Java-已声明方法但无法引用它

Question

I am kind of new to Java, although I've programmed in other procedural langauages for 25 years. So, I'm trying to teach myself Java. Trying to write some silly program consisting of two files: a Class called "Customer", and a main program called "Silly4".

I'm pretending to implement a credit card system for a bank type company (even though the majority of my experience was in defense contracting). I figure this would be a good teaching example for me.

Trying to build a credit card data structure called "Customer" such that (for the time being) it can accomodate 1000 customers. In the main program "Silly4", I instantiate this Customer class as "cust2", and then from there I attempt to work with "cust2". I try to retrieve customer number 5's (j=5) credit card balance. So far, so good.

Then from there I attempt to declare in the class Customer another method (for future use) which I arbitrarily call "bal44", and then I attempt to reference it in the main program Silly4 as " ball44(5541);".

So I compile class Customer, then compile program Silly4, and I'm getting a compile error "java:52: error: cannot find symbol" for the reference to method "bal44(5541)" in main program "Silly4". I'm confused. I've declared and successfully compiled the class Customer with "bal44" in there, but Java is telling me it can't find it. I'm confused.

Please excuse all the extraneous println's, I use them to see how the program is moving along.

Here is class Customer:

// Data Structure for credit card database

public class Customer {
    private String name;
    private int accountNo;
    private double balance;
    private boolean Overdue;

    // Getters, setters, constructor...


    public void setName( String new_name )
        {  name = new_name;  }


    public void setAccount( int new_account )
        {  accountNo = new_account;  }


    public void setBalance( double new_bal )
        {  System.out.println( " Start proc setBalance ");
           balance = new_bal;
           System.out.println( " Finish proc setBalance ");
            }

    public double getBalance()
        {  System.out.println( " Start proc getBalance ");
           System.out.println( " proc getBalance , balance= " + balance + " end print");
           return balance;
           // how to specify which element of array[1000] ? balance I want ?
           // System.out.println( " Finish proc getBalance ");
            }

    // Add new customer to credit card system
    //   (note - index in array Customer[i] is worry of main program
    //
    public void addCustomer( String name2, int account2, double bal2 )
         { name      = name2;
           accountNo = account2;
           balance   = bal2;
             }

    public void bal44 ( int account3 )
         { accountNo = account3; }


    // Constructor

    Customer ()
        {   name      = "John Smith";
            accountNo = 1005;
            balance   = 125.43;
            Overdue   = false;    }

    // see page 1032 Liang for definition complex Object and get-Procs for it


}    

Here is main program/class Silly4:

class Silly4

{ // Program for credit card database

public static void main(String[] args)
 {
     double bal2, bal3;
     int i;                 // loop counter
     int j;

     bal2 = 151.47;
     bal3 = 5.0;           // just initialize

    // And then you can create an array of it:

    System.out.println("** Program Silly4 Running **"); 

    Customer[] cust2 = new Customer[1000];

    System.out.println("** Array cust2 instantiated **"); 

    for(i=0; i<=999; ++i)
    {
         cust2[i] = new Customer();
    }

    System.out.println("** Array2 cust2 Obj initialized **"); 



    //  try to code this eventually  -   cust2.balance = 151.47 ;
    //

    j = 5;                 // customer no. 5

    cust2[j].setBalance( bal2 );


    bal3 = cust2[j].getBalance() ;


    System.out.println("** Balance Customer " + j + " is " + bal3);

    // Add a new customer
    //  comment out -  addCustomer( "Steve Jones", 5541, 1.0 );

    bal44( 5541 );      // test out declaring new method "bal44"


    System.out.println("** End of Silly4  **");

    }

}

Answer 1

When you call

 bal44( 5541 ); 

Without any object, Java assumes it as a "this" object and searches for that method in Silly4 not in Customer class. Call that method from Silly4 like you are calling other method of customer class. eg

cust2[j]. bal44( 5541 ); or something like that.

Read more about "this" here https://docs.oracle.com/javase/tutorial/java/javaOO/thiskey.html

PS A bonus point to note here is that static method's like main method do not have this reference available to them. So, technically here it is searching for a class-level(static) method in Silly4 rather than instance method. :)

Answer 2

You basically answered yourself in the first sentence of your query. Java is not a purely procedural language, it's an object-oriented language.

That means that when you declare methods, they are not just scoped functions. A method declared in a class is not similar to a function declared in an include d file in C. It's something that can be only called from the scope of an object of the enclosing class . In other words, you don't have a function named bal44 . You have a method bal44 of class Customer , which means that you need to ask an object fo class Customer to execute it for you, similar to how you call setBalance() .

EDIT: As another comment mentions, what might be confusing you more is that whenever you use a method name without qualifiers, Java treats it as a shortcut of asking this (the current object) to execute the method, so technically, method(args) is a shortcut for this.method(args) . Therefore, whenever you are within the scope of a single class, a class's methods will behave almost as traditional functions.

Answer 3

See, here is the problem : You are using the bal44(5541) , but you made that method non static, meaning you need to make an object of your class (Customer) and then say for example Customer customer = new Customer() customer.bal(5541); If you want to call that method without making an object of its class, simply put the keyword "static" in the method's header. public static void bal44(int num)

Further explanation : Since you are from a procedural language, recall the main difference is the usage of objects. An object is something that has several traits, or attributes, and actions or behaviors. For example a car, its attributes are its color, speed, etc. and its main action/behavior is drive. Attributes are variables, and Behaviors/Actions are methods. eg

car.drive(); car.setColor("red");

I hope that helped? If you don't understand you can PM me or respond here and I'll explain in further detail