Printf格式錯誤不兼容的類型

Question

我不斷收到此錯誤，說不兼容的類型：java.io.PrintStream cannot be converted to java.lang.String，我沒有得到如何讓它爲我的toString方法工作。我試着把它分配給一個變量，然後返回它，然後只是打算把它打印出來作爲返回語句，我沒有發現我的printf格式有什麼問題。幫助表示讚賞。

import java.text.NumberFormat; 

    public class Item 
    { 
     private String name; 
     private double price; 
     private int quantity; 


     // ------------------------------------------------------- 
     // Create a new item with the given attributes. 
     // ------------------------------------------------------- 
     public Item (String itemName, double itemPrice, int numPurchased) 
     { 
     name = itemName; 
     price = itemPrice; 
     quantity = numPurchased; 
     } 


     // ------------------------------------------------------- 
     // Return a string with the information about the item 
     // ------------------------------------------------------- 
     public String toString() 
     { 

     return System.out.printf("%-15s $%-8.2f %-11d $%-8.2f", name, price, 
    quantity, price*quantity); 
     } 

     // ------------------------------------------------- 
     // Returns the unit price of the item 
     // ------------------------------------------------- 
     public double getPrice() 
     { 
     return price; 
     } 

     // ------------------------------------------------- 
     // Returns the name of the item 
     // ------------------------------------------------- 
     public String getName() 
     { 
     return name; 
     } 

     // ------------------------------------------------- 
     // Returns the quantity of the item 
     // ------------------------------------------------- 
     public int getQuantity() 
     { 
     return quantity; 
     } 
    } 

Answer 1

return System.out.printf("%-15s $%-8.2f %-11d $%-8.2f", name, price, 
    quantity, price*quantity); 

你試圖返回PrintStream。這是不正確的，因爲toString應該返回一個String。

你應該使用String#format方法，其中fomratting

return String.format("%-15s $%-8.2f %-11d $%-8.2f", name, price, 
     quantity, price*quantity); 

Answer 2

System.out.printf不返回一個字符串，您正在尋找String.format

public String toString() { 
    return String.format("%-15s $%-8.2f %-11d $%-8.2f", name, price, quantity, price*quantity); 
} 

Answer 3

變化

System.out.printf("%-15s $%-8.2f %-11d $%-8.2f", name, price, 
quantity, price*quantity); 

到

String.format("%-15s $%-8.2f %-11d $%-8.2f", name, price, 
quantity, price*quantity); 

因爲System.out.println()如果打印字符串到控制檯後還給一個String 。不創建一個格式化與字符串

Answer 4

錯誤是由

return System.out.printf(....) 

造成如果你真的想返回此方法的字符串，然後嘗試

return String.format(....);