为什么在类中定义数组大小时会出现ArrayIndexOutOfBounds异常？

Question

For my project, a user must define the size of an array, and I created a set method to do this. The variable being changed is the class variable, arraySize . But, when someone sets the array size, and I try adding an array, I receive an indexoutofbounds error.

Here's the snippet of code

    //Size of the array
private int arraySize;
//initializes elemnt count of array
private int arrayElementCount;

//Gets count of SID in array
public int getSIDArrayCount() {

    //returns private variable
    return arrayElementCount;
}

//Sets SID Array
public void setArraySize(int setArraySize) {
    //Array size int
    //checks array if array size is greater than 0
    if (setArraySize > 0) {
        //sets array size
           //SIDArray
         arraySize = setArraySize;
    }

}

//SIDArray
private String[] SIDArray = new String[arraySize];

EDIT: I can't initialize the array in the 'setArraySize' method because I need an accessor method for the array.

here's the rest of the class:

/*

* To change this license header, choose License Headers in Project Properties. * To change this template file, choose Tools | Templates * and open the template in the editor. */ package com.ahellhound.pkg202sidproject;

import java.util.Arrays; import java.util.regex.Pattern;

/** * * @author Colby Leclerc */ public class SIDArrayTest {

//Size of the array
private int arraySize;
//initializes elemnt count of array
private int arrayElementCount;

//Gets count of SID in array
public int getSIDArrayCount() {

    //returns private variable
    return arrayElementCount;
}

//Sets SID Array
public void setArraySize(int setArraySize) {
    //Array size int
    //checks array if array size is greater than 0
    if (setArraySize > 0) {
        //sets array size
           //SIDArray
         arraySize = setArraySize;
    }

}

//SIDArray
private String[] SIDArray = new String[arraySize];
//Gets SID Array
public String[] getSIDArray() {

    //returns array
    return SIDArray;
}

//Validates SID; returns true if SID entry is valid
public boolean validateSID(String SIDEntry) {
    //checks if sid entry is 7, and if starts with s
    if (SIDEntry.length() != 7) {
        //retuns false
        return false;

    }

    //checks if SIDEntry begins with 'S'
    if (!SIDEntry.matches("[sS]\\d{6}")) {

        //returns false
        return false;
    }
    //returns true
    return true;
}

//Adds SID to the Array
public void addSID(String SIDEntry) {

    //checks if SID is valid
    if (validateSID(SIDEntry)) {
        //Adds sid to array
        SIDArray[arrayElementCount] = SIDEntry;
        //increases array size by one
        arrayElementCount++;

    } else {

        System.out.println("test failed 2");
    }

}

//Gets SID array and returns all entrys to strings 
public String getSIDArrayString() {
    //Gets array and converts to string
    String SIDArrayString = Arrays.toString(SIDArray);
    //returns array string
    return SIDArrayString;

}

}

I am truly stumped at how to set the array size, when I've created a class variable for it, while having the array a 'get'-able method.

Answer 1

You are instantiating the array before calling your setArraySize-method, thus it gets size of 0.

//SIDArray
private String[] SIDArray = new String[arraySize];

As the SIDArray variable is in the scope of the class, it gets instantiated as soon as an object of the class is created. Because of this, it uses the arraySize varible, which is uninstantiated at this point(and defaults to 0 in case of an int).

To fix this, simply instantiate the array after calling setArraySize() .

Here is some code to clarify what I meant in the comments.

public class A {
    /* In your code, you instantiate the array at this point, in the scope of the class, while arraySize is still uninitialized */
    String[] array; /* uninitialized, defaults to null */
    int arraySize; /* uninitialized, defaults to 0 */

    /* Simple accessor method for the array */
    public String[] getArray() {
        return array;
    }

    public int getArraySize() {
        return arraySize;
    }


    public void setArraySize(int size) {
        this.arraySize = size;
    }

    public void createArray() {
        array = new String[arraySize];
    }
}

Answer 2

You have to initialize the array variable after you set the new size. Change your method to:

//Sets SID Array
public void setArraySize(int setArraySize) {
    //Array size int
    //checks array if array size is greater than 0
    if (setArraySize > 0) {
        //sets array size
        //SIDArray
        arraySize = setArraySize;
        SIDArray = new String[arraySize];
    }
}

Answer 3

The problem is that you are instantiating an array with a fixed size (0). That's not necessarily bad... the real problem is that in you setArraySize(int setArraySize) method, you update your arraySize variable, but the array is never aware of this call!!! You should use something like

SIDArray = new String[arraySize];

But even if you do this, resizing your array will destroy all its previous contents... just be aware of this.