Java Collections

Pre condition, post condition and Invariants

1. Pre-conditions are the things that must be true before a method is called.The method tells clients "this is what I expect from you".
2. Post-conditions are the things that must be true after the method is complete. The method tells clients "this is what I promise to do for you".
3. Invariants are the things that are always true and won't change. The method tells clients "if this was true before you called me, I promise it'll still be true when I'm done".

Functions in VB

Public Function addSlash(ByVal temp As String)
        If Microsoft.VisualBasic.Right(temp, 1) <> "\" Then
            Return temp & "\"
        Else
            Return temp
        End If
End Function


'VB6
Public Function addSlash(temp As String)
    If Right(temp, 1) <> "\" Then
        addSlash = (temp + "\")
    Else
        addSlash = temp
    End If
End Function

Java Class EASY MODE

package somePackage;
import java.*;

public class someClass
{
   public someClass()
   {
   //Constructor
   }
   public void someMethod()
   {
   //do stuff
   }
}

Java Access Modifiers

Public, Protected, Default & Private

Polymorphism

Polymorphism

When methods understand the same message but it means different things, so, if we have two classes and both classes have a method called toString()

Each classes toString() will do something slightly different.

When methods are overridden we can say that they are Polymorphic

Formal Argument v Actual Argument

Formal Argument...

Formal when declaring and the Actual is when called.

public doSomething (Frog aFrog) //Formal

dosomething(jimmy) // Actual

call a non static method from within a static method

To call a non static method from within a static method, do as below;

Exceptions in Java

ArithmeticException – an instance of this class is thrown when an exceptional arithmetic condition has occurred; for example, when integer division by zero has occurred.

NullPointerException – an instance of this class is thrown when you try to send a message to a reference variable whose value is null; that is, it does not reference an object. (The use of the word ‘pointer’ rather than ‘reference’ is a curiosity of the language, a throwback to C language terminology.)

NumberFormatException – an instance of this class is thrown to indicate that a method that converts a string to one of the numeric types has been passed an argument that does not have the appropriate format.

IndexOutOfBoundsException – an instance of this class is thrown when a program tries to access a string with an index outside the valid limits.

public static final

public static final

public static final int HOURS_IN_DAY = 24;