CS112 2012S 09 More Arrays and Static 1 09 0 Array Review Arrays store a list of objects or primative types Fixed size can t change size of an array Get the length of the array get at elements of an array 09 1 Array Review On board What memory looks like for the following public static void main String args int A int x String B Show memory here A new int 5 B new String 3 Show memory here 09 2 Array Review int x String A A new String 5 x A 0 length What happens here 09 3 Array Review int x String A A new String 5 x A 0 length Null Pointer Exception 09 4 Array Review int A new int 10 for int i 0 i A length i A i i Anywhere we could use an integer variable we can use A i for any integer expression i A i 2 4 foo A x y Of course we will get a run time exception if the value of the expression is not between 0 and A length 1 CS112 2012S 09 More Arrays and Static 09 5 2D Arrays We can create 2D arrays as well as 1D arrays Like matrices 2D array is really just an array of arrays 09 6 2D Arrays int x int y Declare a 2D array Alternate way to declare 2D array x new int 5 10 y new int 4 4 Create 50 spaces create 16 spaces 09 7 2D Arrays int x x new int 5 5 x 2 3 11 x 3 3 2 x 4 5 7 Declare a 2D array Create 25 spaces ERROR Index out of bounds 09 8 2D Arrays How would we create a 9x9 array and set every value in it to be 3 09 9 2D Arrays How would we create a 9x9 array and set every value in it to be 3 int board board new int 9 9 for int i 0 i 9 i for int j 0 j 9 j board i j 3 Show memory 09 10 2D Arrays int intMatrix intMatrx new int 5 for int i 0 i 5 i intMatrx i new int 5 09 11 2D Arrays 2 CS112 2012S 09 More Arrays and Static 3 int intMatrix intMatrx new int 5 for int i 0 i 5 i intMatrx i new int i 09 12 Using Arrays Need to declare array size before using them Don t always know ahead of time how big our array needs to be Allocate more space than we need at first Maintain a second size variable that has the number of elements in the array we actually care about Classes that use arrays often will have an array instance variable and a size instance variable how much of the array is used 09 13 Using Arrays public class StringArrayList String data int listSize o public StringArrayList data new String 10 listSize 0 other methods 09 14 Using Arrays public class StringArrayList String data int listSize int size Fill me in other methods 09 15 Using Arrays public class StringArrayList String data int listSize int size return listSize other methods 09 16 Using Arrays public class StringArrayList String data int listSize void add String newString Fill me in other methods 09 17 Using Arrays CS112 2012S 09 More Arrays and Static 4 public class StringArrayList String data int listSize void add String newString data listSize newString listSize other methods What happens when someone tries to add 11 strings to this StringArrayList 09 18 Using Arrays public class StringArrayList String data int listSize void add int index String newString Fill me in other methods 09 19 Using Arrays public class StringArrayList String data int listSize void add int index String newString for int i listSize i index i data i data i 1 data index newString other methods 09 20 Static Normally can only call methods on classes when we created an instance of the class Methods can rely on instance variables to work properly Need to create an instance of a class before there are any instance variables What would the size method return for an ArrayList if there was not an instance to check the size of 09 21 Static Some methods don t operate on an instance of the class pure functions that don t use instance variables at all Math functions like min or pow parseInt takes a string as an input parameter and returns the integer value of the string parseInt 123 returns 123 Seems silly to have to instantiate an object to use these methods static to the rescue 09 22 Static If we declare a method as static it does not rely on an instance of the class Can call the method without creating an instance first CS112 2012S 09 More Arrays and Static 5 Use the Class Name to invoke call the method double x Math min 3 4 6 2 double z Math sqrt x 09 23 Consants Having 3 14159 appearing all over your code is considered bad style Could end up using different values for pi in different places 3 14159 vs 3 1415926 If you want to change the value of pi to add more digits for instance need to search through all of your code to find it In general any time you have a magic number that is an arbitrary numeric literal in your code it should probably be a symbolic constant instead The final modifier is used to prevent you from changing the value of a variable 09 24 Consants class Calendar final int MONTHS IN YEAR 12 final int DAYS IN WEEK 12 final int DAYS IN YEAR 365 Methods that use the above constants Every instance of class Calendar will contain those 3 variables Somewhat wasteful Need to instantiate an object of type Calendar to access them 09 25 Consants Static We can declare variables to be static as well as methods Typically used for constants Math pi Math e Access them using class name not instance name just like static methods You should only use static variables for constants public static final float pi 3 14159
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