Java Tutorial

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Java Loops

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Java Programs

Java Basic Programs Factorial Program in Java Fibonacci Series Program in Java Prime Number Program in Java Palindrome Number Program in Java Armstrong Number Program in Java Anagram Program in Java Pattern Programs in Java Calculator Program in Java Leap Year Program in Java Addition Program in Java Number Pattern Programs in Java Star Pattern Programs in Java Package Program in Java Pyramid Program in Java Sorting Program in Java String Palindrome Program in Java Even Odd Program in Java For Loop Program in Java If Else Program in Java Switch Case Program in Java GCD Program in Java LCM Program in Java Hello Program in Java Matrix Program in Java Menu Driven Program in Java Series Program in Java Client Server Program in Java Swapping Program in Java Pig Latin Program in Java Tower of Hanoi Program in Java Recursion Program in Java Matrix Multiplication Program in Java Perfect Number Program in Java Classes and Objects in Java Example Programs String Programs in Java Array Programs in Java Constructor Program in Java Inheritance Program in Java Abstract class Program in Java Interface Program in Java Encapsulation Program in Java Polymorphism Program in Java Exception Handling Program in Java Multithreading Program in Java Thread Program in Java Collection Programs in Java ArrayList Program in Java Stack Program in Java Applet Program in Java Swing Program in Java JDBC Program in Java How to run Java program in command prompt How to run Java program in Eclipse

Java Sorting

Sorting Algorithms in Java Merge Sort in Java Quick Sort in Java Bubble Sort in Java Insertion Sort in Java Selection Sort in Java Heap Sort in Java Radix Sort in Java Topological Sort in Java Bucket Sort in Java Counting Sort in Java

Java OOPs Concepts

OOPs - Object Oriented Programming Objects and Classes in Java Java Naming Conventions Constructors in Java Java this keyword Java static keyword Inheritance in Java Aggregation in Java Java super keyword Constructor Chaining and Constructor Overloading Java Polymorphism Static and Dynamic Binding in Java Java Abstraction Abstract class in Java Interface in Java Difference between Abstract class and Interface Java final keyword Packages in Java Access Modifiers in Java Java Wrapper classes Java Numbers Java Characters Java Integer Java Boolean Java Arrays Java Command Line Arguments Java strictfp Keyword Java Math

Java Strings

Java Strings Java String Methods StringBuilder in Java StringBuffer in Java Java Regular Expressions StringBuffer vs StringBuilder String vs StringBuffer String vs StringBuilder String Manipulation in Java Java String Concatenation How to Reverse a String in Java String Array in Java How to Compare Two Strings in Java How to Concatenate Two Strings in Java Why String in Immutable in Java java.lang.NumberFormatException for Input String String Pool in Java Java Generate Random String How to take String Input in Java Java String Interview Questions

Java Exceptions

Exception Handling in Java Java try catch Java throw Java throws Difference between throw and throws Java finally Java Custom Exception Java Exception Propagation

Garbage Collection

Automatic Resource Management in Java Java Garbage Collection Java finalize() Java gc() Difference between final, finally and finalize

Multithreading

Multithreading in Java Process and Thread in Java Basic Terms in Multithreading Java Thread creation Thread Scheduler in Java Java Thread class DeadLock in Java

Java IO

Java IO Java Read File Java BufferedWriter Java InputStreamReader Java File Java Read File Line By Line Java FileOutputStream Jar File in Java

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Java Serialization Java transient

Networking

Java Network Programming Java Socket Programming Java URL Java URLConnection HttpURLConnection in Java Java InetAddress Java DatagramSocket and Java DatagramPacket

AWT

Java AWT

Swing

Swing in Java

Java Collections

Collections in Java Java List Interface ArrayList in Java LinkedList in Java Vector in Java Stack in Java ArrayList vs LinkedList in Java ArrayList vs Vector in Java Java Set Interface HashSet in Java LinkedHashSet in Java TreeSet in Java Java Queue Interface PriorityQueue in Java ArrayDeque in Java Java Map Interface HashMap in Java LinkedHashMap in Java TreeMap in Java Dictionary in Java Hashtable in Java Properties in Java Collections class in Java Java Comparable Java Comparator Comparable vs Comparator in Java

Java Generics

Generics in Java

Java Annotations

Annotations in Java

Java JDBC

Java JDBC Tutorial

Java Differences

Java vs JavaScript Python vs Java Kotlin vs Java Java vs C++ C# vs Java Static vs Non-static in Java C vs Java int vs Integer in Java Stack vs Heap in Java Java Array vs ArrayList

How to

How to run Java program How to set path in Java How to check the Java version in cmd How to install Java in Windows 10 How to run Java program in cmd How to call a method in Java How to sort an array in Java How to iterate HashMap in Java How to write Java program How to create an array in Java How to create a package in Java How to generate random numbers in Java How to input String in Java How to create thread in Java How to find length of String in Java How to sort a string in Java How to use scanner in Java How to achieve multiple inheritance in Java How to run Java program in Eclipse How to call a function in Java How to create array of objects in Java How to create custom exception in Java How to achieve abstraction in Java How to call static method in Java

Java 8 Features

Java 8 Features Lambda Expressions in Java Functional Interface in Java Streams in Java Java Base64 Encoding and Decoding Parallel Arrays Sort in Java

Java 9 Features

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Java Servlets Tutorial

Java JSP Tutorial

Java JSP Tutorial

Hibernate Tutorial

Hibernate Tutorial

Spring Tutorial

Spring Tutorial

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Spring MVC Tutorial

Jenkins Tutorial

Jenkins Tutorial

Java Math Methods

Math.abs() Math.acos() Math.addExact() Math.asin() Math.atan () Math.atan2() Math.cbrt() Math.ceil() Math.copysign() Math.cos() Math.cosh() Math.decrementExact() Math.exp() Math.expm1() Math.floor() Math.floorDiv() Math.floorMod() Math.getExponent() Math.hypot() Math.IEEEremainder() Math.incrementExact() Math.log() Math.log10() Math.log1p() Math.max() Math.min() Math.multiplyExact() Math.multiplyFull() Math.negateExact() Math.nextAfter() Math.nextDown() Math.nextUp() Math.pow() Math.random() Math.rint() Math.round() Math.scalb() Math.signum() Math.sin() Math.sinh() Math.sqrt() Math.subtractExact() Math.tan() Math.tanh() Math.toDegrees() Math.toIntExact() Math.toRadians() Math.ulp()

Java String Methods

toCharArray() copyValueOf() endsWith() equals() equalsIgnoreCase() format() getBytes() getChars() hashCode() indexOf() intern() isEmpty() join() lastIndexOf() length() replace() replaceAll() replaceFirst() split() startsWith() subSequence() substring() toLowerCase() toUpperCase() trim() valueOf()

Java Conversion

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Misc

Functional Interfaces in Java Singleton class in Java Awesome explanation of Strings in Java Object class in Java Static class in Java All the important string methods in Java String Handling Method in Java What are Array strings in Java Advantages and Disadvantages of Strings in Java Big Decimal class in Java Class definition in Java Char and String differences in Java Difference between String, StringBuffer and StringBuilder in java Replace character in string Java String Coding Interview Questions in Java What is String in Java? String isnullorempty in Java String Matches in Java Trim Method in String Java Bean class in Java Libraries in Java Arithmetic Operations on String in Java Convert Char array to string in java Check whether Java is installed or not How to calculate time difference in Java How to stop execution after a certain time in Java Jagged Array in Java Java ArraylistRemove() Time Complexity Java Swing Time Picker Zigzag Array in Java

Classes and Objects in Java Example Programs

Classes and Objects in Java Example Programs

Java is an Object-Oriented programming language, i.e., everything in Java is associated with objects and objects are associated with classes. The classes and objects in Java examples programs teach us about classes and objects and their usages.

What are Classes and Objects?

A set of instructions or blueprint required to create an object is called a class. Classes are the most fundamental elements of Object-Oriented programming. Classes in Java relates to real-life stuff and make Object-Oriented programming easy yet interesting.

The instantiation of a class is called an object. In other words, that element of Object-Orientated programming, which obeys the set of instructions or blueprint defined by a class called is an object.

Class is more like a concept that we can understand but cannot touch or see it. An object implements that concept and becomes a real-time entity. Thus, we can see or touch an object. For example, A woody plant that has roots, elongated stems, leaves, a lot of twigs is called a tree. Here, leaves, roots, elongated stems, etc., are concepts or blueprints. Hence, a tree is a class. A mango tree or a guava tree is one of the objects of the class tree. This is because a mango or guava tree implements all the concepts given in the definition of a tree. Whenever we see a tree, it is basically a type of tree, i.e., an object of the class tree.

Programmatically, a Java class contains variables, methods and their definitions, and data-structures. In order to use the variables, methods or data structures of the class, we create objects. Since a class is just a blueprint, it does not occupy any memory, whereas objects occupy memory. Note that a class can have one or more than one object.

How to create a Java class?

The keyword class is used to create a class in Java. After the keyword class, the name of the class is written. For example, the code to create a class whose name is ABC and contains a variable var of integer type that stores the number 5 is:

FileName: ABC.java

 public class ABC
 {
                int var = 5;
 } 

The keyword public is the access specifier.

How to create an object of a Java class?

To create the object, mention the class name, then the object name, followed by the keyword new.

FileName: ABC.java

 public class ABC
 {
                int var = 5;
                public static void main(String argvs[])
                {
                                ABC ob = new ABC(); // creating an object of class ABC
                                System.out.println( ob.var );
                }
 } 

Output:

5

Explanation: ob is the name or reference of the object, and ob is referring to the object creating by the keyword new. In other words, the new keyword is responsible for the instantiation of the class ABC. The new keyword allocates memory for the object and returns the reference of it. This reference is stored in ob.   

Creating an anonymous object

In the previous example, we have seen that ob is the name/ reference of the object. However, we can also create an anonymous object in Java. An object in Java that has no name or reference is called an anonymous object.

FileName: ABC.java

 public class ABC
 {
                int var = 5;
                public static void main(String argvs[])
                {
                                // printing the value contained in var using an anonymous object
                                System.out.println( (new ABC()).var );
                }
 } 

Output:

5

Explanation: Inside the print statement, we have created the anonymous object               ( new ABC()). Using the anonymous object, we are accessing the value of the variable var. The drawback of anonymous objects is; it can be used only once. Consider the following Java program. This program will print the value of the variable var twice.

FileName: ABC.java

 public class ABC
 {
                int var = 5;
                public static void main(String argvs[])
                {
                                // Printing the value contained in var using an anonymous object
                                System.out.println( (new ABC()).var );
                                // Printing the value of var again using another anonymous object
                                System.out.println( (new ABC()).var );
                }
 } 

Output:

 5
 5 

Explanation: In the code, we have created two anonymous objects for displaying the value of the variable var two times. This is because a reference or name is required to use a Java object more than once. In the case of an object having a reference, we do not need to create two objects. Using the reference, we can display the value of variable var twice. The following code snippet does the same.

 public static void main(String argvs[])
 {
                ABC ob = new ABC(); // creating an object having reference ob.
                System.out.println( ob.var );
                System.out.println( ob.var );
 } 

Creating Multiple Objects

Let us observe how we can create multiple objects of a class in Java.

FileName: XYZ.java

 public class XYZ
 {
                void foo()
                {
                    System.out.println("Inside the method foo. ");
                }
                public static void main(String argvs[])
                {
                               // Creating two objects of the class XYZ
                                XYZ obj1 = new XYZ();
                                XYZ obj2 = new XYZ();
                                // calling the method foo()
                                obj1.foo();
                                obj2.foo();
                }
 } 

Output:

 Inside the method foo.
 Inside the method foo. 

Explanation: In the code, we have created two objects, obj1 and obj2, of the class XYZ. Then, obj1 and obj2 are calling the method foo() separately. Since we have created two objects, there will be two copies of the class XYZ. One is assigned to obj1, and another is assigned to obj2. Therefore, changes done by obj1 does not affect changes done by obj2 (exception: class attributes and methods). Let us understand with the help of an example.

FileName: XYZ1.java

 public class XYZ1
 {
                int x;
                public static void main(String argvs[])
                {
                               // Creating two objects of the class XYZ
                                XYZ1 obj1 = new XYZ1();
                                XYZ1 obj2 = new XYZ1();
                                // assigning different values of x for obj1 and obj2
                                obj1.x = 1;
                                obj2.x = 2;
                                // Printing the value of x for obj1 and obj2
                                System.out.println( obj1.x );
                                System.out.println( obj2.x );
                }
 } 

Output:

 1
 2 

Explanation: We are creating two objects of the class XYZ1. Therefore, two copies of the variable x are created: one for obj1 and another for obj2. For obj1, the variable x contains the value 1. For obj2, x contains the value 2 and the same is displayed on the console.

Creating Multiple Classes

Not only multiple objects, but we can also create multiple classes in Java. Consider the following example.

FileName: Main.java

 class XYZ
 {
     int x;
 }
 public class Main
 {
                // main method   
                public static void main(String argvs[])
                {
                               // Creating an object of the class XYZ
                                XYZ obj1 = new XYZ();
                                obj1.x = 1; // accessing the variable x and assigning value 1
                                System.out.println( obj1.x );
                }
 } 

Output:

1

Explanation: The class XYZ has the variable x. Another class, Main, has the main method. Inside the main method, we have created an object of the class XYZ to access the variable x. Note that a Java program can have multiple classes, objects, methods, and variables. Along with x, we can even create another variable, y, or we can create different methods in the class XYZ. In Java projects, we always have different classes containing different variables and methods.



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