如何使用toString（）-Method获得每个对象数量增加的输出？ 爪哇

Question

I'm using the toString()-Method to get the proper output from my ArrayList.

But I'm still not able to get an Output where the number increases per object.

The current code I have written is as follows:

public class Book {  

    @Override
         public String toString() {

              StringBuilder result = new StringBuilder();

              int i = 1;

              result.append("\n\n-Book "+ i++ +": ");
              result.append("\nTitel: " + this.Titel + "§ ");
              result.append("\nAuthor: " + this.Autor + "§ ");

              return result.toString();
         }

by inserting ' i++ ' I'm trying to get an output,

where the number increases for each object from the public class Book();

Current Output:

-Buch **1**:  
Titel: Hunger Games§ 
Author: Suzanne Collins§ ,

-Buch **1**:   
Titel: Twilight§ 
Author: Stephanie Meyer§ ,

-Buch **1**: 
Titel: Pride and Prejudice§ 
Author: Jane Austen§ ,

Output I'm trying to get:

-Buch **1**:  
Titel: Hunger Games§ 
Author: Suzanne Collins§ ,

-Buch **2**:   
Titel: Twilight§ 
Author: Stephanie Meyer§ ,

-Buch **3**: 
Titel: Pride and Prejudice§ 
Author: Jane Austen§ ,

Answer 1

What you need is to set your i variable as static and class field.

public class Book {  

    private static int i = 1;


    @Override
         public String toString() {

              StringBuilder result = new StringBuilder();


              result.append("\n\n-Book "+ i++ +": ");
              result.append("\nTitel: " + this.Titel + "§ ");
              result.append("\nAuthor: " + this.Autor + "§ ");

              return result.toString();
         }

Answer 2

Exactly, you can use a static variable. But instead of increasing it inside the toString() Method id rather increase it in the constructor since your question suggests you want to count the number of objects, not the number of times the toString() method was called on one of your object instances.

public class Book {  

private static int i = 0;

public Book(){
   i++;
}

@Override
     public String toString() {

          StringBuilder result = new StringBuilder();


          result.append("\n\n-Buch"+ i +": ");
          result.append("\nTitel: " + this.Titel + "§ ");
          result.append("\nAuthor: " + this.Autor + "§ ");

          return result.toString();
     }
}

Edit: Below comment is correct, this will always print out how many books have been created the time you call the toString() method of any Book object.

Here is a second Version, you now have one variable per object instance identifing the books number according the time it was created and the same static variable from above, look at the sample output.

public class Book {

private static int i = 0;
private int i2;

public Book() {
    i2 = i;
    i++;
    // or place 'i2 = i;' here if you want to start with "Book 1 of..."
}

@Override
public String toString() {

    StringBuilder result = new StringBuilder();

    result.append("Book " + i2 + " of " + i + " books in total");

    return result.toString();
}
}

Sample Call:

public class SampleCall{

public static void main(String[] args) {

    Book b = new Book();
    Book b1 = new Book();
    Book b2 = new Book();
    Book b3 = new Book();

    System.out.println(b1);
          System.out.println(b1);

}    
}

Sample Output:

Book 0 of 4 books in total
Book 1 of 4 books in total

Since you just started using Java consider a solution where you are setting the number for each Book instance by yourself just for completion:

public class Book {

private static int i = 0;
private int bookNumber;

public Book(int bookNumber) {
    i++;
    /* this.bookNumber means "the variable of the currently in creation book object instance)
     * you only need it if its not clear because there is a local variable with the same name 
     */
    this.bookNumber = bookNumber;
}

// a second (default) constructor in case you want to create book objects and dont yet know the number of it
// after creation you can use the get/set values to access the variable from "outside"
public Book(){
    i++;
}

@Override
public String toString() {

    StringBuilder result = new StringBuilder();

    // Here you dont need this because there is no local variable bookNumber
    // You can however always use it when referencing such object variables 
    result.append("Book " + bookNumber + " of " + i + " books in total");

    return result.toString();
}

public int getBookNumber(){
    return bookNumber;
}

public void setBookNumber(int bookNumber){
    this.bookNumber = bookNumber;
}
}

Sample Call:

public class SumeMain {

public static void main(String[] args) {

    Book b = new Book(1);
    Book b1 = new Book(2);
    Book b2 = new Book();

    System.out.println(b);
    System.out.println(b1);

    //Note below book not yet has a number:
    System.out.println("Forgotten book: "+b2);

    //But you can set it from "outside" (here)
    b2.setBookNumber(3);

    System.out.println(b2);

    //Also note you now took controll of those numbers so two books can have the same number:
    b2.setBookNumber(2);

    System.out.println("Duplicate book number: "+b2);
}


}

Sample Output:

Book 1 of 3 books in total
Book 2 of 3 books in total
Forgotten book: Book 0 of 3 books in total
Book 3 of 3 books in total
Duplicate book number: Book 2 of 3 books in total

By such examples i think youre on the best way to learn the language basics, keep up and have fun :)

Answer 3

You are calling toString() method on an encapsulated Book object. It has no knowledge of other Book's in ArrayList.

Try removing those lines from toString() method:

int i = 1;
result.append("\n\n-Book "+ i++ +": ");

And iterate over collection like this:

List<Book> books = ...
for(int i = 0; i < books.size(); i++){
    System.out.println("\n\n-Book "+ (i + 1) +": ");
    System.out.println(books.get(i));
}

Strange that you know how to use toString() and StringBuilder, but experience problems in iterating collections. Try going back to basic programming techniques.

Also, a lot of answers recommend using a static field. Don't go with that approach. Static fields are not good for this. It won't work in case you have to iterate the collection of books twice.