Java Topics

• Java Topics
• Introduction
  • Key Features
  • Notes
• Concepts
  • Binary
    • 8-bit
  • Integer Overflow
• Basics of Java
  • Errors/Troubleshooting
  • Variables
  • Unit Tests
  • Formatted Strings
  • Character methods return values. Each method must prepend Character., as in Character.isLetter
  • Do/While Loop
  • Character vs String
  • Input Validation
  • Arrays
    • Array are references
    • Traversing Arrays
    • Summary so far
  • Enhanced Loops
  • Hashmaps
    • import java.util.HashMap;
    • Table: Common HashMap methods.
• Objects
  • Classes
  • Static Fields and Methods
  • Mutable vs Immutable Objects
    • Aliasing
• Summary So Far
  • Loops
• Objects and Classes: Introduction
  • Operator
  • Methods:
  • Wrapper Classes
  • ArrayList
  • Output Formatting
• Objects and Classes: Design
  • Encapsulation
  • Garbage Collection in Inheritance
  • Instance Data vs Local Variable
  • Formal vs Actual Parameter
    • new
    • null
    • toString()
    • hashCode()
  • Primitives Review
  • Example Program
• Inheritance and Polymorphism
  • Concepts
  • Keywords
  • Inheritance
    • Design
  • Polymorphism
  • Programming and Common Errors
• Algorithms
  • O-Notation
  • Abstract Data Types (ADT)
  • Search Algos
    • Linear Search
    • Binary Search (Divide and Conquer)
  • • AVL Balanced Tree (Adelson-Velsky and Landis)
• Data Structures
  • Hash Tables
    • Hash Function
      • Collisions

Introduction

Java is a high-level, object-oriented programming language designed for platform independence, allowing it to run on various devices without modification. Developed by James Gosling and his team at Sun Microsystems, Java was released in 1995. Known for its simplicity, portability, and robustness, Java has become a popular choice for building diverse applications, from web and mobile apps to large-scale enterprise systems.

Key Features

1. Write Once, Run Anywhere (WORA): Java’s “Write Once, Run Anywhere” philosophy enables the execution of Java programs on any device with a Java Virtual Machine (JVM).

2. Object-Oriented: Java is a pure object-oriented programming language, emphasizing the use of classes and objects for modular and scalable code.

3. Automatic Memory Management: Java features automatic memory management through garbage collection, simplifying memory handling and reducing the risk of memory leaks.

4. Security: Java provides a secure runtime environment, with features such as sandboxing applets and a robust set of APIs for encryption and authentication.

5. Rich Standard Library: Java comes with a comprehensive standard library that simplifies common programming tasks, including input/output operations, networking, and GUI development.

6. Multithreading: Java supports multithreading, allowing developers to build concurrent and scalable applications efficiently.

7. Community Support: Java has a large and active community, providing extensive documentation, tutorials, and support through various forums and resources.

Java remains a prominent language in software development, widely used in web development, mobile app development (Android), enterprise systems, and cloud computing.

Notes

• At its core level, computers are switches, and store information in bytes (8 bits).
  • memory stores information in binary (1 or 0, like a switch!)
  • instructions go to the processor, and turns executable (in binary 1101 1011 ...), then an assembler creates the high level instruction.
    • Java takes the high level language, runs it through a compiler, which takes it through a virtual processor that turns it executable (bytecode), and finally executes in the Virtual Machine within a processor
      • Java this method allows it to execute on different processors, its design which is reflected in the intention of creating desktop applications.
• pointer: variables that store addresses of other variables. Typically occupies 4 bytes of memory. Think of this like a reference to an originally stored variable.
  • in Java, this may be like string1 = string2. This does not create a new string, but references string2.

Concepts

Binary

8-bit

• represented in 8 binary, or 10101010 where each number would symbolize: $$ 12^8 + 02^7 + 12^6 + 02^5 + 12^4 + 02^3 + 12^2 + 02^0 $$

Integer Overflow

• Integer overflow in Java occurs when the result of an arithmetic operation on integers exceeds the maximum value that can be represented by the data type. In Java, integer types (int and long) have fixed sizes and ranges.

Basics of Java

import java.util.Scanner; // import scanner

public class Salary {                       
  public static void main (String [] args) {
    // Program starts in main and executes in main's braces {}
    int wage; // declare variable, does not assign 
    // semicolon ends statement, and declaring a variable is paramount to object manipulation

    // wage = 20; assigns the variable's value
    Scanner scnr = new Scanner(System.in);
    // create object scanner, from System.in (keyboard input)
    wage = scnr.nextInt();                      

    System.out.print("Salary is \n");
    // \n creates a new line       
    System.out.println(wage * 40 * 52 + " this is example text");
    /*
     * This is weirdly valid
     *
    */
  }
}

• print vs println: ln encloses the output in its own line, sending the next line to a blank line

Errors/Troubleshooting

• sometimes a ; can be put out as an error when its the parenthesis or something earlier up the program line

• Many programmers recommend the good practice of configuring compilers to be more picky with warnings than the default. Ex: javac can be run as javac -Xlint yourfile.java to enable all recommended warnings

• CamelCase should be used for Java classes

• / can be used as an escape sequence like:

("Hello \"Hello\" \nWorld")

Hello "Hello" 
World

• Dereferencing: Dereferencing means the action of accessing an object’s features through a reference. Performing any dereferencing on a primitive will result in the error “X cannot be dereferenced”, where X is a primitive type

Variables

• To declare a constant variable, use final: final double varName = 111

Unit Tests

Unit tests test a programs intended function by inputing data, its expected output, and its real output, or a FAILED statement.

Single Unit Tests:

// assert testExpression: detailedMessage;
assert (myVal(0,1) == 0) : "Assertion (0,1) failed"; 

Class-wide: You can test across a class using multiple methods and printing FAIL statements.

Formatted Strings

%a	floating point (except BigDecimal)	Hex output of floating point number
%b	Any type	“true” if non-null, “false” if null
%c	character	Unicode character
%d	integer (incl. byte, short, int, long, bigint)	Decimal Integer
%e	floating point	decimal number in scientific notation
.n%f	floating point	decimal number with n trailing
%g	floating point	decimal number, possibly in scientific notation depending on the precision and value.
%h	any type	Hex String of value from hashCode() method.
 %n	none	Platform-specific line separator.
%o	integer (incl. byte, short, int, long, bigint)	Octal number
%s	any type	String value
%t	Date/Time (incl. long, Calendar, Date and TemporalAccessor)	%t is the prefix for Date/Time conversions. More formatting flags are needed after this. See Date/Time conversion below.
%x	integer (incl. byte, short, int, long, bigint)	
Hex string.

Character methods return values. Each method must prepend Character., as in Character.isLetter

isLetter(c) true if alphabetic:
a-z or A-Z  
isLetter('x') // true
isLetter('6') // false
isLetter('!') // false
  toUpperCase(c)  Uppercase version 
toUpperCase('a')  // A
toUpperCase('A')  // A
toUpperCase('3')  // 3
isDigit(c)  true if digit: 0-9. 
isDigit('x') // false
isDigit('6') // true
  toLowerCase(c)  Lowercase version 
toLowerCase('A')  // a
toLowerCase('a')  // a
toLowerCase('3')  // 3
isWhitespace(c) true if whitespace. 
isWhitespace(' ')  // true
isWhitespace('\n') // true
isWhitespace('x')  // false

Do/While Loop

• separate the while loop from executing a code block (i.e. a menu)
• for something to happen regardless of condition
• do/while vs while:
  • do/while executes a code block once before condition checking

count = 0;
num = 6;

do {
   num = num - 1;
   count = count + 1;
} while (num > 4); 
// executes twice, but DO always executes!
// for all num, DO will execute!

Character vs String

• ASCII - merican Standard Code for Information Interchange
• ‘char’ vs “String”

<String>.length(): returns length of *String*
<String>.indexOf(<char/string>): returns index of occurrence
<String>.subString(n, m): returns characters in the form of a string at n and before m
word.toLowerCase();
word.toUpperCase();

Input Validation

if (!in.**hasNextDouble()**) {

    String word = in.next();
    System.err.println(word + "is not a number");
}

• De Morgans Law:

!(A && B)  is the same as   !A || !B

!(A || B)  is the same as   !A && !B

Arrays

// Create Arrays
String[] names;
double[] values;

values = new double[size]; 
// new operator allocates memory for the array and automatically initializes all of its elements to a default value
// values refer to an array of doubles; 
// how many elements depends on the value of size 
// (at the time the array is created)

• toString(), returns a string representation of an array.
• copyOf() , copies an array
  • double[] data = Arrays.copyOf(values, 3);

Array are references

When you make an assignment to an array variable, it simply copies the reference. But it doesn’t copy the array itself. For example:

double[] values = new double[3];
double[] data = values;

Traversing Arrays

int[] array = {1, 2, 3, 4, 5};
for (int i = 0; i < array.length; i++) {
    array[i] *= array[i];
}

// Values after the loop: {1, 4, 9, 16, 25}

• Search (returning index of occurence), Reducing (i.e. Sum),

Summary so far

• Definition of methods besides the main method
  • A method is a list of statements executed by invoking the method’s name (this is known as a method call). There are user defined methods and built in methods.
    • i.e. sqrt(x), pow(x,y), abs(x)
  • These methods may receive arguments/parameters and may or may not return values.
  • Values passed as arguments to methods may be of any type, such as, but not limited to: -
    • int, double, String.
  • Values returned by methods may be of any type, such as, but not limited to:
    • boolean, int, double, String.

<access_modifier> <return_type> <method_name>( list_of_parameters) {
    //body
}


public int subtractNumbers(int m, int n) {
    int p = 0;
    p = m - n;
    System.out.println(p);
    return p;
}

we can see that public corresponse to the access modifier, int to return type, and subtractNumbers to method name. You can infer what the other parts are.

• public static:
  • public means the method may be called from any class in the program
  • static means the method only uses values that are passed to the method

Enhanced Loops

• for-each: reduces programming burden but removes incrementing capabilities
  • cannot change your ‘value’ within the for each loop.

int[] values = { 1, 2, 3, 4, 5 };

for (int value:values) {
  System.out.println(value);
}

Hashmaps

import java.util.HashMap;

• The HashMap type is an ADT implemented as a generic class (discussed elsewhere) that supports different types of keys and values. Generically, a HashMap can be declared and created as HashMap<K, V> hashMap = new HashMap<K, V>(); where K represents the HashMap’s key type and V represents the HashMap’s value type.

Table: Common HashMap methods.

What is a Hashmap?

// Map now: Tom->14,  
exMap.put("John", 86); 
// Map now: Tom->14, John->86
putIfAbsent() putIfAbsent(key, value) 
Associates key with specified value if the key does not already exist or is mapped to null. 
// Assume Map is: Tom->14, John->86
exMap.putIfAbsent("Tom", 20); 
// Key "Tom" already exists. Map is unchanged.
exMap.putIfAbsent("Mary", 13); 
// Map is now: Tom->14, John->86, Mary->13
get() get(key)
Returns the value associated with key. If key does not exist, return null.  
// Assume Map is: Tom->14, John->86, Mary->13
exMap.get("Tom")  // returns 14 
exMap.get("Bob")  // returns null
containsKey() containsKey(key) 
Returns true if key exists, otherwise returns false.  
// Assume Map is: Tom->14, John->86, Mary->13
exMap.containsKey("Tom")  // returns true 
exMap.containsKey("Bob")  // returns false
containsValue() containsValue(value) 
Returns true if at least one key is associated with the specified value, otherwise returns false. 
// Assume Map is: Tom->14, John->86, Mary->13
exMap.containsValue(86)  // returns true (key "John" associated with value 86)
exMap.containsValue(17)  // returns false (no key associated with value 17)
remove()  remove(key) 
Removes the map entry for the specified key if the key exists.  
// Assume Map is: Tom->14, John->86, Mary->13
exMap.remove("John");  
// Map is now: Tom->14, Mary->13
clear() clear() 
Removes all map entries.  
// Assume Map is: Tom->14, John->86, Mary->13
exMap.clear();  
// Map is now empty
keySet()  keySet() 
Returns a Set containing all keys within the map. 
// Assume Map is: Tom->14, John->86, Mary->13
keys = exMap.keySet();
// keys contains: "Tom", "John", "Mary"
values()  values() 
Returns a Collection containing all values within the map.  
// Assume Map is: Tom->14, John->86, Mary->13
values = exMap.values();
// values contains: 14, 86, 13

Objects

• objects have features, qualities, functions
• abstraction: interaction at a high level with no lower level understanding (encapsulation) i.e. tech priest
• objects highly support abstraction

Classes

• defines new type that can group data and methods to create objects
  • public member methods: operations a class user can perform on an object
• A field is a variable of any type that is declared directly in a class or struct
  • private field: accessible to class method but not class users
• constructor: class member method that initializes fields

Static Fields and Methods

• static indicates the variable is allocated once in memory. However due to being stored in static memory, it has global scope.
• static field is the field of a class instead of distributively to each class object

add()	add(element) 
Create space for and add the element at the end of the list.	
// List originally empty
valsList.add(31); // List now: 31 
valsList.add(41); // List now: 31 41

get()	get(index) 
Returns the element at the specified list location known as the index. Indices start at 0.	
// List originally: 31 41 59. Assume x is an int. 
x = valsList.get(0);  // Assigns 31 to x
x = valsList.get(1);  // Assigns 41
x = valsList.get(2);  // Assigns 59
x = valsList.get(3);  // Error: No such element

set()	set(index, element) 
Replaces the element at the specified position in this list with the specified element.	
// List originally: 31 41 59 
valsList.set(1, 119);  // List now 31 119 59

size()	size()
Returns the number of list elements.	
// List originally: 31 41 59. Assume x is an int. 
x = valsList.size();  // Assigns x with 3

Mutable vs Immutable Objects

• mutable: may be changed after creation
• immutable: cannot be changed after creation

Aliasing

• more than one reference to the same object. Below is an example where a new object is not created, but the object box1 is referenced by box2.

Rectangle box1 = new Rectangle(0, 0, 100, 200);
Rectangle box2 = box1;

Summary So Far

• Definition of methods besides the main method
  • A method is a list of statements executed by invoking the method’s name (this is known as a method call). There are user defined methods and built in methods.
    • i.e. sqrt(x), pow(x,y), abs(x)
• These methods may receive arguments/parameters and may or may not return values.
  • Values passed as arguments to methods may be of any type, such as, but not limited to: -
    • int, double, String.
  • Values returned by methods may be of any type, such as, but not limited to:
    • boolean, int, double, String.
• Call methods in main to perform a calculation or an action.
• To call a method:

public class VeganClass { 
   public static double calcNum() {
      // some calculations and logic
   }

   public static void main(String[] args) {
      // Calls method calcNum and prints the result
      System.out.println(calcNum());
   }
}

• Input values from the standard input using the Scanner object.
• Output values on the screen.

import java.util.Scanner;

public class MyClass {
  public static void main (String [] args) {
    Scanner input = new Scanner(System.in);
    // create new Scanner object called 'input' which takes 'System.in'
  }
}

• Strings and string methods, such as: charAt(), indexOf, length(), equals(), equalsIgnoreCase(), compareTo()
  • charAt(): determines the character at index x of a string

String userWord = "myWord";

• One-dimensional arrays
  • myArray = [1, 2, 3, 4, 5];
  • Two-dim: int [][] myArray = new int[3][3] // 3x3, 9 elements
• if-else statements

if (myThing == myNum) {
  // do stuff
}

• switch statements

switch (userVal) {
   case 1: // case userVal == 1
      numItems = 5; 
      break;

   case 3: // case userVal == 3
      numItems = 12; 
      break;

   default:
      numItems = 55;
      break;
}

Loops

• for and for-each (Find: Enhanced Loops)
• while and do-while (Find: Do/While Loop)
• Mutable and Immutable Objects
  • Mutable: can be changed
    • StringBuilder, Point origin, Point blank, double dist
  • Immutable: cannot change the content of object once created
    • String
      • creating for loop that creates new String objects and concatenates them can create a lot of garbage, because a new String is declared and stored for use.
      • i.e. This String + This String and more + This String and more more
  • Aliasing: more than one reference to the same object
• Designing your own classes (recall lab):

public class myObject {
  // declare private variable (accessible only to method)
	private double thing;

	public myObject() {
    // initialize value
		this.thing = // some value;
	}
	
	public double myMethod(double myInput) {
    // example method
		thing = thing + myInput;
		return thing;
	}
  // get and set methods
}

• Creating objects of classes and calling methods on those objects myObject myInstanceObject = new myObject();
• The basics of GUI (Swing/SWT)
  • use Eclipse to generate default Swing application (Final Lab):
    • File»New»Create Java Project»New»Project Explorer»New»Other»Swing Designer»Application Window
• It is helpful to review Discussion posts, homework assignments, group projects and quizzes.

Objects and Classes: Introduction

Operator

• instanceof
  • example: if(obj instanceof MyObject) { }

Methods:

• equals(): checks if two objects have the same reference.
  • equalsIgnoreCase()

Wrapper Classes

Java’s Collectin library only works with objects. For example, you can create an ArrayList of Integer elements (not int)

ArrayList

Common Methods:

• add(): Create space for and add the element at the end of the list.
• get(): Returns the element at the specified list location known as the index. Indices start at 0.
• set(): Returns the element at the specified list location known as the index. Indices start at 0.
• size(): Returns the number of list elements.

Output Formatting

Sub-specifier	Description	Example
width	Specifies the minimum number of characters to print. If the formatted value has more characters than the width, the value will not be truncated. If the formatted value has fewer characters than the width, the output will be padded with spaces (or 0's if the '0' flag is specified).	printf("Value: %7.2f", myFloat);
Value:   12.34
.precision	Specifies the number of digits to print following the decimal point. If the precision is not specified, a default precision of 6 is used.	printf("%.4f", myFloat);
12.3400

printf("%3.4e", myFloat);
1.2340e+01
flags	-: Left aligns the output given the specified width, padding the output with spaces.
+: Prints a preceding + sign for positive values. Negative numbers are always printed with the - sign.
0: Pads the output with 0's when the formatted value has fewer characters than the width.
space: Prints a preceding space for positive value.	printf("%+f", myFloat);
+12.340000
printf("%08.2f", myFloat);
00012.3

width	Specifies the minimum number of characters to print. If the formatted value has more characters than the width, the value will not be truncated. If the formatted value has fewer characters than the width, the output will be padded with spaces (or 0's if the '0' flag is specified).	printf("Value: %7d", myInt);
Value:     301
flags	-: Left aligns the output given the specified width, padding the output with spaces.
+: Print a preceding + sign for positive values. Negative numbers are always printed with the - sign.
0: Pads the output with 0's when the formatted value has fewer characters than the width.
space: Prints a preceding space for positive value.	printf("%+d", myInt);
+301

printf("%08d", myInt);
00000301

printf("%+08d", myInt);
+0000301

Objects and Classes: Design

Encapsulation

• single class in charge of respective data/functionality. Self-governing, and implementation hidden from user

Garbage Collection in Inheritance

• Local and formal released to garbage collecion

Instance Data vs Local Variable

• Instance Data Variables exist as long as the object does
  • Local variables are released for garbage collection for method runtime

Formal vs Actual Parameter

• Formal is declared by method header with method scope, for external info
• Actual passed during invocation for sending info to a method

new

• allocates memory, executes constructor, and returns reference to object

null

• allowed as assignment to Primitives

Allowed/!Allowed Operations

Compare with ==. Can be short-circuiting as null-check. Cannot invoke methods on null.

toString()

• String representation of object

hashCode()

• returns unique identifier for an object

Primitives Review

In Java, primitives are basic data types that represent the most fundamental values. They are built-in types provided by the Java language itself. Java has eight primitive types, which include:

• byte:
  • A 1-byte integer type representing values from -128 to 127.
• short:
  • A 2-byte integer type representing values from -32,768 to 32,767.
• int:
  • A 4-byte integer type representing values from -2,147,483,648 to 2,147,483,647.
• long:
  • An 8-byte integer type representing larger integer values.
• float:
  • A 4-byte floating-point type representing single-precision decimal values.
• double:
  • An 8-byte floating-point type representing double-precision decimal values.
• char:
  • A 2-byte type representing a single character using the Unicode encoding scheme.
• boolean:
  • A 1-bit type representing true or false values.

Example Program

public class myObject {
	private double myInstanceDataVar; 

	public myConstructor(FormalParameter myData) {
    myInstanceDataVar = myData;
	}
	
	public double myMethod(double myInput) {
    // example method
		thing = thing + myInput;
		return thing;
	}

  public static void main(String[] args) {
    FormalParameter myActualParameter = new FormalParameter();
    myClassMethod(myActualParameter);
  }

}

Inheritance and Polymorphism

Concepts

• Downcasting/Upcasting:
  • Casting an object down and up a hierarchy
  • Animal myAnimal = new Dog()
    • Dog is subclass of animal, therefore Dog is upacasted to Animal Animal

Keywords

• final: restricts Inheritance

Inheritance

Design

1. Always is-a
2. Optimize Reuse–push methods high in the hierarchy
3. Purposeful Overriding
4. Encapsulation
5. Override general Object methods (toStirng, equals)
6. Optimize visibility modifiers–zero trust

Polymorphism

Treating different classes as objects of one superclass. Objects have declared and actual types. Utilizes instanceof and downcasting. Uses a single interface.

There are two main types of polymorphism: compile-time polymorphism (overloading) and runtime polymorphism (overriding)

public class Animal {
    public void makeSound() {
        System.out.println("Animal makes a sound");
    }
    
    // Overloading
    public int add(int a, int b) {
        return a + b;
    }

    public double add(double a, double b) {
        return a + b;
    }
}

public class Dog extends Animal {
    @Override
    public void makeSound() {
        System.out.println("Dog barks");
    }
}

Programming and Common Errors

• equals Method header (Overriding equals()):

public boolean Object(Employee e) {
// Should be type Object, not Employee i.e. (Object obj)
  if (obj instanceof Employee) {
    Employee otherEmployee = (Employee) obj; // Casting to type Employee
  }
}

Algorithms

O-Notation

O-Notation notes

Abstract Data Types (ADT)

Stacks are a Last In First Out (LIFO) ADT.

Search Algos

Linear Search

Sequentially evaluating each element. Pseudocode:

public static int linearSearch {
  for (start; end; increment) {
    if (element == target) {
      return element;
    } 
  }
  return -1; // not found
}

Binary Search (Divide and Conquer)

Utilizes sorted inputs and divides the list in half repeatedly. For $N$ elements, max steps to reduce searches pace is $[log_{2}N] + 1$.

public static int binarySearch {
  int left = beginning;
  int right = end;
  while (left <= right) {
    // take middle value
    // if middle is target, FOUND!
    // compare and either:
    // set left and right to left partition
    // left and right to right partition
  }
  return -1
}

AVL Balanced Tree (Adelson-Velsky and Landis)

An AVL tree is a BST with a height balance property and specific operations to rebalance the tree when a node is inserted or removed. This section discusses the balance property; another section discusses the operations. A BST is height balanced if for any node, the heights of the node’s left and right subtrees differ by only 0 or 1. Guarantees $O(log\ n)$ search, insertion, and delete.

Data Structures

Hash Tables

$O(1)$ time complexity.

Hash Function

index = key % table_size

Collisions

Separate Chaining

• Index points to a linked list for same-index values Open Addressing
• Look for next available slot (linear/quadratic probing or double hashing)