用变量声明数组大小会导致超出范围的异常 [英] Declaring array size with variable leads to out of bounds exceptiong

问题描述

我正在创建一个int数组secretNumber.当我将数组大小声明为数字时，没有超出范围的异常，但是当我使用变量(numDigits)声明数组大小时，我在索引0处针对'secretNumber [i] = val这行得到了超出范围的异常. '.这是课程:

import java.util.Random;
import java.util.Scanner;

public class Engine {
    public int numDigits;
    public int[] secretNumber = new int[numDigits]; //this is the array
    public Random randomNumberGenerator;

    public void setNumDigits() {
        Scanner setNumDigits = new Scanner(System.in);
        System.out.println("Enter the number of digits to use");
        String numDigits = setNumDigits.nextLine();
        this.numDigits = Integer.parseInt(numDigits);
    }

    public int getNumDigits() {
        return this.numDigits;
    }

    public void generateNewSecret() {
        int val;
        for (int i = 0; i < numDigits - 1; i++) {
            Random rand = new Random();
            val = rand.nextInt(9);
            secretNumber[i] = val; //out of bounds exception is here
        }
    }

    public void setSecretNumber(int[] secretNumberCopy) {
        secretNumberCopy.equals(this.secretNumber);
    }

    public int[] getSecretNumber() {
        return secretNumber;
}



}

这是执行方法的主要方法，在设置数组之前，我运行numDigits setter:

import java.util.Scanner;

public class Bagels {
    public static void main(String[] args) {

        Player playerOne = new Player();

        playerOne.setName();

        System.out.println(playerOne.getName());

        Engine engine = new Engine();

        engine.setNumDigits();
        engine.setSecretNumber(engine.secretNumber);
        engine.generateNewSecret();

        System.out.println(engine.getSecretNumber());


    }
}

如果我设置了numDigits，为什么索引0会超出范围?

解决方案

虽然我们已经对出现异常的原因做了一些澄清，但仍然存在一个问题，即如何解决它以及将来如何避免此类情况.

让我们逐步介绍您的代码:

Engine engine = new Engine();

创建新的engine对象，将所有类变量实例化为其默认值.原始int的默认值为0;

在初始化时，您已经:

public int numDigits; // 0
public int[] secretNumber = new int[numDigits]; // arrays object of size 0
public Random randomNumberGenerator; // null

如何进行?

问题部分出在对象设计中-您需要确定约束状态变量的不变量.您需要在numDigits初始化期间设置数组的大小:

public int[] secretNumber; // null at the moment of object initialization

public void setNumDigits() {
    Scanner setNumDigits = new Scanner(System.in);
    System.out.println("Enter the number of digits to use");
    numDigits = Integer.parseInt(setNumDigits.nextLine());
    secretNumber = new int[numDigits];
}

I'm creating an int array secretNumber. When I declare the array size as a number, there's no out of bounds exception, but when I declare the array size with a variable (numDigits) I get the out of bounds exception at Index 0 for the line ' secretNumber[i] = val '. This is the class:

import java.util.Random;
import java.util.Scanner;

public class Engine {
    public int numDigits;
    public int[] secretNumber = new int[numDigits]; //this is the array
    public Random randomNumberGenerator;

    public void setNumDigits() {
        Scanner setNumDigits = new Scanner(System.in);
        System.out.println("Enter the number of digits to use");
        String numDigits = setNumDigits.nextLine();
        this.numDigits = Integer.parseInt(numDigits);
    }

    public int getNumDigits() {
        return this.numDigits;
    }

    public void generateNewSecret() {
        int val;
        for (int i = 0; i < numDigits - 1; i++) {
            Random rand = new Random();
            val = rand.nextInt(9);
            secretNumber[i] = val; //out of bounds exception is here
        }
    }

    public void setSecretNumber(int[] secretNumberCopy) {
        secretNumberCopy.equals(this.secretNumber);
    }

    public int[] getSecretNumber() {
        return secretNumber;
}



}

This is the main that executes the methods, and I run the numDigits setter before setting the array:

import java.util.Scanner;

public class Bagels {
    public static void main(String[] args) {

        Player playerOne = new Player();

        playerOne.setName();

        System.out.println(playerOne.getName());

        Engine engine = new Engine();

        engine.setNumDigits();
        engine.setSecretNumber(engine.secretNumber);
        engine.generateNewSecret();

        System.out.println(engine.getSecretNumber());


    }
}

Why would Index 0 be out of bounds if I've set numDigits??

解决方案

While we have already some clarification to the reason you got the exception there is still a question how to fix it and how to avoid such cases in future.

Let's go through your code step-by-step:

Engine engine = new Engine();

New engine object is created an all class variables instantiated to their default values. Default value for primitive int is 0;

At the moment of initialization you have:

public int numDigits; // 0
public int[] secretNumber = new int[numDigits]; // arrays object of size 0
public Random randomNumberGenerator; // null

How to proceed with that?

The issue is partially in object design - you need to identify the invariants that constrain the state variables. You need to set the size of the array during numDigits initialization:

public int[] secretNumber; // null at the moment of object initialization

public void setNumDigits() {
    Scanner setNumDigits = new Scanner(System.in);
    System.out.println("Enter the number of digits to use");
    numDigits = Integer.parseInt(setNumDigits.nextLine());
    secretNumber = new int[numDigits];
}

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