java get missing field

Question

public static void main(String[] args) { 
    // create a book named "Java is Fun" with 200 in stock costing 55.00 each. The stock number is 1234 
    Book javafun = new Book("Java is Fun", 200, 55.00, 1234); 
    //print out the book using the toString() method 
    System.out.println (javafun.toString()); 

    //create a book calling the empty constructor. 
    Book newbook2 = new Book(); 
    //set the title to "Databases R Us" 
    String title = "Databases R Us"; 
    //set the number in stock to 50 
    int numInStock = 50; 
    // set the cost each to $35.00 
    double cost = 35.00; <------ this 
    // set the stock number to 5555 
    int stockNum = 5555; 
    //print out the book. 
    System.out.println(newbook2.toString()); 
    //change the price to $55.00 
    double cost = 55.00; <-- and this gives me duplicate error 
    //print out the book 
    System.out.println (newbook2.toString()); 

} 

我該如何做空間下面的步驟？

我有點做了我認爲應該但它給了我我有重複。並且輸出不出來正確的

輸出應該是：

Java是有趣的ID號1234在股票200本書的成本每$ 55,00。

ID爲5555的數據庫R Us有50本書，每本價值35美元。

ID爲5555的數據庫R Us有50本書，每本55美元。

Answer 1

您有一個重複的局部變量。 double cost = 55.00;將導致語法錯誤。

通過程序中的註釋來判斷，它看起來像只是想要更改值，不要在數據類型前面加上變量名稱。

即cost = 55.00;

如果你的意思是要改變與特定Book對象相關的成本，作爲另一個用戶建議..只是去book1.setCost/Title/Whatever(...)我假設在你的書類，你有setter方法和你變量不公開。

Answer 2

除了重複的變量創建，你需要在這裏將成員cost的值更改爲55或50，這將是這樣的。

// change the value of the cost member 
newBook2.cost = 55; 

現在，一旦你輸出這個（以字符串符號），你將在書中得到成本。到目前爲止，您只是初始化新變量，而不是更改Book對象成員的值。比方說，你得到一本書，你想改變這些對象的屬性，你可以這樣做類似下面的代碼，

// create the object 
Book myBook = new Book(); 

// now we will change the values for the members 
myBook.title = "Databases R Us"; 
myBook.numInStock = 50; 
myBook.cost = 50; 
myBook.stockNum = 5555; 

// Above were the settings for the Book object 
System.out.print(myBook.toString()); 

上面的代碼就會有你的書對象指定的屬性而不是變量的值。欲瞭解更多關於類成員，請通過這個Java開發人員的鏈接：http://docs.oracle.com/javase/tutorial/java/javaOO/classvars.html

Answer 3

public class BookDriver { 

public static void main(String[] args) { 
    // create a book named "Java is Fun" with 200 in stock costing 55.00 each. The stock number is 1234 
    Book javafun = new Book("Java is Fun", 200, 55.00, 1234); 
    //print out the book using the toString() method 
    System.out.println (javafun.toString()); 
    //create a book calling the empty constructor. 
    Book newbook2 = new Book(); 

    //set the title to "Databases R Us" 
    newbook2.setTitle("Databases R Us"); 
    //set the number in stock to 50 
    newbook2.setNumInStock(50); 
    // set the cost each to $35.00 
    newbook2.setCost(35.00); 
    // set the stock number to 5555 
    newbook2.setStockNum(5555); 
    //print out the book. 
    System.out.println(newbook2.toString()); 
    //change the price to $55.00 
    newbook2.setCost(55.00); 
    //print out the book 
    System.out.println (newbook2.toString());