in Syrup?”Will Humans Swim Faster or Slower in Syrup?”

For cataloging the odors of 131 different stressed frogsFor cataloging the odors of 131 different stressed frogs

To Dr. Yoshiro Nakamats who catalogued and analyzed To Dr. Yoshiro Nakamats who catalogued and analyzed every meal he ate for the last 34 years (and every meal he ate for the last 34 years (and counting)counting)

““Pressures Produced When Penguins Pooh – Pressures Produced When Penguins Pooh – Calculations on Avian Defaecation”Calculations on Avian Defaecation”

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More on parameter passingMore on parameter passing

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Modifying parameters Consider the following code

public class Methods5 {

public static void changeValue (int x) { x = 7;

}

public static void main (String args[]) { int y = 5; changeValue(y); System.out.println (y);

}

}

What gets printed? 5 is printed

5y

5x 7x

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Program DemoProgram Demo

Methods5.javaMethods5.java

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Pass by value Java is a pass-by-value language

This means that a COPY of the parameter’s value is passed into the method If a method changes that value, only the COPY is changed Once the method returns, the copy is forgotten And thus the change is not visible outside the method

There are other manners of returning values that are used in other languages Pass by reference Pass by name (nobody uses this anymore)

We will see about trying to change object parameters later in this slide set

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Modifying parameters Consider the following code

import java.awt.*;

public class Methods6 {

public static void changeValue (Rectangle r) {

r.setSize (10,20); }

public static void main (String args[]) {Rectangle rect = new Rectangle(1, 2);changeValue(rect);System.out.println (rect.getWidth());

}

}

What gets printed? 10 is printed

+ Rectangle ()+ Rectangle (int width, int height)+ setSize (int width, int height)+ getWidth ()

Rectangle

- width = 1 - height = 2

rect

+ Rectangle ()+ Rectangle (int width, int height)+ setSize (int width, int height)+ getWidth ()

Rectangle

- width = 10 - height = 20r

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Program DemoProgram Demo

Methods6.javaMethods6.java

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Fan-supplied demotivators!Fan-supplied demotivators!

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Pass by value Java is still a pass-by-value language

This means that a COPY of the parameter’s value is passed into the method But the parameter is a REFERENCE to an object The object itself is not passed So any changes to the reference are forgotten about But you can modify the object it refers to

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Modifying parameters Consider the following code

import java.awt.*;

public class Methods7 {

public static void changeValue (Rectangle r) {

r = new Rectangle (10,20); }

public static void main (String args[]) {Rectangle rect = new Rectangle(1, 2);changeValue(rect);System.out.println (rect.getWidth());

}

}

What gets printed? 1 is printed

+ Rectangle ()+ Rectangle (int width, int height)+ setSize (int width, int height)+ getWidth()

Rectangle

- width = 1 - height = 2

rect

+ Rectangle ()+ Rectangle (int width, int height)+ setSize (int width, int height)+ getWidth()

Rectangle

- width = 10 - height = 20

r

The only change!

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Program DemoProgram Demo

Methods7.javaMethods7.java

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Pass by value Java is still a pass-by-value language

This means that a COPY of the parameter’s value is passed into the method But the parameter is a REFERENCE to an object The object itself is not passed So any changes to the reference are forgotten about But you can modify the object it refers to

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How well do you feel you How well do you feel you understand parameter understand parameter

passing?passing?20% 20% 20%20%20%1.1. Very well! This Very well! This

stuff is easy!stuff is easy!2.2. Fairly well – with Fairly well – with

a little review, I’ll a little review, I’ll be goodbe good

3.3. Okay. It’s not Okay. It’s not great, but it’s not great, but it’s not horrible, eitherhorrible, either

4.4. Not well. I’m Not well. I’m kinda confusedkinda confused

5.5. Not at all. I’m Not at all. I’m soooooo lostsoooooo lost

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A frisbee demo of parameter passing…

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How well do you feel you How well do you feel you understand parameter understand parameter

passing?passing?20% 20% 20%20%20%1.1. Very well! This Very well! This

stuff is easy!stuff is easy!2.2. Fairly well – with Fairly well – with

a little review, I’ll a little review, I’ll be goodbe good

3.3. Okay. It’s not Okay. It’s not great, but it’s not great, but it’s not horrible, eitherhorrible, either

4.4. Not well. I’m Not well. I’m kinda confusedkinda confused

5.5. Not at all. I’m Not at all. I’m soooooo lostsoooooo lost

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Today’s demotivatorsToday’s demotivators