Equals Method in Java
As the name suggests in java, .equals() is a method used to compare two objects for equality. The .equals() method in java is used to check if the two strings have similar values. It checks them character by character. One should not confuse .equals() method with == operator. The String equals() method compares the two given strings based on the content of the string, whereas the == operator is used for address comparison. If all the contents of both the strings are the same, then .equals() returns true otherwise, it returns false. If all characters are not matched, then it returns false.

Let us understand this with the help of an example:

public class Demo {
public static void main(String[] args)
{
String s1 = "GreatLearning";
String s2 = "GreatLearning";
String s3 = new String("GreatLearning");
System.out.println(s1 == s2); // true
System.out.println(s1 == s3); // false
System.out.println(s1.equals(s2)); // true
System.out.println(s1.equals(s3)); // true
}
}

Even though s1 and s3 are created with the same field(content), they are pointing to two different objects in memory. Hence at different addresses. Therefore == operator gives false and .equals() method gives true as both contain similar content greatLearning.

Message Passing in Java
Message Passing in terms of computers is a communication phenomenon between the processes. It is a kind of communication used in object-oriented programming. Message passing in Java is the same as sending an object, i.e., a message from one thread to another thread. It is utilized when threads do not have shared memory and are not able to share monitors or any other shared variables to communicate. In message passing calling program sends a message to a process and relies on that process to run its own functionality or code. Message passing is easy to implement, has faster performance, and we can build massive parallel models by using it.

There are two types of it: Synchronous and Asynchronous.

Synchronous message passing occurs when the objects run at the same time.
In the case of an Asynchronous message passing, the receiving object can be down or busy when the requesting object sends the message.

Constructors in java
In Java, a constructor is a block of codes similar to the method. It is called when an instance of the class is created. At the time of calling constructor, memory for the object is allocated in the memory.

It is a special type of method which is used to initialize the object.

Every time an object is created using the new() keyword, at least one constructor is called.

It calls a default constructor if there is no constructor available in the class. In such case, Java compiler provides a default constructor by default.

There are two types of constructors in Java: no-arg constructor, and parameterized constructor.

Note: It is called constructor because it constructs the values at the time of object creation. It is not necessary to write a constructor for a class. It is because java compiler creates a default constructor if your class doesn't have any.

Rules for Creating Java constructor
There are two rules defined for the constructor.
1.Constructor name must be the same as its class name
2.A Constructor must have no explicit return type
3.A Java constructor cannot be abstract, static, final, and synchronized

Note: We can use access modifiers while declaring a constructor. It controls the object creation. In other words, we can have private, protected public or default constructor in java

Types of Java constructors
There are two type of constructors in java:
1. Default constructor (no-arg constructor)
2. Parameterized constructor

Java Default Constructor
A constructor is called "Default Constructor" when it doesn't have any parameter.
Syntax of default constructor:

<class_name>() {}

Example of default constructor
In this example, we are creating the no-arg constructor in the Bike class. It will be invoked at the time of object creation.

// java program to create and call a default constructor
class Bike1 {
// creating a default constructor
Bike1() {System.out.println("Bike is created");}
// main method
public static void main(String args[]) {
// calling a default constructor
Bike1 b = new Bike1();
}
}

output

Rule: if there is no constructor in a class, compiler automatically creates a default constructor.

Q) What is the purpose of a default constructor ?
The default constructor is used to provide the default values to the object like 0, null, etc., depending on the type.

// Let us see another example of default constructor
// which displays the default values
class Student3 {
int id;
String name;
// method to display the value of id and name
void display(){System.out.println(id+" "+name);}

public static void main(String args[]){
// creating object
Student3 s1=new Student3();
Student3 s2=new Student3();
// displaying values of the object
s1.display();
s2.display();
}
}

output

Explanation:In the above class,you are not creating any constructor so compiler provides you a default constructor. Here 0 and null values are provided by default constructor.

Java Parameterized Constructor
A constructor which has a specific number of parameters is called a parameterized constructor.

Why use the parameterized constructor ?
The parameterized constructor is used to provide different values to distinct objects. However, you can provide the same values also.

Example of parameterized constructor
In this example, we have created the constructor of Student class that have two parameters. We can have any number of parameters in the constructor.

// java program to demonstrate the use of the parameterized constructor
class Student4 {
int id;
String name;
// creating a parameterized constructor
Student4(int i, String n) {
id = i;
name = n;
}
// method to display the values
void display(){System.out.println(id+" " + name);}

public static void main(String args[]) {
// creating objects and passing values
Student4 s1= new Student4(111, "Karan");
Student4 s2 = new Student4(222, "Aryan");
// calling method to display the values of object
s1.display();
s2.display();
}
}

output

Constructor Overloading in Java
In Java, a constructor is just like a method but without return type. It can also be overloaded like Java methods.

Constructor overloading in Java is a technique of having more than one constructor with different parameter lists. They are arranged in a way that each constructor performs a different task. They are differentiated by the compiler by the number of parameters in the list and their types.