CORE JAVA CHEATSHEET, Summaries of Object Oriented Programming

Download CORE JAVA CHEATSHEET and more Summaries Object Oriented Programming in PDF only on Docsity! 1 CORE JAVA CHEATSHEET Object Oriented Programming Language:​ based on the concepts of “objects”. Open Source: ​Readily available for development. Platform-neutral: ​Java code is independent of any particular hardware or software. This is because Java code is compiled by the compiler and converted into byte code. Byte code is platform-independent and can run on multiple systems. The only requirement is Java needs a runtime environment i.e, JRE, which is a set of tools used for developing Java applications. Memory Management: ​Garbage collected language, i.e. deallocation of memory. Exception Handling: ​Catches a series of errors or abnormality, thus eliminating any risk of crashing the system. THE JAVA BUZZWORDS Java was modeled in its final form keeping in consideration with the primary objective of having the following features: Simple, Small and Familiar Object-Oriented Portable and Platform Independent Compiled and Interpreted Scalability and Performance Robust and Secure Architectural-neutral High Performance Multi-Threaded Distributed Dynamic and Extensible 2 DATA TYPES IN JAVA Primitive Data Types Data Type Default Value Size (in bytes) 1 byte = 8 bits boolean FALSE 1 bit char “ “ (space) 2 byte byte 0 1 byte short 0 2 byte int 0 4 byte long 0 8 byte float 0.0f 4 byte double 0.0d 8 byte Non-Primitive Data Types Data Type String Array Class Interface ​TYPECASTING It is a method of converting a variable of one data type to another data type so that functions can process these variables correctly. Java defines two types of typecasting: ● Implicit Type Casting (Widening) 5 Keywords Usage abstract used to declare an abstract class. catch used to catch exceptions generated by try statements. class used to declare a class. enum defines a set of constants extends indicates that class is inherited final indicates the value cannot be changed finally used to execute code after the try-catch structure. implements used to implement an interface. new used to create new objects. static used to indicate that a variable or a method is a class method. super used to refer to the parent class. this used to refer to the current object in a method or constructor. throw used to explicitly throw an exception. throws used to declare an exception. try block of code to handle exception METHODS IN JAVA The general form of method : type name (parameter list) { //body of the method //return value (only if type is not void) } Where type - return type of the method name - name of the method parameter list - sequence of type and variables separated by a comma return - statement to return value to calling routine 6 CONDITIONAL STATEMENTS IN JAVA 1. if-else Tests condition, if condition true if block is executed else the else block is executed. class TestIfElse { public static void main(String args[]) { int percent = 75; if(percent >= 75) { System.out.println("Passed"); } else { System.out.println("Please attempt again!"); } } } 2. Switch Test the condition, if a particular case is true the control is passed to that block and executed. The rest of the cases are not considered further and the program breaks out of the loop. class TestSwitch { public static void main(String args[]) { int weather = 0; switch(weather) { case 0 : System.out.println("Sunny"); 7 break; case 1 : System.out.println("Rainy"); break; case 2 : System.out.println("Cold"); break; case 3 : System.out.println("Windy"); break; default : System.out.println("Pleasant"); } } } LOOPS IN JAVA Loops are used to iterate the code a specific number of times until the specified condition is true. There are three kinds of loop in Java : For Loop Iterates the code for a specific number of times until the condition is true. class TestForLoop { public static void main (String args[]) { for(int i=0;i<=5;i++) System.out.println("*"); } } While Loop If condition in the while is true the program enters the loop for iteration. class TestWhileLoop { public static void main (String args[]) { int i = 1; while(i<=10) { System.out.println(i); i++; } } } Do While Loop 10 System.out.println("Contents of objB: "); objB.showij(); objB.showk(); System.out.println(); System.out.println("Sum of i, j and k in objB:"); objB.sum(); } } Some limitations in Inheritance : ● Private members of the superclass cannot be derived by the subclass. ● Constructors cannot be inherited by the subclass. ● There can be one superclass to a subclass. 4. Polymorphism Defined as the ability to take more than one form. Polymorphism allows creating clean and readable code. In Java Polymorphism is achieved by the concept of method overloading and method overriding, which is the dynamic approach. ● Method Overriding In a class hierarchy, when a method in a child class has the same name and type signature as a method in its parent class, then the method in the child class is said to override the method in the parent class. In the code below, if we don’t override the method the output would be 4 as calculated in ParentMath class, otherwise, it would be 16. class ParentMath { void area() { int a =2; System.out.printf("Area of Square with side 2 = %d %n", a * a); System.out.println(); } } class ChildMath extends ParentMath { 11 void area() { int a =4; System.out.printf("Area of Square with side 4= %d %n", a * a); } public static void main (String args[]) { ChildMath obj = new ChildMath(); obj.area(); } } ● Method Overloading Java programming can have two or more methods in the same class sharing the same name, as long as their arguments declarations are different. Such methods are referred to as overloaded, and the process is called method overloading. Three ways to overload a method : 1. Number of parameters example: add(int, int) add(int, int, int) 2. Data type of parameters example add(int, int) add(int, float) 3.Sequence of data type of parameters example add(int, float) add(float, int) Program to explain multilevel inheritance and method overloading : class Shape { void area() { System.out.println("Area of the following shapes are : "); } } 12 class Square extends Shape { void area(int length) { //calculate area of square int area = length * length; System.out.println("Area of square : "+area); } } class Rectangle extends Shape { //define a breadth void area(int length,int breadth) { //calculate area of rectangle int area = length * breadth; System.out.println("Area of rectangle : " + area); } } class Circle extends Shape { void area(int breadth) { //calculate area of circle using length of the shape class as radius float area = 3.14f * breadth * breadth; System.out.println("Area of circle : " + area); } } class InheritanceOverload { public static void main(String[] args) { int length = 5; int breadth = 7; Shape s = new Shape(); //object of child class square Square sq = new Square(); //object of child class rectangle 15 System.out.println("Area of Rectangle = "+ result1); float result2 = cir.compute(x,y); System.out.println("Area of Circle = "+ result2); } } CONSTRUCTORS IN JAVA ● A constructor initializes an object on creation. ● They have the same name as the class. ● They do not have any return type, not even void. ● Constructor cannot be static, abstract or final. ● Constructors can be : ● Non - Parameterized or Default Constructor: Invoked automatically even if not declared class Box { double width; double height; double depth; // This is the constructor for Box. Box() { System.out.println("Constructing Box"); width = 10; height = 10; depth = 10; } // compute and return volume double volume() { return width * height * depth; } } class BoxVol { public static void main(String args[]) { // declare, allocate, and initialize Box objects Box mybox1 = new Box(); Box mybox2 = new Box(); double vol; vol = mybox1.volume(); System.out.println("Volume is " + vol); 16 vol = mybox2.volume(); System.out.println("Volume is " + vol); } } Parameterized : Used to initialize the fields of the class with predefined values from the user. class Box { double width; double height; double depth; Box(double w, double h, double d) { width = w; height = h; depth = d; } double volume() { return width * height * depth; } } class BoxVolP { public static void main(String args[]) { Box mybox1 = new Box(10, 20, 15); Box mybox2 = new Box(3, 6, 9); double vol; vol = mybox1.volume(); System.out.println("Volume is " + vol); vol = mybox2.volume(); System.out.println("Volume is " + vol); } } ARRAYS IN JAVA 17 Array is a group of like-type variables that are referred by a common name, having continuous memory. Primitive values or objects can be stored in an array. It provides code optimization since we can sort data efficiently and also access it randomly. The only flaw is that we can have a fixed-size elements in an array. There are two kinds of arrays defined in Java: 1. Single Dimensional: Elements are stored in a single row import java.util.Scanner; class SingleArray { public static void main(String args[]) { int len = 0; //declaration int [] numbers = {3,44,12,53,6,87}; Scanner s = new Scanner(System.in); System.out.println("The elements in the array are: "); for(int i=0;i<numbers.length;i++) { System.out.print(numbers[i] + " "); } System.out.println(); System.out.println("The sum of elements in the array are: "); int sum =0; for(int i=0;i<numbers.length;i++) { sum = sum + numbers[i]; } System.out.println("Sum of elements = " + sum); } } 2. Multi-Dimensional: Elements are stored as row and column class MatrixArray 20 Method Task Performed toLowerCase() converts the string to lower case toUpperCase() converts the string to upper case replace(‘x’ , ‘y’) replaces all appearances of ‘x’ with ‘y’ trim() removes the whitespaces at the beginning and at the end equals() returns ‘true’ if strings are equal equalsIgnoreCase() returns ‘true’ if strings are equal, irrespective of case of characters length() returns the length of string CharAt(n) gives the nth character of string compareTo() returns negative if string 1 < string 2 positive if string 1 > string 2 zero if string 1 = string 2 concat() concatenates two strings substring(n) returns substring returning from character n substring(n,m) returns a substring between n and ma character. toString() creates the string representation of object indexOf(‘x’) returns the position of first occurrence of x in the string. indexOf(‘x’,n) returns the position of after nth position in string ValueOf (Variable) converts the parameter value to string representation Program to show Sorting of Strings: class SortStrings { static String arr[] = { "Now", "the", "is", "time", "for", "all", "good", "men", "to", "come", "to", "the", "aid", "of", "their", "county" }; public static void main(String args[]) { for(int j = 0; j < arr.length; j++) { for(int i = j + 1; i < arr.length; i++) { if(arr[i].compareTo(arr[j]) < 0) { String t = arr[j]; arr[j] = arr[i]; 21 arr[i] = t; } } System.out.println(arr[j]); } } } String Buffer and String Builder ● For mutable strings, we can use StringBuilder and StringBuffer classes which as well implement CharSequence interface. ● These classes represent growable and writable character interface. ● They automatically grow to make room for additions , and often has more characters preallocated than are actually needed, to allow room for growth. Difference between length() and capacity() length() : To find the length of StringBuffer capacity() : To find the total allocated capacity /* StringBuffer length vs. capacity */ class StringBufferTest { public static void main(String args[]) { StringBuffer sb = new StringBuffer("Hello"); System.out.println("buffer = " + sb); System.out.println("length = " + sb.length()); System.out.println("capacity = " + sb.capacity()); } } StringBuilder versus StringBuffer String Builder String Buffer Non-Synchronized : hence efficient. Synchronized Threads are used, multithreading. Thread Safe MULTITHREADING Multitasking: ​Process of executing multiple tasks simultaneously, to utilize the CPU. This can be achieved in two-ways: 22 ● Process-based multitasking.(Multitasking) ● Thread-based multitasking (Multithreading) Multitasking versus Multithreading Multitasking Multithreading OS concept in which multiple tasks are performed simultaneously. Concept of dividing a process into two or more subprocess or threads that are executed at the same time in parallel. Multiple programs can be executed simultaneously. Supports the execution of multiple parts of a single program simultaneously. Process has to switch between different programs or processes. Processor needs to switch between different parts or threads of the program. less efficient highly efficient program or process in the smallest unit in the environment thread is the smallest unit cost effective expensive Life Cycle Of Thread A thread is always in one of the following five states, it can move from state to another by a variety of ways as shown. New thread:​ Thread object is created. Either it can be scheduled for running using start() method. Runnable thread :​ Thread is ready for execution and waiting for processor. Running thread: ​It has got the processor for execution. Blocked thread: ​ Thread is prevented from entering into runnable state. Dead state: ​Running thread ends its life when it has completed executing its run() method. 25 new A().start(); new B().start(); new C().start(); } } Implementing Runnable Interface The run( ) method that is declared in the Runnable interface which is required for implementing threads in our programs. Process consists of following steps : ● Class declaration implementing the Runnable interface ● Implementing the run() method ● Creating a thread by defining an object that is instantiated from this “runnable” class as the target of the thread. ● Calling the thread’s start() method to run the thread. Using Runnable Interface class X implements Runnable { public void run() { for(int i=0;i<=10;i++) { System.out.println("Thread X " + i); } System.out.println("End of thread X "); } } class RunnableTest { public static void main(String args[]) { X runnable = new X (); Thread threadX = new Thread(runnable); threadX.start(); System.out.println("End of main Thread"); } } 26 Thread Class Versus Runnable Interface Thread Class Runnable Interface Derived class extending Thread class itself is a thread object and hence, gains full control over the thread life cycle. Runnable Interface simply defines the unit of work that will be executed in a thread, so it doesn’t provide any control over thread life cycle. The derived class cannot extend other base classes Allows to extend base classes if necessary Used when program needs control over thread life cycle Used when program needs flexibility of extending classes. EXCEPTION HANDLING Exception is an abnormality or error condition that is caused by a run-time error in the program, if this exception object thrown by error condition is not caught and handled properly, the interpreter will display an error message. If we want to avoid this and want the program to continue then we should try to catch the exceptions.This task is known as exception handling. Common Java Exceptions Exception Type Cause of Exception ArithmeticException caused by math errors ArrayIndexOutOfBoundException caused by bad array indexes ArrayStoreException caused when a program tries to store wrong data type in an array FileNotFoundException caused by attempt to access a nonexistent file IOException caused by general I/O failures. NullPointerException caused by referencing a null object. NumberFormatException caused when a conversion between strings and number fails. OutOfMemoryException caused when there is not enough memory to allocate StringIndexOutOfBoundException caused when a program attempts to access a non-existent character position in a string. Exceptions in java can be of two types: Checked Exceptions : ● Handled explicitly in the code itself with the help of try catch block. ● Extended from java.lang.Exception class Unchecked Exceptions : 27 ● Not essentially handled in the program code, instead JVM handles such exceptions. ● Extended from java.lang.RuntimeException class TRY AND CATCH Try keyword is used to preface a block of code that is likely to cause an error condition and “throw” an exception. A catch block defined by the keyword catch “catches” the exception “thrown” by the try block and handles it appropriately. A code can have more than one catch statement in the catch block, when exception in try block is generated, multiple catch statements are treated like cases in a switch statement. Using Try and Catch for Exception Handling class Error { public static void main(String args[]) { int a [] = {5,10}; int b = 5; try { int x = a[2]/b-a[1]; } catch(ArithmeticException e) { System.out.println("Division by zero"); } catch(ArrayIndexOutOfBoundsException e) { System.out.println("ArrayIndexError"); } catch(ArrayStoreException e) { System.out.println("Wrong data type"); } int y = a[1]/a[0]; System.out.println("y = " + y); } } 30 ● Input Stream: ​which extracts i.e. reads data from source file and sends it to the program. ● Output Stream:​ which takes the data from the program and send i.e writes to the destination. STREAM CLASSES They are contained in​ ​java.lang.io​ package. They are categorized into two groups 1.​ Byte Stream Classes:​ provides support for handling I/O operation on bytes. 2. ​Character Stream Classes: ​provides support for managing I/O operations on characters. BYTE STREAM CLASSES Designed to provide functionality for creating and manipulating streams and files for reading/writing bytes. Since streams are unidirectional there are two kinds of byte stream classes : ● Input Stream Classes ● Output Stream Classes INPUT STREAM CLASSES They are used to read 8-bit bytes include a super class known as InputStream. InputStream is an abstract class and defines the methods for input functions such as : Method Description read( ) Reads a byte from input stream read(byte b [ ]) Reads an array of bytes into b read(byte b [ ], int n, int m) Reads m bytes into b starting from the nth byte of b available( ) Tells number of bytes available in the input skip(n) Skips over n bytes from the input stream reset ( ) Goes back to the beginning of the stream close ( ) Closes the input stream OUTPUT STREAM CLASSES 31 These classes are derived from the base class OutputStream. OutputStream is an abstract class and defines the methods for output functions such as : Method Description write( ) Writes a byte to the output stream write(byte b[ ]) Writes all the bytes in the array b to the output stream write(byte b[ ], int n, int m) Writes m bytes from array b starting from the nth byte close( ) Closes the output stream flush( ) Flushes the output stream READING/WRITING BYTES Two common subclasses used are FileInputStream and FileOutputStream that handle 8-bit bytes. FileOutputStream is used for writing bytes to a file as demonstrated below: // Writing bytes to a file import java.io.*; class WriteBytes { public static void main(String args[]) { bytes cities [] = {'C','A','L','I','F','O','R','N','I','A', '\n', 'V','E','G','A','S','\n','R','E','N','O','\n'}; //Create output file stream FileOutputStream outfile = null; try { //connect the outfile stream to "city.txt" outfile = new FileOutputStream("city.txt"); //Write data to the stream outfile.write(cities); outfile.close(); } catch(IOException ioe) { System.out.println(ioe); 32 System.exit(-1); } } } FileIntputStream is used for reading bytes from a file as demonstrated below: //Reading bytes from a file import java.io.*; class ReadBytes { public static void main(String args[]) { //Create an input file stream FileInputStream infile = null; int b; try { //connect the infile stream to required file infile = new FileInputStream(args [ 0 ]); //Read and display while( (b = infile.read ( ) ) !=-1) { System.out.print((char) b ); } infile.close(); } catch(IOException ioe) { System.out.println(ioe); System.exit(-1); } } } CHARACTER STREAM CLASSES Two kinds of character stream classes : Reader Stream Classes 35 SortedMap (extends Map) sorted collection of key value pairs Iterator object used to traverse through a collection List (extends Iterator) object used to traverse through a sequence CLASSES The classes available in the collection framework implement the collection interface and sub-interfaces. They also implement Map and Iterator interfaces. Classes and their Corresponding interfaces are listed : Class Interface AbstractCollection Collection AbstarctList List Abstract Queue AbstractSequentialList List LinkedList List ArrayList List, Cloneable and Serializable AbstractSet Set EnumSet Set HashSet Set PriorityQueue Queue TreeSet Set Vector List, Cloneable and Serializable Stack List, Cloneable and Serializable Hashtable Map, Cloneable and Serializable Array List Implementation // Using the methods of array list class import java.util.*; class Num { public static void main(String args[]) { ArrayList num = new ArrayList (); num.add(9); 36 num.add(12); num.add(10); num.add(16); num.add(6); num.add(8); num.add(56); //printing array list System.out.println("Elements : "); num.forEach((s) -> System.out.println(s)); //getting size System.out.println("Size of array list is: "); num.size(); //retrieving specific element int n = (Integer) num.get(2); System.out.println(n); //removing an element num.remove(4); //printing array list System.out.println("Elements : "); num.forEach((s) -> System.out.println(s)); } } Linked List Implementation import java.util.Scanner; class LinkedList { public static void main (String args[]) { Scanner s = new Scanner(System.in); List list = new List(); System.out.println("Enter the number of elements you want to enter in LL : "); int num_elements = s.nextInt(); 37 int x; for(int i =0;i<=num_elements;i++) { System.out.println("Enter element : "); x = s.nextInt(); list.insert(x); } System.out.println(">>>>> LINKED LIST AFTER INSERTION IS : "); list.print(); int size = list.count(); System.out.println(">>>>> SIZE OF LL => "+size); System.out.println("Enter the node to be inserted in the middle: "); int mid_element = s.nextInt(); list.insertMiddle(mid_element); System.out.println(">>>> LL AFTER INSERTING THE NEW ELEMENT IN THE MIDDLE "); list.print(); } } HashSet Implementation import java.util.*; class HashSetExample { public static void main(String args[]) { HashSet hs = new HashSet(); hs.add("D"); hs.add("W"); hs.add("G"); hs.add("L"); hs.add("Y"); System.out.println("The elements available in the hash set are :" + hs); } } Tree Set Implementation import java.util.*; class TreeSetExample { public static void main(String args[]) 40 ● Process of allocating new objects ● Properly removing unused objects( garbage collection) Example Illustrating Memory Management 41 1. When a method is called , frame is created on the top of the stack. 2. Once a method as completed execution, the flow of control returns to the calling method and its corresponding stack frame is flushed. 3. Local variables are created in stack. 4. Instance variables are created in the heap and are part of the object they belong to. 5. Reference variable are created in stack. SOME COMMON JAVA CODING QUESTIONS 1. Enter radius and print diameter, perimeter and area import java.util.Scanner; class Circle { public static void main (String args []) { double r,dia,peri,area ; System.out.println("Enter the radius of circle : "); 42 Scanner s = new Scanner (System.in); r = s.nextDouble(); dia = 2*r; peri = 2*Math.PI*r; area = Math.PI*r*r; System.out.printf("The dia of circle is : %.2f \n", dia); System.out.printf("The peri of circle is : %.2f \n", peri); System.out.printf("The area of the circle is : %.2f \n", area); } } 2. Print all the even numbers between x and y. import java.util.Scanner; class EvenOdd { public static void main (String args[]) { int x,y; Scanner s = new Scanner (System.in); System.out.println("Enter the values x , y : "); x = s.nextInt(); y = s.nextInt(); System.out.println(" **** EVEN NUMBERS BETWEEN GIVEN RANGE ARE **** >> "); int count = x; while(count <=y) { if(count % 2 == 0) { System.out.println(count); } count ++; } } } 3. To check if given number is prime