Java Constructors

What is constructor?

• Constructor is similar like a method, constructor name should be same as class name.
• Constructor is used to perform the automatic initialization for class instance members.
• Constructor initializes object upon object creation.
• Once constructor is defined, constructor is automatically called immediately after the object is created but before the new operator completes.
• Constructor does not have return type and not even void, implicit return type of a constructor is a class type itself.

What are the different types of constructors in java?

There are two types of constructors:

• Default constructor (no argument constructor)
• Parameterized constructors

Default constructor - No argument constructor

class Student {                                                                 
 String name;                                                                   
 int mark1, mark2, mark3;                                                       
                                                                                
 //Constructor                                                                  
 Student() {                                                                    
  System.out.println("Initializing student objects");                           
  name = "test1";                                                               
  mark1 = 75;                                                                   
  mark2 = 80;                                                                   
  mark3 = 65;                                                                   
 }                                                                              
                                                                                
 int getTotalMarks() {                                                          
  return (mark1 + mark2 + mark3);                                               
 }                                                                              
}                                                                               
class JavaConstructor {                                                         
 public static void main(String args[]) {                                       
  Student student1 = new Student();                                             
  Student student2 = new Student();                                             
                                                                                
  System.out.println("Student1: " + student1.getTotalMarks());                  
  System.out.println("Student2: " + student2.getTotalMarks());                  
 }                                                                              
}    

Output:

$ java JavaConstructor
Initializing student objects
Initializing student objects
Student1: 220
Student2: 220

Parameterized Constructors

class Student {                                                                 
 String name;                                                                   
 int mark1, mark2, mark3;                                                       
                                                                                
 //Constructor                                                                  
 Student(String n, int m1, int m2, int m3) {                                    
  System.out.println("Initializing student objects");                           
  name = n;                                                                     
  mark1 = m1;                                                                   
  mark2 = m2;                                                                   
  mark3 = m3;                                                                   
 }                                                                              
                                                                                
 int getTotalMarks() {                                                          
  return (mark1 + mark2 + mark3);                                               
 }                                                                              
}                                                                               
class JavaConstructor1 {                                                        
 public static void main(String args[]) {                                       
  Student student1 = new Student("test1", 75, 45, 89);                          
  Student student2 = new Student("test2", 87, 78, 98);                          
                                                                                
  System.out.println(student1.name + ": " + student1.getTotalMarks());          
  System.out.println(student2.name +": "  + student2.getTotalMarks());          
 }                                                                              
}   

Output:

$ java JavaConstructor1
Initializing student objects
Initializing student objects
test1: 209
test2: 263

What is the use of this keyword?

this keyword is used to refer the current instance or object. this is always a reference to the object on which method is invoked.

 //Constructor                                                                  
 Student(String name, int mark1, int mark2, int mark3) {                        
  System.out.println("Initializing student objects");                           
  this.name = name;                                                             
  this.mark1 = mark1;                                                           
  this.mark2 = mark2;                                                           
  this.mark3 = mark3;                                                           
 }                                                                              

What is instance variable hiding?

• It is not legal in java declaring two local variables in same name in the same or enclosing scopes.
• If instance variable and local variable have same name, then local variable hides the instance variable.

class Student {                                                                 
 String name;                                                                   
 int mark1, mark2, mark3;                                                       
                                                                                
 //Constructor                                                                  
 Student(String name, int mark1, int mark2, int mark3) {                        
  System.out.println("Initializing student objects");                           
  name = name;                                                                  
  mark1 = mark1;                                                                
  mark2 = mark2;                                                                
  mark3 = mark3;                                                                
 }                                                                              
                                                                                
 int getTotalMarks() {                                                          
  return (mark1 + mark2 + mark3);                                               
 }                                                                              
}                                                                               
                                                                                
class JavaConstructor2 {                                                        
 public static void main(String args[]) {                                       
  Student student1 = new Student("test1", 75, 45, 89);                          
  Student student2 = new Student("test2", 87, 78, 98);                          
                                                                                
  System.out.println(student1.name + ": " + student1.getTotalMarks());          
  System.out.println(student2.name +": "  + student2.getTotalMarks());          
 }                                                                              
}  

Output:

$ java JavaConstructor2
Initializing student objects
Initializing student objects
null: 0
null: 0

Accessing instance variable using this keyword

this keyword is used to access instance variable and differentiates with local variable in same scopes.

class Student {                                                                 
 String name;                                                                   
 int mark1, mark2, mark3;                                                       
                                                                                
 //Constructor                                                                  
 Student(String name, int mark1, int mark2, int mark3) {                        
  System.out.println("Initializing student objects");                           
  this.name = name;                                                             
  this.mark1 = mark1;                                                           
  this.mark2 = mark2;                                                           
  this.mark3 = mark3;                                                           
 }                                                                              
                                                                                
 int getTotalMarks() {                                                          
  return (this.mark1 + this.mark2 + this.mark3);                                
 }                                                                              
}                                                                               
                                                                                
class JavaConstructor2 {                                                        
 public static void main(String args[]) {                                       
  Student student1 = new Student("test1", 75, 45, 89);                          
  Student student2 = new Student("test2", 87, 78, 98);                          
                                                                                
  System.out.println(student1.name + ": " + student1.getTotalMarks());          
  System.out.println(student2.name +": "  + student2.getTotalMarks());          
 }                                                                              
}     

Output:

$ java JavaConstructor2
Initializing student objects
Initializing student objects
test1: 209
test2: 263

Overloading Constructors

Overloaded constructor is called based upon the parameters specified when new is created for object creation.

class Student {                                                                 
 String name;                                                                   
 int mark1, mark2, mark3;                                                       
                                                                                
 //Constructor                                                                  
 Student() {                                                                    
  System.out.println("Initializing student objects");                           
  name = "test1";                                                               
  mark1 = 75;                                                                   
  mark2 = 80;                                                                   
  mark3 = 65;                                                                   
 }                                                                              
                                                                                
 Student(String name, int mark1, int mark2, int mark3) {                        
  System.out.println("Initializing student objects");                           
  this.name = name;                                                             
  this.mark1 = mark1;                                                           
  this.mark2 = mark2;                                                           
  this.mark3 = mark3;                                                           
 }                                                                              
                                                                                
 int getTotalMarks() {                                                          
  return (this.mark1 + this.mark2 + this.mark3);                                
 }                                                                              
}                                                                               
                                                                                
class JavaConstructor2 {                                                        
 public static void main(String args[]) {                                       
  Student student1 = new Student();                                             
  Student student2 = new Student("test2", 87, 78, 98);                          
                                                                                
  System.out.println(student1.name + ": " + student1.getTotalMarks());          
  System.out.println(student2.name +": "  + student2.getTotalMarks());          
 }                                                                              
}     

Output:

$ java JavaConstructor2
Initializing student objects
Initializing student objects
test1: 220
test2: 263

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