无参数构造函数和带参数的构造函数之间的关系

Question

Please have a look at the following class:

public class Loan {

    private double annualInterestRate;
    private int numberOfYears;
    private double loanAmount;
    private java.util.Date loanDate;

    // Default constructor
    public Loan() {
        this(2.5, 1, 1000);
    }

}

For the codeline this(2.5, 1, 1000); , I get the following error message in Eclipse: "The constructor Loan(double, int, int) is undefined". This error disappears when adding a new constructor with arguments:

// Construct a loan with specified annual interest rate, number of years and loan amount
    public Loan(double annualInterestRate, int numberOfYears, int loanAmount) {
        this.annualInterestRate = annualInterestRate;
        this.numberOfYears = numberOfYears;
        this.loanAmount = loanAmount;
        loanDate = new java.util.Date();
    }

Why does the creation of the constructor with arguments remove the "undefined" error from the default constructor? How are these two different constructors related to each other?

How do I know that the values in this(2.5, 1, 1000) are assigned to the right data fields? I assume 2.5 should be assigned to annualInterestRate , 1 to numberOfYears , and 1000 to loanAmount .

Answer 1

In your no-arg constructor, the line

        this(2.5, 1, 1000);

explicitly means "call another constructor for the same class as the current constructor, but which takes these arguments".

So that's why adding the other constructor fixes the problem. The order in which you pass the arguments needs to match the order in which the parameters appear on that constructor, that constructor's parameter list defines the order you need to put the arguments in when calling it.

The relationship between these two constructors is that they are chained. The one with the 3 arguments is the primary constructor, the other calls the primary constructor with default values. Designing constructors in this way helps to initialize your objects consistently, because a single primary constructor always gets called. (For instance, the loanDate instance field gets set regardless of which constructor you call.)

Answer 2

You got the error

"The constructor Loan(double, int, int) is undefined"

Because if you take a look at:

this(2.5, 1, 1000);

You can see that the arguments are as follow 2.5 which is a double , 1 which is an int and 1000 which is also an int , so it calls a constructor with these arguments and there was no such constructor defined .

And with this keyword we are calling a constructor of the current class , so we are expecting a constructor with these arguments, that's why you neeed to implement this constructor:

// Construct a loan with specified annual interest rate, number of years and loan amount
public Loan(double annualInterestRate, int numberOfYears, int loanAmount) {
    this.annualInterestRate = annualInterestRate;
    this.numberOfYears = numberOfYears;
    this.loanAmount = loanAmount;
    loanDate = new java.util.Date();
}

And as stated by David in comments the error disappeared because you defined the stated constructor.

Answer 3

In order to answer your query the first thing is that Java is a strongly typed language. Refere Wiki

Now coming to your query, when you right this(2.5, 1, 100) , then it tries to find out the constructor referred by this() with the arguments types double,int,int (this is acheived in Java using implicit type conversions) in the sequence. As the sequence of the arguments matters along with type matching so it is assured that the right value is assigned to right argument. I hope it answers your queries.