Java Notes

OOP Basics

Static Method

a method that preforms an operation for the entire class, not its individual objects aka class method. Static method invoked by using class name with dot operator:

double interestRate = BankAccount.getInterestRate();

Overloaded

same name diffrent parms

formal parameters

place holders for actual parameters

Information Hiding and Encapsulation

class interface

headings for public methods and public named constants

ADT

abstract data type specification for a set of data and operations on that data

UML Class Diagrams

(+) sign before public methods and instance variables (-) sign before private methods and instance variables

UML Example:

-------------------------------------------------
|   Purchase                                    |  <- class name
-------------------------------------------------
|   -name:string                                |  <- instace
|   -groupcount:int                             |       variables
|   -groupPrice:double                          |
|   -numberBought:int                           |
-------------------------------------------------
|   +setName(string newName):void               |  <- methods
|   +setPrice(int count):void                   |
|   +gettotalcost(): double                     |
-------------------------------------------------

Defining an equals method for a class

if you do not define one, Java will use a default often not returning a desired result

public class Speicies {
    public boolean equals (Species otherObject){
        return (this.name.equalsIgnoreCase(otherObject.name)&&
                (this.population == otherObject.population)&&
                (this.growthRate == otherObject.growthRate);
    }
}

Parameters of a Class Type

public class DemoSpecies {
    public void tryToReplace(Demospecies otherObject){
        otherObject = this;
    }
}

sending a DemoSpecies object to the method tryToReplace will use Java’s call by value parameter mechanism copies the value of the object to the formal parameter.

Assignment to the parameter does not affect the argument. You cannot replace an object passed to it as an argument with another object.

ArrayList

ArrayList is a parameterized class

Linked list

contains data and a refrence to another object in the collection.

ex:

head -> ["Duey" | -]-> ["cheatem" | null ]
//node of a linked list, two instace variables data, and refrence

The getLink method of the ListNode class refrences the private instance variable of a class type. To keep private make ListNode a private inner class of StringLinkedList.

Inner cass definition local to the outher class definition usable anywhere wthin definition of outer class.

I/O for txt files/charecters

creating a PrintWriter object can throw an exception so it must be inside a try/catch statement.

PrintWriter out = null;
try {
    out = new PrintWriter("out.txt");
}
catch(FileNotFountException e){
    System.out.print("Fail");
    System.exit(0);
}

Scanner reader = new Scanner(System.in);
int i = reader.nextInt();
string s1 = reader.nextLine();
string s2 = reader.nextLine();

If you enter:

5 hello
world

i would equal 5 s1 would equal hello s2 would equal world

If you enter:

5
hello
world

i would equal 5 s1 would equal “n” s2 would equal hello

Adding a reader.nextLine() after the nextInt() will avoid problems.

Scanner class

scanner object_name
nextInt()	reads an int
next()	reads until delimiter (whitespace)
nextDouble()	reads double
nextLine()	reads entire line
useDelimiter(Delim_word)	separator aka delimiter

Strings

• immutable
• charAt(Position/index)
• substring (start, end)

Wrapper Classes

also immutable like string always require new for instantiation unlike string

Integer myInteger = new Integer(s);

Integer Class
    int comparTo
    boolean equals
    int intValue
    String toString

Precedence Rules

1. unary operators:

   +,-,!.++,--

2. binary arithmetic operators:

   *,/,%

3. binary arithmetic operators:

   +,-

4. boolean operators:

   <,>,<=,>=

Conditional Operator

(op1 > op2) ? r1:r2 ; If (op1>op2) is true do r1, else do r2