Java興趣計算器不會編譯，不知道爲什麼

Question

我正在用Java編寫一個興趣計算器。該程序提示用戶輸入，並使用該輸入計算某個銀行賬戶的利息（支票，儲蓄或CD）。

這是我的程序的要點，它非常簡單。但是現在我被困在了爲什麼return語句不能在createAccount方法中工作。任何幫助，將不勝感激。

Banker.java：

import java.awt.*; 
import java.awt.event.*; 
import javax.swing.*; 

public class Banker { 

// Array for type of bank account 
public static void createAndShowGUI() { 


    // Declare strings for period, balance, rate 
    String period; 
    String balance; 
    String rate; 
    String type; 
    String input; 

    // Prompt for account type 
    String[] accttype = {"Checking", "Savings", "CD"}; // Array of bank acct types 
    input = (String) JOptionPane.showInputDialog(null, "Choose account...", 
      "Choose bank account type", JOptionPane.QUESTION_MESSAGE, null, 
      accttype, // Array of acct types 
      accttype[0]); // First choice 


    // Prompt user for input 
    period = JOptionPane.showInputDialog(null, "Number of periods (length):"); 
    balance = JOptionPane.showInputDialog(null, "Beginning balance:"); 
    rate = JOptionPane.showInputDialog(null, "Interest rate (use decimal, example: .05 = 5%):"); 

    // Make Calculate button 
    JButton calculate = new JButton("Calculate"); 

    // Make 2 Labels 
    JLabel blabel = new JLabel("Period: " + period); 
    JLabel plabel = new JLabel("Balance: " + balance); 

    // Setup window with flow layout and exit on close 
    JFrame frame = new JFrame("Interest Savings Calculator Plus"); 
    frame.setLayout(new FlowLayout()); 
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 

    // Add combo box, calc button and labels 
    frame.add(calculate); 

    frame.add(plabel); 
    frame.add(blabel); 

    //Display the window. 
    frame.pack(); 
    frame.setVisible(true); 

} 

public static Account createAccount(String type, String checkno, String lengthm, String input) { 
    String message = "Would you like to open another account?"; 
    String title = "Are you sure?"; 
    if (input == "Checking") { 
     checkno = JOptionPane.showInputDialog(null, "First check number:"); 

     // display the JOptionPane showConfirmDialog 
     int reply = JOptionPane.showConfirmDialog(null, message, title, JOptionPane.YES_NO_OPTION); 
     if (reply == JOptionPane.YES_OPTION) { 
      System.exit(0); 

     } 
    } else if (input == "CD") { 
     lengthm = JOptionPane.showInputDialog(null, "Length until maturity:"); 

     // display the JOptionPane showConfirmDialog 
     int reply = JOptionPane.showConfirmDialog(null, message, title,  JOptionPane.YES_NO_OPTION); 
     if (reply == JOptionPane.YES_OPTION) { 
      System.exit(0); 
      return input; 
     } 
    } 
} 

public static void main(String[] args) { 
    createAndShowGUI(); 
} 
} 

Acccount.java

import java.awt.*; 
import java.awt.event.*; 
import javax.swing.*; 

public class Account implements ActionListener { 

JButton calculate; 
private int period; 
private int balance; 
private int fbalance; 
private int rate; 
private int monthlyFee; 
private String printstring; 

@Override 
public String toString() { 
    return String.format("Period: " + period + ", Balance: " + balance); 
} 

public int getPeriod() { 
    return period; 
} 

public void setPeriod(int period) { 
    this.period = period; 
} 

public int getBalance() { 
    return balance; 
} 

public void setBalance(int balance) { 
    this.balance = balance; 
} 

public int getRate() { 
    return rate; 
} 

public void setRate(int rate) { 
    this.rate = rate; 
} 

public int getFbalance() { 
    return fbalance; 
} 

public void setFbalance(int fbalance) { 
    this.fbalance = fbalance; 
} 

public String getPrintstring() { 
    return printstring; 
} 

public void setPrintString(String printstring) { 
    this.printstring = printstring; 
} 

public void calculate() { 
    for (int i = 0; i < period; i++) { 
     fbalance = balance + balance * rate - monthlyFee; 
    } 

} 

public void actionPerformed(ActionEvent e) { 
    calculate(); 
} 
} 

Answer 1

首先的createAccount返回類型爲Account，你正在返回從它String。它會在那裏失敗。

因此，請將您的退貨類型更改爲String。並確保您的方法始終返回value。您應該從您的代碼可能遵循的每個路徑中返回一個值。或者，您可以在方法的末尾添加return null;（但您應該考慮前面的說明）。

但是，很難理解爲什麼你要返回string，從createAccount方法。事實上，你根本沒有在該方法中創建任何帳戶。請重新命名您的方法以反映其確切目的。

其次，您使用的==運營商，這將使你的問題，一旦你走出編譯器錯誤的比較，你strings。您應該使用equals方法來比較字符串： -

if (input == "Checking") 

應該是： -

if (input.equals("Checking")) 

Answer 2

還必須添加return條款時，裏面的if-else的一個分支沒有創建帳戶。

Answer 3

那麼首先，按照Rohit的建議。

另一個主要問題是並不是所有的代碼路徑都返回一個值。 （如果輸入「檢查」，會發生什麼？）

二，也就是返回值被放置在系統退出後的路徑：

public static Account createAccount(String type, String checkno, String lengthm, String input) { 
    String message = "Would you like to open another account?"; 
    String title = "Are you sure?"; 
    if (input == "Checking") { 
     checkno = JOptionPane.showInputDialog(null, "First check number:"); 

     // display the JOptionPane showConfirmDialog 
     int reply = JOptionPane.showConfirmDialog(null, message, title, JOptionPane.YES_NO_OPTION); 
     if (reply == JOptionPane.YES_OPTION) { 
      System.exit(0); 

     } 
     // Return if input == checking not here 
    } else if (input == "CD") { 
     lengthm = JOptionPane.showInputDialog(null, "Length until maturity:"); 

     // display the JOptionPane showConfirmDialog 
     int reply = JOptionPane.showConfirmDialog(null, message, title,  JOptionPane.YES_NO_OPTION); 
     if (reply == JOptionPane.YES_OPTION) { 
      System.exit(0); 
      return input; // never gets here 
     } 
    } 
} 

是否使用一個IDE？或只使用記事本？你關注編譯器警告嗎？始終照顧警告，他們可能是潛在的運行時錯誤。