Java Methods

Topics  Declaring fields vs. local variables  Primitive data types  Strings  Compound Assignment  Conversions from one value to another  Arithmetic operators

Declaration vs. initialization  The following is JUST declaration  int sum;  double x,y;//note, 2 variables declared  Declaration including initialization  int mileage = suburban.getMileage();  Declaration and initialization are often separate  UrRobot karel;  karel = new UrRobot(1,1,East,0);

Declaration vs. initialization  Additional notes:  int x = 5; //assigning x the value of 5  x is referencing the value of 5  final double gravity = 9.8;  The reserved word final makes the variable become a constant  Useful for values that should never be changed

Fields vs. Local Variables  Fields – declared within a class, but outside of any constructor or method  - fields are accessible to the entire class  - fields are typically declared private // let us start observing this style rule  Local variables – temporary variables declared inside a constructor or method  Every time a method is called, that variable is re- initialized  Note: your program can have fields and local variables of the same name  avoid this!

Parameters  Parameters are considered a third type of variable in addition to fields and local variables  Parameters – variables passed to constructors or methods  - act like local variables  public void moveSteps (int step); public static void main(String[] args) { moveSteps(int x = 6);}  It is bad convention to declare and initialize when you call a method, but this demonstrates that each method creates a new reference for the parameter, so you can give it a new name

Primitive Data types  There are 8 – listed on page 128  We will use: boolean, char, int, double  Boolean – true or false  int – for use of integers – cuts off any decimal approximations (no rounding!  5.9 = 5)  double – for use of decimals  char – unicode character set- ASCII code  If you had char x = 46, what does System.out.println(x) do?

Unicode values  When declaring and initializing a char type variable, use single quotations:  char initial = ‘a’;  Fact!  unicode values are 16 bit  To see all unicode values, copy the following: for(x=0; x<26; x++) { for(y=0; y<10; y++) //will do values from 0 to 260 { System.out.printf(" %4d = %4c", 10*x+y, 10*x+y); } System.out.println(); }

Casting  Suppose you had the following:  double hour = 3.5;  int miles = 20;  Check to see if the following compiles:  double mph = miles/hour;  Int and double are apples and oranges!  The fix  a cast  double mph = (double)miles/hour;

Arithmetic Expressions  Arithmetic expressions  You should know the following arithmetic operators: +,-,*,/  An important one you need to know:   modulus - %   modulus will give you the remainder of a quotient  Ex) 10%3 will give you 1

Arithmetic Expressions  You can use modulus as an effective even or odd evaluator  If (x%2==0)  if (x is even)  if( x%2!=0)  if(x is odd)  The modulus of a number is never larger than it’s divisor  i.e. (255%16 = 15)

Compound Assignment  When doing arithmetic operations to update a variable, it is recommended to use compound assignment  Take x = x +y;   x+=y;  You can do this for any operator

Pre/Post Increment  It is also recommended to use pre and post increment/decrement operators  Post increment: operator applied after the expression is used  x++ (x= x +1) & x-- (x = x-1)  Pre increment: operator applied before  ++x (x=x+1)  If y = 0, x =3, what is the difference between:  y = x++ and y = ++x ?  Use a System.out.println(y) to check  Can you explain it?

Strings  Strings are not primitive data types; they are objects derived from the String class  We call them “literals”  Ex): String str = “karel”;   the variable str references the String “karel”  Concatenation: String str = “karel” + “J”;  System.out.println(str);  karelJ

Escape sequences  These are specific character combinations in a String that the compiler recognizes  ex) \n  newline (check page 131 for the rest)  System.out.println(“Hi, how \n are you”);  What does that look like?

Converting to a string  For primitive values:  Just concatenate an empty string!  Ex) int amount = 15;  System.out.println(“” + amount);  Or  String amount = “” + amount;

Work  Page 143 – finish the Pie Chart program!  Exercises: 1,2,4,5,6,7,10,12,13,18,19