Java Programming Notes PDF

## JAVA into notes ### What is a class? - All Java programs are contained in a class. - The start of a class is indicated by the reserved word `class`. - There are about fifty (50) reserved words in Java. - **Brackets `{}`**: Brace brackets are used to indicate the beginning and end of any sec...

## JAVA into notes ### What is a class? - All Java programs are contained in a class. - The start of a class is indicated by the reserved word `class`. - There are about fifty (50) reserved words in Java. - **Brackets `{}`**: Brace brackets are used to indicate the beginning and end of any section of a Java program. - **Comments `//`**: Two slashes indicate a comment in Java. - Anything on a line after the `//` is ignored by the compiler. - It is also possible to enclose longer sections of code in a comment using the older style of comments: - `/* beginning of comments end of comments */` ### JAVA 1 #### What is Programming? What does a comp program do? - A computer program tells a computer in minute detail the sequence of steps that are needed to fulfill a task. - The act of designing and implementing these programs is called computer programming. #### The Java Programming Language - In 1991 a group led by James Gosling and Patrick Naughton at Sun Microsystems designed a language that they code named "Green" for use in consumer devices such as intelligent television "set-top boxes". - The language was designed to be simple and architecture-neutral, so that it could be executed on a variety of hardware. - Gosling realized in 1994 "We could write a really cool browser" - The HotJava browser was shown to an enthusiastic crowd at the SunWorld exhibition in 1995, and had one unique property. - It could download programs called Applets, from the web and run them. - Applets written in the language now called Java, let web developers provide a variety of animations and interaction that can greatly extend the capabilities of a web page. - In 1996 both Netscape and Microsoft supported Java. #### Java Programming - System.out.println Method - `System.out.print("\n")` - will take me to the next line (this is a reserved character in Java- there are many reserve characters - we will look at this later in the course) ### Object #### What is an object? - An Object is an entity that you can manipulate in your program by calling Methods. - For example you saw in yesterday's class that `System.out` refers to an Object and you saw manipulate it by using the Method (`println`). #### What are considered as 'black bones'? - You should think of the objects as a "black box" with a public interface (the methods that call) and a hidden implementation (the code and data that are necessary to make these methods work). #### What does a class do? - Every Object belongs to a class. - The class defines the methods for the objects. - There the String class defines the `length` method and many other methods. #### String class defines which method? - The `system.out` method is created automatically what a Java program loads the System class. #### system.out Method is related to which class? - Let us create an Object. - We are going to use the Rectangle Class in the Java library. **Note:** - The Rectangle object isn't a rectangular shape - it is a set of numbers that describe the rectangle. - Each rectangle is described by the x and y coordinates of its top left corner, its width and its height. - i.e. (5, 10, 20, 30) **To implement a rectangle object use the following:** - `System.out.println(new Rectangle ((5, 10, 20, 30));` - you can also use `Rectangle namer = new Rectangle (5, 10, 20, 30);` - this will give the object the name namer **Try both** #### What is your output for the System.out? #### What is the pro of creating new? #### New command causes the creation of what? - The `new` command causes the creation of an object of type Rectangle. - This process of creating a new object is called construction. - The four values 5,10,20,30 are called the construction parameters. - Different classes will require different construction parameters #### A class definition contains two types of elements: variables and methods - Variables are used to store the Objects information. - Methods are used to process the information. #### Instance field - A Variable defined in a class for which every object of the class has its own value. #### Variable - A Symbol in a program that identifies a storage location that can hold different values. ## Example: - `public class Rectangle` - `private double length;` // instance variables - `private double width;` - `public Rectangle (double I, double w){` - `ength =l;` - `vidth = w;` // rectangle constructor - `public double calculateArea()` - `return length*width;` ## Data Type #### What is a variable? - A variable is a named piece of memory that you use to store information in your java program. - Each variable has three important attributes: - **a name that you choose** (`Identifier`) - **a value that you give it and that you can change.** - **A type which limits what kind of value you can hold.** #### What 3 attributes a variable has? #### What is an identifier? - The name that you choose for a variable is called an identifier. - An identifier can be any length, but must start with a letter and underscore or a dollar sign. - An identifier must start with `Eg. int numberOfstudents = 98;` #### How many basic types of data type? #### categories of datatype? **Data Types - 8 basic types** **Defined in three categories.** 1. **Numeric** - `Integers` - `Placting points` - `int` - `short` - `long` - `float` - `double` - `Type char (single)` 2. **Boolean** - `true` - `false` - `Byte` 3. **Characters** #### How many kinds of numeric data type are there? - There are two kinds of numeric data types (integers (no fractions) and floating points - There are four types of integers (byte, Short, int and Long). - Within Floating point data type there are float and double) #### Name types of integers and floating points? ## Numeric Values that can be store from Numeric Data Type | Type | Storage | Min Value | Max Value | | :-------- | :-------------------- | :---------- | :---------- | | byte | 8 bits | -128 | 127 | | short | 16 bits | -32 768 | 32 767 | | int | 32 bits | -2 147 483 648 | 2 147 483 647 | | long | 64 bits | -9 223 372 036 854 | 9 223 372 036 854 | | float | 32 bit | -3.40E+038 | 3.40E+037 | | double | 64 bit | -1.70E+308 | 1.70E+308 | **Examples:** - `int intValue = 4;` - `double doubleValue =1.493233434;` ## Characters - A variable of type `char` holds a single character. **Example:** - `Char charValue = 'c';` - `Char charVall = '\t';` **Special Characters:** - `\b` = backspace - `\t` = tab - `\n` = newline - `\r` = Carriage return - `\"` = Double Quote - `\'` = Single Quote - `\\` = Backslash - **Is it possible to specify a character by quoting its number within the Unicode character set. The system runs from 0 to 65535, but it must be specified in hexadecimal following the prefix?** - `\u, so it runs from \u0000 to \uFFFF` **Example:** - `char CharValue2='\u0045';` ## Booleans - The Boolean data type has only two valid values: true and false. - A Boolean variable is usually used to indicate if a particular condition is true or not. **Example** - `boolean found = true;` ## Arithmetic Expressions - Programming statements often involve expressions which are combinations of operators and operands used to perform calculations. - All expressions are evaluated according to an operator precedence hierarchy that establishes the rules that govern the order in which the operations are evaluated. - **What is operator precedence hierarchy?** - Any arithmetic operator at the same level of precedence are performed left to right. - Any expression in parentheses is evaluated first. - Parentheses can be nested, and the innermost expressions are evaluated first. ## Constants - Constants are identifiers and are similar to variables except that they hold a particular value for the duration of their existence. - **What are constants and how are they different from variables?** **the syntax:** - `nal typeName variableName = expression.` **Example** - `nal double NickelValue = 0.98;` ## Java User Input - The `Scanner class` is used to get user input. - When using the Scanner class, create an object of the class and use one of the available methods. **Example:** - `mport java.util.Scanner; // Use this command to import the Scanner` - `class` - `class testing_input {` - `public static void main(String[] args) {` - `Scanner object = new Scanner(System.in); // This will create a Scanner object` - `System.out.println("Enter username");` - `String Name = object.nextLine(); // This will read the input by the user` - `System.out.println("Username is: " + Name); // This will output the Name` - `}` - `}` ## Method | Method | Description | | :--------------- | :-------------------------------- | | `nextBoolean()` | Reads a boolean value from the user | | `nextByte()` | Reads a byte value from the user | | `nextDouble()` | Reads a double value from the user | | `nextFloat()` | Reads a float value from the user | | `nextInt()` | Reads a int value from the user | | `nextLine()` | Reads a String value from the user | | `nextLong()` | Reads a long value from the user | | `nextShort()` | Reads a short value from the user | ## Getting Started With Java – Variables - In order to create useful programs, we need the ability to store information and retrieve that information as needed. - Information is stored in the memory of the computer in specially reserved areas known as variables. - **Why are identifiers used?** - In order to keep track of these variables, we use identifier names (or simply identifiers) for each variable, method, or class in our program. - The rules for creating any type of identifier are: - Any character used in an identifier must be a letter or the alphabet, a digit (0, 1, ..., 9), or an underscore character (_). - The first character cannot be a digit - **By convention, classes are given identifiers using PascalCase, where each word begins with a capital letter.** - **By convention, variables and methods are given identifiers using camelCase, where the first letter is lowercase, and any other words in the identifier are uppercase.** - You cannot use any of the following reserved words, each of which has a predefined meaning in the Java language. | | | | | | | | | | | | :-------- | :---------- | :------- | :------ | :-------- | :------- | :-------- | :------ | :-------- | :-------- | | abstract | assert | boolean | break | byte | case | catch | const | continue | default | | double | do | else | enum | extends | false | final | for | goto | if | | implements | import | instanceof | int | interface | long | native | new | null | package | | private | protected | public | return | short | static | strictfp | super | switch | synchronized | | this | throw | throws | transient | true | try | void | volatile | while | | ## Declaring Variables - **Why do we need declaration statements?** - To reserve space in memory for variables, we need to write a declaration statement, where we specify the type of a variable and its identifier (i.e., name). - **What do you specify when declaring?** - A simple declaration might look like `<type> <identifier>;` **For example:** - `int count;` - `float area;` - `boolean finished;` - Note the a declaration statement, like any other statement in Java is terminated (i.e., ended) by the semicolon character. - It is possible to declare multiple variables of the same type using a single declaration statement of the form `<type> <identifier₁>, <identifier2>, .... <identifierk>;` **For example:** - `float radius, circumference, area;` ## Assigning Values to Variables - Declaring a variable reserves space in the computer's memory for information of a particular data type (e.g., int, or float, or char). - This variable does not become useful until we assign a value to it. - **How and what are the ways to assign variables?** - **Until when declaring variable becomes useful?** 1. **When declaring the variable** - `int total = 15;` 2. **Immediately after declaring the variable** - `int total;` - `total = 15;` 3. **After declaring the variable, but later in the program** - `int total;` - `total = 15;` 4. **Using the value from another variable** - `int a = 5;` - `int b;` - `b = a;` ## Converting Between (Numeric) Data Types - **What is important when assigning a value to a variable?** - The assigning a value to a variable, it is important that the data type match the value. - For numeric data types, it is possible to convert from a smaller data type to a larger data type, but not the reverse. - **What is the old of legal conversions ?** - **Is it possible to forme Java to accept a conversion?** - **legal conversions:** byte → short → char → int → long → float → double - It is possible to force Java to accept a conversion, even if it breaks Java's own rules. This is called a cast, and it is essentially a promise to the compiler that you know what you are doing. - It has the form: `<variable> = (<data type>) <questionable data>;` **For example,** - `short a;` - `a = (short) 75000;` - `System.out.println(a);` - // range of -30000 to +30000 - // 75000 doesn't fit in short - override! - // output will be 9464 since 75000 - // doesn't fit! ## Dutput of Variables - The `print` and `printin` methods can be used to output variables or combinations of variables. - Keep in mind that the variable must have been given a value before you try to output it, or you will get an error. ## Class PrintValue - `public static void main(String[] args)` - `{ - `int length = 5;` - `int width = 10;` - `System.out.print("The length is " + length); ` - `System.out.println(" and the width is " + width);` - `}` - The length is 5 and the width is 10 ## Constants - Java allows us to associate an identifier with a constant value through the use of the `final` modifier in the declaration of a variable. - By convention, constants are given identifiers which are all upper case, which helps easily identify them when reading code. - `final int CLASS_SIZE = 30;` - `final char TERMINATOR = '*';` - `final float PI = 3.1415;` - Once an identifier has been declared to be final, its value can never be changed. ## Numeric Operations | Order | Operator | Operation | | :---- | :------- | :----------------------------------- | | 1 | `()` | Parentheses | | 2 | `++ --` | Increment, Decrement | | 3 | `new` | Object creation | | 4 | `*/%` | Multiplication, Division, Modulus | | 5 | `+-` | Addition, Subtraction | | 6 | `><== >= =` | Relational Operators, Logical Operations | | 7 | `==!=` | Equality Operators | | 8 | `&` | Boolean AN | | 9 | `^` | Boolean XOR | | 10 | `\|` | Boolean OR | | 11 | `&&` | Logical AND | | 12 | `\|\|` | Logical OR | | 13 | `?:` | Conditional Operator | | 14 | `= += -= *= /= %=` | assignment | **Example:** - `9+6<=25*4+2 ` - `Step 1: (9+6) < = ((25 * 4) + 2 )` - `Step 2: (9+6)<= (100 + 2)` - `Step 3: 15 <= 102` - `Step 4: true` ## Arithmetic promotion - **What is arithmetic promotion?** - Occurs automatically when certain arithmetic operators need to modify their operands in order to perform the operation. **Example:** - `X=9+6/5` - `1. 16/5 = 3` - `2. 9+3=12` - `3. 12` - **What happens when 2 different types of are involved?** - In general when two different types are involved in a operation, the smaller type (the one with fewer bits) is converted to the larger type before the operation is performed. ## Class Library - **What is class library?** - The class library is made up of several clusters of related classes, which are called JAVA API. API stands for Application Programmer Interface. - The classes of the Java Standard class library are also grouped into packages, which like the APIs; lets us group related classes by one name. - The String class for example is part of the `java.lang` package. #### Examples: | Package | Provides support to: | | :---------- | :------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | `Java.Applet` | Create programs (Applets) that are easily transported across the web | | `Java.awt` | Draw graphics and create graphical user interfaces | | `Java.io` | Perform a wide variety of input and output functions | - **Syntax:** `import java.Applet.*; // opens everything from Applet` #### Exiting an Application: - Use the following code: - `System.exit(0);` ## Reading Console Input/Alternative - **What does console input read?** - Console input reads from the `System.in` object, which can only read bytes. - Keyboard consists of characters. - **What do you have to do in order to read characters?** - To get a reader for characters, you have to turn `System.in` into an `InputStreamReader` object: **Syntax:** - `InputStreamReader reader = new InputStreamReader (System.in);`<br> - An input stream reader can read characters, but can't read a whole String at a time. - **How to overcome limitation of InputStreamReader?** - To overcome this limitation you turn an `InputStreamReader` into a `BufferReader..` **Syntax:** - `BufferedReader console = new BufferedReader(reader);` **Example Coding** - `import java.io.*; // opens everything from java.io` - `Public class ReadingInput` - `{` // open ReadingInput - `public static void main(String[] args) throws IOException` - `{` // open main - `InputStreamReader reader = new InputStreamReader(System.in);` ## The If Statement - **What does an if statement do?** - The if statement lets a program carry out different actions depending on outcome of a condition. - An if statement consist of ther reserved word `if` followed by a boolean expression, or condition. - The condition is enclosed in brackets and must evaluate true or false. - If the condition is true then the statement is executed - If the condition is false then the statement is skipped **Syntax** - `if (expression/condition )` - `statement;` - `next statement;` **Example:** - `ublic class example_1 {` - `ublic static void main (String args [])` - `{ - `int num = 5;` - `(num %2==1)` - `ystem.out.println(num + " is odd number");` - `(num%2 == 0)` - `System.out.println (num + " is even number");` - `}` - `}` ## The if - else Statement - **What does an if/else statement do?** - An if – else statement allows a program to do one thing if a condition is true and different thing if the condition is false. **Syntax** - `if (expression/condition)` - `{ - `Statement;` - `}` - `else` - `{` - `statement;` - `}` **Example:** - `public static void main (String args[])` - `{ - `int num = 5;` - `if (num % 2 == 1)` - `System.out.println (num + " is odd number");` - `else` - `System.out.println(num + “is even number");` - `}` - `}` ## If - Elseif **Syntax** - `If (condition/expression) // lower case if` - `{ - `statement;` - `}` - `elseif (condition/expression)` - `{ - `statement;` - `}` - `elseif (condition/expression)` - `{ - `statement;` - `}` - `else` - `{ - `statement;` - `}` - `} // close if` ## Iterations - **What is the purpose of a while loop?** - A while statement executes a block of code repeatedly. - A termination condition controls how often the loop is executed. **Syntax** - `while (expression)` - `{ - `// statements;` - `}` **Example:** - `public class example_1` - `{ - `public class static void main (String args[])` - `{ - `final int limit =5;` - `int count =0;` - `while (count<limit)` - `{ - `System.out.println(++count );` - `}` - `}` - `}` - **What happens when given logical expression becomes false?** - This loop executes as long as the given logical expression between brackets is true. - When the expression is false, execution continues with the next statement outside of the loop. **Example_2:** - `public class example2` - `{ - `public static void main(String args[])` - `{` - `int count =1;` - `while count <= 10)` - `{ - `String display = count % 2 == 1 ? " ****" : " ***";` - `System.out.println (display);` - `++ count;` - `}` - `}` - `}` ## The do Statement - **Difference b/w while and do loop?** - The do statement is similar to the while statement, except that its termination condition is at the end of the body of the loop. - Like the while loop, the do loop executes the statements in the loop body until the condition becomes false. - The condition is written at the end of the loop to indicate that is not evaluated until the body of the loop is executed. - Therefore the body of a do loop is always execuated at least once. **Example:** - `public class example3` - `{ - `public static void main(String args [])` - `{ - `final int limit =5;` - `int count =0;` - `do` - `{ - `System.out.println (++count);` - `}` - `while (count<limit)` - `}` - `}` **Syntax** - `do` - `{ - `// statements;` - `}` - `while (expression)` ## The for Statement - **What is the fundamental diff b/w while and do staments and for statement?** - The while and do statements are good to use when you don't initially know how many times you want to execute the loop body. - The for statement is another repetition statement that is particularly well suited for looping a specific number of times. **Syntax:** - `for (initialization_expression; loop_condition; incrament_expression)` - `{ - `//statements;` - `}` - **what are the 8 parts of for statement?** - The first part - initialization_expression: - This is executed before execution of the loop starts - `loop_condition` – loop will continue as long as this statement is true. The condition is checked at the beginning of each loop cycle - `incrament_expression` - used to incrament loop counter. **Example:** - `public class example_4` - `{ - `public static void main(String args [])` - `{ - `final int limit = 6;` - `for (int count =0; count < limit; count++) - `System.out.println (count);` - `}` - `}` - `}` **Example:** - `public class example_5` - `{ - `public static void main (String args[])` - `{ - `for (int i=0; <= 10; i++)` - `{ - `for (int j = 0; j <= 5; j++)` - `{ - `System.out.println("*");` - `}` - `System.out.println();` - `}` - `}` - `}` ## Arrays - **What is an array?** - An array is a data structure that defines an ordered collection of a fixed number of the same data type. - The size of an array is fixed and cannot increase to accommodate more elements. - In Java, arrays are objects. - **Simple arrays are one-dimensional arrays → simple list of values.** ### Declaring array variables **Syntax:** - `<elementType> [] arrayName;` - or - `<elementType><arrayname> [];` **where `<elementType>` is the variable type → int, String, long, float etc...** ### Examples - `int a [], b;` - `Rectangle [] medium, large;` - **Is variable b an array of type int?** - When `[]` follows the type, all variables in the declaration are arrays. - Otherwise the `[]` notation must follow each individual array name in the declaration. ### Constructing an Array - **How can an array be constructed?** - An array can be constructed for a specific number of elements type, using the `new` operator: - `<arrayname> = new <elementtype> [ <NumberofElements>]` - The minimum number of elements is 0. **Example:** - `int a [], b;` - `Rectangle [] medium, large;` **Array construction:** - `a= new int [10]; // array for 10 integers` - `medium = new Rectangle [5]; // array of 5 Rectangles;` **The array construction can be combined:** `<elementType> <arrayName> [] = new <elementType> [<numberOfElements>];` **Example:** - `Int a[] = new int [10];` - `Rectangle medium [] = new Rectangle [5];` ### Initializing an Array - `<elementType> [] <arrayName. = {< values>];` **Examples:** - `Int a [] = { 1, 256, 123};` - `String courses [] = {"com", "sci"};` - We can also assign values: - **How is the whole array referenced?** - **What are individual elements of an array called?** ### Using an Array - The whole array is referenced by the array name, but the individual array elements (index) are accessed using the `[]` operator. - **How are indexes accessed?** - The lower bound of any array is 0, the upper bound is one less than the number of elements. - **What are the upper and lower bounds of an array?** **Example:** - `Int a [] = new int [10];` - // has 10 elements - **To access the first element** - `a [0]` - **To access the last element** - `a [9]` - **There is no a [10];** - We can also assign values using our element index. - `Int a[] = new int [10];` - `a [0] = 10;` - `a [9] = 20;` - **This is very useful in a looping structure.** - **assigning values by using element index** ## Class Exercise: - Given the following, write a method within the Example class, called `Maximum` to determine the maximum value of the array `b`. - **Things to consider:** - Do I need an array? - Should I create another array to check and store the information? - **To determine the length of an array use the .length Method** **Example:** - `a.length` - where a is the array - `length` is the method which will return an int – make sure you save this a variable if needed. - `Public class Example` - `{ - `Public static void main (String[] args)` - `{ - `Example ex = new Example ();` - `double b [] ={10,12,5,-1,2};` - `System.out.println("The maximum is: + ex.Maximum(b));` - `}` - `}` ## Sorting Algorithms in Java - **Why programmers use sorting algorithms?** - There are many different sorting algorithms that programmers use to sort information in a list (array), most often in numerical or alphabetical order. - **Name one method to sort info in an away?** - One of the more common and easy to understand, although less efficient, methods for sorting information in a list is called the Bubble Sort. - **commen,** - **eary,** - **less efficient** - **How does Bubble Sort method work?** - Bubble Sort repeatedly compares side by side items in a list and swaps them if they are in the wrong order. This is repeated until no additional swaps are needed. **Example** - Here is an example of an array that we would like to sort in numerical order. - `8 4 6 3` - The bubble sort algorithm compares adjacent items from left to right. - Since we want to sort numerically, we want to check if the number on the right is greater than the number on the left. The program would check: - Is 8>4? - Since 8 is greater than 4, the program would swap the two numbers, making the new list appear as so: - `4 8 6 3` - The program would then keep checking the numbers, until it reaches the last item. - Is 8>6? Yes. Swap them! - Is 8>3? Yes. Swap them! - After the first iteration of the sort, the list would appear as follows: - `4 6 3 8` - At this point, the last number in the array will be the highest, but the rest of the array might not yet be sorted. - The algorithm will continue to compare values (except for the last one) until it has been sorted. ## Method - **When will a method run?** - A method will only run when it is called. - Within the construct of a method, you can pass data, which are referred to as parameters into a method. - They perform "actions" and they are commonly known as functions. - **What are parameters?** - Parameters. - **How do we create a Method?** - A method must be declared within the construct of a class. - It must be defined with a name followed by (). - Java has some pre-defined methods `System.out.println()` - **Example_Create your own method:** - `public class MyClass {` - `static void myMethod() { // method creation with myMethod being the name` - `// code to be executed` - `}` - `}` - **Example_explained:** - **What does static mean?** - 1. `static` means that the method belongs to the MyClass class and not an object of the MyClass class. - 2. `void` means that this method does not have a return value. - You will learn more about return values later in this chapter. - **What does void mean?** - **How do I call a method?** - To call a method, write the method's name followed by () and ; **Example:** - `public class MyClass {` - `static void myMethod() {` - `System.out.println("I just got executed!");` - `}` - `}` - `public static void main(String[] args) {` - `myMethod();` - `}` - `}` ## What are method parameters? - Method Parameters are information which can be passed into