Here are the tools you need for your Midterm

• Create a new program

public class MyClassName
{
     // you can define a class variable here
     public static void main( )
     {
           // your statements will go here
           // your calls to method1 will have the form: method1()
     }
     public static void method1( )
     {
           // your statements will go here
     }
}

Reminder: After every line, place a ; or {

• create a variable
  • In Java: <type> <variable name> ;  example: int myIntVar;
  • declare with correct type: int, double, char, boolean, (not capitalized)
  • declare Object variables with types such as : String, Scanner, Random, (capitalized)

   

• change variable contents:
  • In Java: <variable name> = <some expression>; example:  myIntVar = 5 ;.

   

  Reminder: Do not create a variable more than once in the same method

• Say something on the screen: 
  • In Java: System.out.println(<whatever you want to print> + <whatever you want to print> );
    
  • example: System.out.println("Hello, my name is " + myName + "My number is " + myNumber);
  • Remember: the semi-colon; capitalize System, use both parentheses;
    
  • Remember quotes around text and not around variable names
  • variations: printf and print

Loops

For loop - repeats when you know the number of times to repeat

for (statement to start the loop; condition that evaluates to true or false; statement to run when the loop repeats)
{
// do something
}

What you need to know:

• loop control - know how to set up a loop to run a certain number of times
• scope - variables created in a loop are destroyed at the end of the loop
• only what is in the {} will repeat.
• to accumulate - initialize a variable before the loop, add to itself during the loop, and then print the total at the end of the loop
• tracing - be able to trace all variables in the program including the counter. Know that the first time through the loop it executes the starter statement, and every time follwing it executes the incrementer.) From this you can tell how many times a loop will run.
• inner loop - should not use same counter as outer loop; know what it is, but will not have an inner loop question on the quiz.

FOR coding helps: (but not needed explicitly for the test)

• Don't put a semi-colon at the end of the for condition
• Remember to create the variable used to count the loop with an int statement somewhere in your program. (either in the for statement or before it)
• You can add to the counter between the brackets as well.
• After the first time through, the for loop does the "statement to run when the loop repeats" first, and then checks whether the condition fails.
• When the condition fails, it goes to the bottom bracket of the loop and continues on
• You can have loops inside loops
• Stopping when equal to a single number is dangerous - go with < or > instead when you can.

---

Work with methods

• Define a method

  public static return_type method_name (type parm1, type parm2)
  {
  ...
  ...
  return expression;
  }

• add a method right before the class closing curly braces
• a method header is immediately before the method's curly braces
  • ex: public static double myMethod(int x){ // your statements go here}
• a method's parameter variables are created inside the parentheses (ex: in the statement above, int x is the parameter creation
• a method's return type is defined before the method name (ex: in the statement above, double is the return type)
• a method with a return type needs to end with a return statement.
• a method with no return type (void) cannot have a return statement
• Class with methods creation step by step guide

 

Reminder: Ensure your method is defined to always return the correct type.

• Call a method:
  • object_or_class.method_name(parm1, parm2);
  • You need the class or object if the method is not in your own class.
  • Then, you need the object if the method needs to know some information that is not passed in.
  • for static methods you create: <method name> (any parms). ex: myMethod(3);
  • for static methods that are not in your class: <class>.<method name> (any parms) ex: Math.pow(5,2);
  • for instance methods that are not in your class: <object>.<method name> (any parms) ex: myScanner.nextDouble();
  • parameters must match in type and number
  • for methods that return a variable: set it = to something. ex: myDoubleVar = myMethod(3);
  • Call Math methods (and save their results into variables)
    • absolute value ex: myDoubleVar= Math.abs(#);
    • random ex: myDoubleVar = Math.random();
    • minimum: Math.min(#,#)
    • maximum: Math.max(#,#)
    • raise to a power: Math.pow(#,#) : Note: The first number is raised to the second number
      
    • knows PI and E

   

  Reminder: Containers cannot reach into other container's values; only pass values using input/output variables.

---

• Setup to read something from the screen:
  • In Java: there are 2 commands needed:
    
  • before the class starts, import the scanner class using: import java.util.Scanner;
  • inside the method, create a scanner object using: Scanner kbd = new Scanner(System.in);
• Read something from the screen:
  • In Java: <your variable> = < name of the scanner you created above>.nextLine();
    
  • Ask the user for something first, or the system will appear to hang waiting for the user to answer a question you never asked.
  • Use the scanner method that matches your variable type.
  • example to get the whole line: myStringVar = kbd.nextLine();
  • example to get the next word: myStringVar = kbd.next();
  • example to get the next double: mydoubleVar = kbd.nextDouble();
  • example to get the next int: myIntVar = kbd.nextInt();
  • See Sun's page on scanner for much more

---

• Create boolean expressions:
  
  • use when you want to save the result of a true/false question
  • use comparison operator: ==, <,>,<=,>=,!=
  • use logical indicator
    • OR is ||
    • AND is &&
  • example: myBooleanVar = x > 5 && x < 10 (x can be between 5 and 10)
  • example: myBooleanVar2 = x <5 || x >10 (x can be less than 5 or greater than 10)

If / if else / else

if (condition that evaluates to true or false)
{
// do something
}
else if (condition that evaluates to true or false) // remember that everything that met first criteria doesn't get here
{
// do something else
}
.
. // repeat else if as needed
.
else // remember to put no condition
{
// do something
}

If facts:

• else is not needed
• when the condition is met, it executes inside the parentheses and then goes to the end of the if and continues on down
• You can have ifs inside the brackets for another if

Methods to know how to use: (You will not be asked to memorize these method names. If you need the method name, the test will give you possible methods to use)

Decimal Format: Library: import java.text.DecimalFormat;
Create instance: DecimalFormat percentage2 = new DecimalFormat(“0.00%”);
Call methods: System.out.println(percentage2.format(.308)); // will print 30.80

0 = required position
# = show it if you have it
% = make it a % (mult by 100)
E = E notation (scientific)

String: •stringvar.charAt(3) // will be a character
•stringvar.length() // will be an integer
•stringvar.equals(somethingOtherString) // will be either true or false (a boolean)
•stringvar.toUpperCase() // will be a String that is all uppercase
•stringvar.toLowerCase() // will be a String that is all lowercase
•stringvar.subString(2) // will be a String starting in position 2 to the end
•stringvar.subString(2, 5) // will be a String starting in position 2 and ending in position 4
•ex: newString = myOriginalString.toLowerCase();

Picture Creation basics

• import picture knowledge: javalib.worldimages.*;
• import color knowledge: java.awt.Color;
• Make WorldImages using AImage.make<shape> , AImage.makeFromFile
• Place images on top of each other using <your world image>. place (<your other world image> , x,y)
• x indicates how far to the right; y indicates how far down (positive)
• Show your image with <your world image>.show()

How to approach a problem

• Reorganize problem into sections, inputs, outputs and processes; Flowchart complex processes
• Write test cases on paper for all methods that return values
• code class & method headers only with one valid return value for each method
• Write comments explaining what each method must do
• Import any knowledge you need (pictures, Random, DecimalFormatter, Scanner)
• Translate comments into code

  Reminder: code, then trace