“the method ** is undefined for the type string” in Eclipse

问题

I am currently completing a project and am trying to create a menu. The menu itself works alright but when I try and link to the other java file that the method itself is in I get the error "the method method name is undefined for the type String". Below is the code that I am using to try and link to the method in the other java file.

        {
        if (menuChoice.equals("A"))
        {
            System.out.print("Enter the Movie ID: ");
            movieID = sc.nextLine();
            movieID.borrowMovie();
        }
        if (menuChoice.equals("a"))
        {
            System.out.print("Enter the Movie ID: ");
            movieID = sc.nextLine();
            movieID.borrowMovie();
        }
        if (menuChoice.equals("B"))
        {
            System.out.print("Enter the Movie ID: ");
            movieID = sc.nextLine();
            movieID.returnMovie();
        }
        if (menuChoice.equals("b"))
        {
            System.out.print("Enter the Movie ID: ");
            movieID = sc.nextLine();
            movieID.returnMovie();
        }

Further to, below is the code of the 2 methods that I am trying to link to in the other java file. It is a requirement of the project to have 2 separate class files and to demonstrate how to use them both together but this part has got me stumped!

// Implement borrowMovie method
public double borrowMovie(String memberID)
{
    if (this.isOnLoan = false)
    {
        this.isOnLoan = true;
    }
    else 
    {
        return Double.NaN;
    }
    }

// Implement returnMovie method
public double returnMovie(int daysBorrowed)
{
    if (this.isOnLoan = false)
    {
        return Double.NaN;
    }
    else
    {
        this.isOnLoan = false;

    }
}

Any help at all will be appreciated :)

回答1:

Looking at your code, I guess that movieID is a String.

Class String does not have methods borrowMovie() and returnMovie(), so you'll get an error from the compiler that tells you so.

Instead of movieID.borrowMovie() you want: borrowMovie(movieID).

Here's another bug in your borrowMovie() and returnMovie() methods:

if (this.isOnLoan = false)

You are assigning the value false to this.isOnLoan here. That is most likely not what you want. Use == for comparison, = for assignment. Better yet, to check if a boolean is false, use !:

if (!this.isOnLoan)

Why are your methods returning double values?

回答2:

This line assigns the value false to isOnLoan and as such will always yield true.

if (this.isOnLoan = false)

Apart from this there's no return value on the else branch for example:

public double returnMovie(int daysBorrowed)
{
    if (this.isOnLoan = false)
    {
        return Double.NaN;
    }
    else
    {
        this.isOnLoan = false;
        return this.id;
    }
}

来源：https://stackoverflow.com/questions/17810669/the-method-is-undefined-for-the-type-string-in-eclipse