如何使用Scanner / System.in中尚未設置的字符串的值-Java

Question

我正在嘗試創建一個計算器。 這可能不是最有效的方法，但是我想對到目前為止的工作有所幫助。

    package me.Nelsin.Calculator;


    import java.text.DecimalFormat;
    import java.util.Scanner;


public class Calculator {


public static void main(String args[]) {

    double answer;
    //2 numbers
    System.out.println("Enter your first number: ");
    Scanner fnumb = new Scanner(System.in);
    double fnum = fnumb.nextDouble();

    System.out.println("Enter your operation, *, /, +, -: ");
    Scanner operation = new Scanner(System.in);
    String op = operation.nextLine();

    System.out.println("Enter your second number: ");
    Scanner snumb = new Scanner(System.in);
    double snum = snumb.nextDouble();

    //Answers
    if(op.equals("*")) {
        answer = fnum * snum;
        DecimalFormat df = new DecimalFormat("#.##");
        System.out.println(df.format(answer));
    }   
    if (op.equals("/")) {
        answer = fnum / snum;
        DecimalFormat df = new DecimalFormat("#.##");
        System.out.println(df.format(answer));
    }
    if (op.equals("+")) {
        answer = fnum + snum;
        DecimalFormat df = new DecimalFormat("#.##");
        System.out.println(df.format(answer));
    } 
    if (op.equals("-")) {
        answer = fnum - snum;
        DecimalFormat df = new DecimalFormat("#.##");
        System.out.println(df.format(answer));  
        }
    System.out.println("Enter your operation, *, /, +, -: ");
    Scanner operation2 = new Scanner(System.in);
    String op2 = operation2.nextLine();

    System.out.println("Enter your third number here: ");
    Scanner tnumb = new Scanner(System.in);
    double tnum = tnumb.nextDouble();

    if (op2.equals("*")) {
    double answer2 = answer * tnum;
    DecimalFormat df = new DecimalFormat("#.##");
    System.out.println(answer2);
         }
    }
}

所以我的錯誤在這里：

    double answer2 = answer * tnum;

我相信這是因為直到用戶運行該程序才設置答案。 錯誤：

The local variable may have not been initialised.

Answer 1

發生此錯誤的原因是因為您在if語句中設置了變量“ answer”。 修復錯誤，在條件如if后面添加else語句

...

else{
answer=0.0;
}

或更簡單的解決方案是說：

double answer=0;

Answer 2

因為尚未初始化局部變量且局部變量沒有默認值，您會得到錯誤。 聲明變量時將其賦值。

double answer = 0.0;

Answer 3

Instance variables被初始化為null或為其默認的原始值（如果它們是原始值）。

Local variables未定義且未初始化，因此您有責任設置初始值。

所以改變這個double answer; double answer = 0;