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Java- Constructor

Constructor

At the time of Object Creation some peace of code will execute automatically to perform initialization that peace of code is nothing but -Constructor”. Hence the main Objective of constructor is to perform initialization.

Rules for writing Constructor

  • The name of the constructor and name of the class must be same.

  • The only allowed modifiers for the constructors are public, private,protected, <default>. If we are using any other modifier we will get C.E(Compiler Error). 
    class Test
{
static Test(){
----
}
}
C.E:- modifier static not allowed here.

  • return type is not allowed for the constructors even void also. If we are declaring return type, then the compiler treats it as a method and hence there is no C.E and R.E(RuntimeError).

    class Test
{
void Test(){
System.out.println("Hai .....");
}
public static void main(String arg[]){
Test t = new Test();
}
}

  • If we are not writing any constructor, then the compiler always generate default constructor.

  • If we are writing at least one constructor, then the compiler won’t generate any constructor. Hence every class contains either programmer written constructor or compiler generated default constructor but not both simultaneously. constrctor

  • super() & this() in constructor

    • we should use as first statement in constructor.

    • We can use either super or this but not both simultaneously.

    • we can invoke a constructor directly from another constructor only

  • Inheritance concept is not applicable for constructor, so overriding is also not applicable

  • Recursive Constructor invocation leads to Compile-time Exception. 
    class Test {
  Test() {
    this(10);
  }
    
  Test(int i) {
    this();
  }
}
---------------------------------------
Test.java:6: error: recursive constructor invocation
        Test(int i) {
        ^
1 error

  • whenever we are writing parameterized constructor, it is recomended to provide no-arument constructor as well. If parent class contains parameterized constructor, then while writing child class constructor we should take a bit care super

  • If the parent class constructor throws checked exception, Compulsory the child class constructor should throw the same checked exception or it’s parent other wise we will get compile time error 
    class p {
  p() throws IOException {}
}
    
class c extends p {
  c() {
    super();
  }
}
Test.java:10: error: unreported exception IOException; must be caught or declared to be thrown
                super();
                     ^
  • If the parent class constructor throws unchecked exception, then child class constructor not required to throw that exception.
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