CODEWITHSUNDEEP
javaswingobjectconstructor
user3698066
user3698066

Reputation: 21

Difference between creating object in the constructor and creating object in declaration

what is the difference of this two codes snippet

first one

import javax.swing.*;

public class Mainframe extends JFrame {

    private JLabel nameLbl = new JLabel( "Name" );
    private JTextField nameTf = new JTextField( "10" );

}

second

import javax.swing.*;

public class Mainframe extends JFrame {

    private JLabel nameLbl;
    private JTextField nameTf;

    public Mainframe() {
        nameLbl = new JLabel( "Name" );
        nameTf = new JTextField( "10" );
    }

}

Upvotes: 0

Views: 118

Answers (5)

raggsss
raggsss

Reputation: 41

Creating object of any variable in constructor and in declaration is similar

see the example below:

public class DemoClass {

  String str;
  String newStr = new String("I am initialized outside");

  public DemoClass() {

    System.out.println(newStr);
    str = new String("I am initialized inside");
    System.out.println(this.str+"\n");

  }

 public static void main(String[] args) throws Exception {

    DemoClass dc = new DemoClass();

  }
}

In the above example you can see -- In the constructor the variables are getting initialized, as the object of DemoClass is already created in the memory by JVM before calling the constructor.

Constructors are meant only for initializing any instance variable.

Flow of creating object: Before creating the DemoClass object, JVM will create the depended object i.e. newStr will be created first, and then DemoClass object will be created.

Upvotes: 1

Richard Le
Richard Le

Reputation: 669

The explicit initialization of object fields is copied into every constructor (non-argument constructor or argument constructors) by compiler.

Upvotes: 0

Kumar Abhinav
Kumar Abhinav

Reputation: 6675

Internally ,the instance variable declaration and your constructor code becomes same in the bytecode.Even the initializer block gets merged in the following order :-

1)Instance member declaration
2)All initializer block declarations in the order of their occurence
3)Constructor code

After compilation,bytecode treats it as one

Upvotes: 0

TheLostMind
TheLostMind

Reputation: 36304

There is not much difference in your particular case.

But generally, if you wanted to initialize your object with some custom values, then you do it in a constructor.

example : 

public Mainframe(String name, String number) {
    nameLbl = new JLabel( name );
    nameTf = new JTextField( number );
}

Upvotes: 1

Aniket Thakur
Aniket Thakur

Reputation: 68945

Functionality wise there is none. But if you have multiple constructors in second case and if you are creating Object via non default constructor then the instance variables will remain null.

Upvotes: 0

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