Review notes

Characteristics of Java

• Simple, object oriented,robust, secure, architecture neutral and portable, high proformance,interpreted dynamic, distributed.
• write once, run everywhere

how java works

• To compile, use javac XXX.java.
• To run, use java XXX .
• Not every program has a main method.
• You compile the program into bytecode.
• The bytecode is interpreted by Java virtual machine.

Comments

• // : single line
• /* ....... */ : multiple lines.
• /** ...... */ : multiple lines, can be used to generate documementation.
• You should know the difference between them and how to use them.

main

• public static void main(String[] args)
• It is the entry point of an application.
• Not every class has this method. If you has this method in the class, you can run the program by java ClassName .
• You can overload the main method. But it is not the entry point of the application.
• This method is a class method, with argument of type String[]. The return tpye is void. It is no different from other methods.
• Since args is a String array. You can use
  • args.length to access the length of args.
  • You can use args[0], args[1]... to access the elements in the array
  • If you try to access a non-existent args[i] (i.e. i >= args.length ) an error will occur during runtime.

• you can run the program by java ClassName string0 string1 ..
• String[] args = {string0, string1 , ... } is created
• args pass to main(String[] args) and the method body is executed
• if no argument is given, args is not null but args.length is 0.
• arguments are seperated by space
• if you want to have an arguemnt with space in between, use quotation. e.g. java ClassName "Charles Li".

System.out.println(String s)

• Use System.out.println(string) to print out a string and then go to next line.
• Use System.out.print(String) to print out a string without going to a new line at the end.
• System.out.println(....) can print out int, float,... as well.

class

• Objects has states(members) and behavior(methods)
• Everything in java is a class.
• The filename must be the classname.java

Identifies

1. An identify is a squence of letters , digits, dollar sign $, underscore _.
2. It cannot begin with a digit.
3. It cannot be any reserved keywords like , public, class, void ....
4. Actually it can be in other language (other than English) if you use unicode.
5. They are case senstive . i.e. myName is different from MyName.

Primitive type

• Primitive types are byte, short, int, long, float, double, char, boolean.
• You don't have to memorize the size of the primitive. But you should know that they are platform independent. And long is more precise than int, double is more precise than float etc etc.
• You can define more than more than one variable of the same primitive on the same line, eg.

   int a, b, c

int

• If you don't initiate the value for an integer, it is 0
• You dont't need to understand the octal number and hexadecimal number. They won't appear in the midterm

float and double

• Different ways to declare double are

   double a = 3.14, b = 3, c = 2.713e10, d = 34.665e5d;
   

• Ways to declare float are very similar to double. You have to add f at the end of the number.

  float a = 3.14f, b= 34.665e5f;
  

• If you don't initate the values, they are 0.
• float a = 3.14 causes an compiling error because 3.14 is a double by defalt

long

• The way to declare it is long x = 58819394L; .
• more precious then int

char

• char represents a character.
• example char a = 'a';

boolean

• example

  boolean a = true;
  boolean b = false;
  

Arithemetic

• You should know how to do arithemetic on primitive type.
• The arithmetic operators are very similar to C++.
• When the operands are of 2 different types, the more precious type will be returned.
  example : when a integer add to a float, the result is a float.
  

Flow control

• Refer to my notes on flow control for more detail
• you should know how to use if, else, while, do-while, for, switch .

Array

• To declare an array use ClassName[] arrayName
• To allocate an array with given size, use arrayName = new ClassName[size].
• You should initiate the elements of the array, otherwise they all are null.
• A quick way to initalize array is :
  Example

   
  int[] array1 = {1,2,3,4,5};
  String[] array2 = {"this", "is" , "review" , "notes"}
  

• use array.length to get the size of the array.
• Try to understand arrays of array from my handout

String

• initiate String by String s = "Hi";
• You can add 2 strings together by +
• You should not memorize all the methods in the String class. But you should understand the methods. Documentation will be provided.

class and inheritance

• how to use constructor.
• how to use new.
• how to use private, protected and public and their meaning.
• understand the difference between static variables and non-static variables.
• how to use this and super.
• understand overriding and overloading
• understand 3 ways to use final.
• understand subclass and superclass and how to extends classes.
• understand what is abstract class.
• understand how Java pass information in a method. Java pass the arguments by value.

Import

• Understand the meaning of import
• Import by wild character , example import java.awt.*.
• Import by full name, example import javax.swing.JFrame.
• Import statements are not necessary. Example
  javax.swing.JFrame f = new javax.swing.JFrame("title").

Graphics

• know how to extend JFrame and override paint(Graphics g) method
• understand Font class and Color class, how to set/get font and color
• how to use drawLine, drawRect, fillRect, drawOval, fillOval, drawString methods

JFrame

• getContentPane()
• how to set layout
• constructors
• setSize, setVisible, setLocation....