this worked but need improvement

I don't understand why the argument of printIdea method in main method is (new Idea())?

because the printIdea method takes an idea object. You can do it this way: Idea temp = new Idea(); printIdea(temp); But if you look at that code, temp equals new Idea(), you could save a whole line of code by not declaring the separate variable and passing it to the method and just do what the OP did.

Thank you for the answer, it's really help. But I think I still having a question. Is there any difference between Idea temp = new Idea(); and new Idea();? Because if I write Idea temp = new Idea(); then Idea object has a reference, which is temp, while if I just write new Idea(); then Idea object doesn't have reference.

You are correct. In the code above there is not a need for a reference, but a lot of times there is a need for one. I will do it both ways depending on that need for the reference. Basically if you don't need to modify the data then there is no need for a reference. A good example of a need to change data is changing a string if it contains a certain value. Take for example this:

if("The answer is: foolish".contains("foolish")){
   /*cant do anything with that string here because there is no reference to it*/
}

or

String answer = "The answer is: foolish";
if(answer.contains("foolish"))
   answer = answer.replaceall("foollish", "**BLOCKED**");

The second example there is a reference to the string and the code can modify that reference. The first one you just cant do it. Its a small example but i think you get the point.

I think I get the point. Thank you for the explanation. :)