java - 使用扫描仪出现 NullPointerException

Question

我是编程新手，我开始用 Java 创建一个简单的计算器，但我不断收到错误

    for (int i = 0; i < user_input.length(); i++)

错误提示:

Exception in thread "main" java.lang.NullPointerException

如何解决这个问题？

import java.util.*;
import java.awt.*;
import javax.swing.*;
import java.util.Scanner;

public class stringCalculator
{
    public static ArrayList<String> input = new ArrayList<String>();
    public static ArrayList<String> calcOperators = new ArrayList<String>();
    public static ArrayList<Integer> calcOperands = new ArrayList<Integer>();
    public static String user_input;
    public static String first;
    public static int int1;
    public static String char1;
    public static int int2;
    public static String next;
}

public static void input()
{
    Scanner user_input = new Scanner(System.in);
    int1 = user_input.nextInt();
    char1 = user_input.next();
    int2 = user_input.nextInt();
    next = user_input.nextLine();
}

public void calcOperators()
{
    for (int i = 0; i < user_input.length(); i++)
    {
        if (char1 == "+")
        {
            calcOperators.add(char1);
        }
        else if (char1 == "-")
        {
            calcOperators.add(char1);
        }
        else if (char1 == "char1")
        {
            calcOperators.add(char1);
        }
        else if (char1 == "/")
        {
            calcOperators.add(char1);
        }
    }


public void calcOperands()
{
    for (int i = 0; i < user_input.length(); i++)
    {
        if (int1 == 1 || int2 == 1)
        {
            calcOperands.add(1);
        }
        else if (int1 == 2 || int2 == 2)
        {
            calcOperands.add(2);
        }
        else if (int1 == 3 || int2 == 3)
        {
            calcOperands.add(3);
        }
        else if (int1 == 4 || int2 == 4)
        {
            calcOperands.add(4);
        }
        else if (int1 == 5 || int2 == 5)
        {
            calcOperands.add(5);
        }
        else if (int1 == 6 || int2 == 6)
        {
            calcOperands.add(6);
        }
        else if (int1 == 7 || int2 == 7)
        {
            calcOperands.add(7);
        }
        else if (int1 == 8 || int2 == 8)
        {
            calcOperands.add(8);
        }
        else if (int1 == 9 || int2 == 9)
        {
            calcOperands.add(9);
        }
        else if (int1 == 0 || int2 == 0)
        {
            calcOperands.add(0);
        }
    }
}

}

public class Main
{
    public static void main(String[] args)
    {
        stringCalculator c = new stringCalculator();
        c.input();
        c.calcOperators();
        c.calcOperands();
    }
}

Answer 1

我有点困惑为什么你有

public static String user_input

然后

Scanner user_input = new Scanner(System.in);
int1 = user_input.nextInt();
char1 = user_input.next();
int2 = user_input.nextInt();
next = user_input.nextLine();

最后，您在循环中对字符串和整数使用相同的变量名称:

for (int i = 0; i < user_input.length(); i++)
for (int i = 0; i < user_input.length(); i++)

我强烈建议重构此代码。

        .....
    private ArrayList<String> input = new ArrayList<String>();
    private ArrayList<String> calcOperators = new ArrayList<String>();
    private ArrayList<Integer> calcOperands = new ArrayList<Integer>();
    private String user_input;
    private String first;
    private String next;
    private String char1;
    private int integer2;
    private int integer1;
}

public static void input()
{
    Scanner input = new Scanner(System.in);
    int int1 = input.nextInt();
    int int2 = input.nextInt();
    string str = input.next();
    string nxt = input.next();

    setInteger1(int1);
    setInteger2(int2);
    setStringFirst(str);
    setStringNext(...);
    ....
    // And so on
}

private void setInteger1(int int1) {
    this.integer1 = int1;
}

private Integer getInteger1() {
    return this.integer1;
}

private void setInteger2(int int2) {
    this.integer2 = int2;
}

private Integer getInteger2() {
    return this.integer2;
}

private void setStringFirst(String fst) {
    this.first = fst;
}

private String getStringFirst() {
    return this.first;
}

// And so on. Create all get and set methods for each global variable and for future
// reference do not use variable names that are the same as method, names
// and try to use more meaningful variable names. In fact, if you look at
// Java naming conventions it would do you good.

您能否告诉我们这些循环的用途？因为我实在不明白这是什么？您想迭代“事物”数组并将每个操作与给定的数组元素匹配吗？或者您只想用一个元素来匹配其中一个操作？

旁注:类名应大写:

public class StringCalculator

================================================== =====================================

好的，我制作了一个简单的程序，允许您将字符串添加到数组中，然后显示它。它基于您正在做的事情，尽管您会发现它的结构不同。这应该会给您一个良好的开端，并允许您进一步实现并完成您正在做的任何事情。

    public class Thing {

        private String  operator;

        private void getUserInput() {
            Scanner input = new Scanner(System.in);
            if(input.hasNextInt()) {
                System.out.println("I have entered an integer: " + input.nextInt());
            }
            else {
                setOperator(input.nextLine());
                addOperators();
                System.out.println("I have entered a string: " + getOperator());
            }
            displayThing();
        }

        private ArrayList<String> addOperators() {
            ArrayList<String> operatorsList = new ArrayList<String>();

            if(getOperator().equals("+")) {
                operatorsList.add(operator);
            }
            if(getOperator().equals("-")) {
                operatorsList.add(operator);
            }
            else {
                operatorsList.add(getOperator());
            }
            return operatorsList;
        }

        private void displayThing() {
            System.out.println(addOperators());
        }

        public static void main(String[] args) {
            Thing program = new Thing();
            program.getUserInput();
        }

        // Setters and getters
        private void setOperator(String operator) {
            this.operator = operator;
        }

        private String getOperator() {
            return this.operator;
        }

    }