Methods  in  Java  

Jussi  Pohjolainen  Tampere  University  of  Applied  Sciences  

Problem  public class HugeProgram { public static void main(String [] args) {

1. codeline 2. codeline 3. codeline ... 10 000. codeline } }

How  are  you  going  to  

manage  this  huge  

codeblock?  

Problem  

•  It's  possible  that  your  whole  program  is  implemented  inside  the  main  –  method  

•  Result:  No  one  will  understand  the  code!  •  You  have  to  create  the  code  that  is  – Reusable  – You  and  others  can  manage  it  

•  Well  how?  

How  to  divide  your  code?  

•  Best  pracJce:  Use  Object-­‐Orientated  Programming  – Classes,  Methods,  Inheritance,  ...  

•  Lot  of  possibiliJes  with  OO!  •  But  first  let's  concentrate  on  methods!  

Methods:  Divide  your  Code?  public class HugeProgram { public static void main(String [] args) { doSomething1(); doSomething2(); doSomething3(); } public static void doSomething1() { // something } public static void doSomething2() { // something } public static void doSomething3() { // something } }

Methods:  Divide  your  Code?  public class HugeProgram { public static void main(String [] args) { doSomething1(); doSomething2(); } public static void doSomething1() { System.out.print("Hello "); } public static void doSomething2() { System.out.println("World!"); } } java HugeProgram

Hello World!

Method  Call  

Method   Method  

Examples  

•  Flash  demo:  Method  1  (no  parameters,  no  return)  

•  MethodDemo1.java  •  MethodDemo2.java  •  MethodDemo3.java  •     

IT'S  NOT  THAT  EASY:  PARAMETERS  AND  RETURN  

Real  Life  Example  

I  really  want  to  buy  christmas  gi\s  for  my  children.  The  doll  costs  5  

euros  and  the  toy  car  costs  6  euros.  I  have  12  euros,  is  that  enough?  Why  didn't  I  listen  to  the  math  teacher  at  school?  Why  didn't  I  learn  sum  –  calculaJons?  Why  oh  

why?  

Real  Life  Example  

Hello  Hello!  My  name  is  Mr.  Sum  and  I  can  sum  everything!  Please  use  my  knowledge!  

Real  Life  Example  

Well  Mr  Sum.  I  have  a  problem  I  don't  know  what  is  5  plus  6.    

Real  Life  Example  Don't  understand  your  

problem.  I  only  do  simple  calculaJons,  I  really  need  to  have  two  integer  values  to  be  able  to  do  the  sum.  Please  just  

give  my  two  numbers!  

Parameters!  

5  6  

Return!  

11  

Real  Life  Example  

Well  thanks,  at  least  I  know  my  numbers  and  11  is  less  than  12,  so  great!  My  children  will  be  

very  happy!  

Mr.  Sum?  

Mr.  Sum  

5  

6  

11  

parameter   return  

Real  Life  Example  in  the  Nerd  World  

I  am  implemenJng  a  applicaJon  and  I  really  need  a  random  floaJng  point  value  between  

[0,1[  

Well  Howdy,  my  name  is  Mr.  Random  and  I  live  in  a  town  called  Math.  I  can  give  you  a  random  floaJng  point  value  

between  [0,1[  

Real  Life  Example  in  the  Nerd  World  

Give  random  number.  

0.34523456  

Mr.  Random?  

Mr.  Random     0.34523456  

return  

Real  Life  Example  in  the  Nerd  World  

Did  you  know  that  in  the  town  Math  there  other  guys  living  too?  For  

example:  Mr.  Round,  Mr  Max,  Mr  Min,  Mr  Sin...  

LET'S  START  CODING  

The  usage  of  Class  Library  import java.lang.Math;

public class Example {

public static void main(String [] args) {

double myRandomNumber = Math.random();

System.out.println(myRandomNumber);

}

}

Import  the  class  

class   method  

Mr.  Max  in  Code  import java.lang.Math;

public class Example {

public static void main(String [] args) {

int maxNumber = Math.max(5,10);

System.out.println(maxNumber );

}

}

Import  the  class  

class   method   parameters  

Your  own  Methods  public class Example {

public static void main(String [] args) {

printSomething();

}

public static void printSomething() {

System.out.println("Hello World!");

}

}

Your  own  Methods  public class Example { public static void main(String [] args) { PrintClass.printSomething(); } } class PrintClass { public static void printSomething() { System.out.println("Hello World!"); } }

Parameters  public class Example { public static void main(String [] args) { MyMath.max(5, 10); } } class MyMath { public static void max(int a, int b) { if(a > b) System.out.println(a); else System.out.println(b); } }

Return  public class Example { public static void main(String [] args) { int maxNumber = MyMath.max(5, 10); System.out.println(maxNumber); } } class MyMath { public static int max(int a, int b) { if(a > b) return a; else return b; } }

In  One  class  public class Example { public static void main(String [] args) { int maxNumber = max(5, 10); System.out.println(maxNumber); } public static int max(int a, int b) { if(a > b) return a; else return b; } }

Examples  

•  Flash-­‐demo:  Method  2  •  Flash-­‐demo:  Method  3  •  Flash-­‐demo:  Method  4  •  MethodDemo4.java  

VISIBILITY  OF  VARIABLES  

This  does  not  work,  why?  import java.util.Scanner; class Example { public static void main(String [] args) { Scanner input = new Scanner(System.in); int x = input.nextInt(); if(x == 5) { int b = 5; } System.out.println(b); } }

This  does  not  work,  why?  import java.util.Scanner; class MethodDemo3 { public static void main(String [] args) { askName(); printName(); } public static void askName() { Scanner input = new Scanner(System.in); String name = input.nextLine(); } public static void printName() { System.out.println(name); } }

SoluJon  import java.util.Scanner; class MethodDemo3 { public static void main(String [] args) { String n = askName(); printName(n); } public static String askName() { Scanner input = new Scanner(System.in); String name = input.nextLine(); return name; } public static void printName(String name) { System.out.println(name); } }

EXAMPLE  

Exercise  

•  Implement  a  Java-­‐app  that  prints  square.  The  height  of  the  square  is  asked  from  the  user.  

java Rectangle Height? 4 XXXX X X X X XXXX

import java.util.Scanner; class Rectangle { public static void main(String [] args) { Scanner input = new Scanner(System.in)a; int height = input.nextInt(); // Go all the lines for(int line=0; line<height; line++) { // If the line is the first or last if(line == 0 || line == height-1) { // Print 'X' height amount of times } else { // Print 'X' // Print ' ' height-2 amount of times // Print 'X' } // Print enter System.out.println(); } } }

Let's  use  Methods!  class Rectangle {

public static void print(char mark, int amount) {

for(int i=0; i<amount; i++) {

System.out.print(mark);

}

}

...

}

public static void main(String [] args) {

Scanner input = new Scanner(System.in);

int height = input.nextInt();

// Go all the lines

for(int line=0; line<height; line++) {

// If the line is the first or last

if(line == 0 || line == height-1) {

print('X', height); // METHOD CALL!

} else {

print('X', 1); // METHOD CALL!

print(' ', height-2); // METHOD CALL!

print('X', 1); // METHOD CALL!

}

// Print enter

System.out.println();

}

}