递归和迭代二进制搜索中的空指针异常

Question

当我搜索元素时，错误出现在空指针处。它成功接受输入并显示数组，但是当我输入搜索元素并按Enter键时，它会显示错误。

它显示空指针异常，带有以下堆栈跟踪。

搜索元素时的错误如下。

Exception in thread "main" java.lang.NullPointerException
at BinarySearchClass.binarySearch(BinarySearchClass.java:26)
at BinarySearchIterative.main(BinarySearchIterative.java:25)

即;它在这一行中说

BinarySearchClass.java:26

while ((array[indexToLook] != search) && (maxIndex > minIndex)) {

和另一个文件中的这一行

BinarySearchIterative.java:25

int found = BSI.binarySearch(search);

请帮我纠正。

import java.util.Arrays;
import java.util.Scanner;

public class BinarySearchClass {

    private  Integer[] array;

    public  BinarySearchClass(int size){
        this.array = new Integer[size];
    }

    public BinarySearchClass(){
        this(100); //default size of array.
    }

    public  int binarySearch(int search){

        int maxIndex = size()-1;
        int minIndex = 0;

        int indexToLook = (int) Math.floor((minIndex + maxIndex)/2);
        while ((array[indexToLook] != search) && (maxIndex > minIndex)) {
            if(array[indexToLook] > search) {
                maxIndex = indexToLook - 1;
            }
            else{
                minIndex = indexToLook + 1; 
            }
            indexToLook = (int) Math.floor((minIndex+maxIndex/2));
        }
        if(array[indexToLook] == search)
            return indexToLook;

        return -1;
    }

    public  int binarySearch( int search, int minIndex, int maxIndex){
        if (minIndex == maxIndex){
            if(array[minIndex] == search) 
                return minIndex;
            return -1;
        }
        int indexToLook = (int) Math.floor((minIndex + maxIndex)/2);
        if(array[indexToLook] == search){
            return indexToLook;
        }
        if(array[indexToLook] < search)
            return binarySearch( search, indexToLook+1, maxIndex );
        return binarySearch( search, minIndex, indexToLook-1);
    }

    public String toString() {
        return Arrays.deepToString(this.array);
    }

    private  int size() {
        int i = 0;
        while (((array[i] != null) && i < array.length) ) {
            i++;
        }
        return i;
    }
}

class BinarySearchIterative：

import java.util.Scanner;

public class BinarySearchIterative   {

    public static void main(String[] args){
        BinarySearchClass BSI = new BinarySearchClass(10);
        Scanner scan = new Scanner(System.in);
        System.out.println("Iterative");

        System.out.println("Enter the number of elements: ");
        int size = scan.nextInt();
        int[] array = new int[size];
        int i;
        for(i =0; i<=size-1; i++){
            System.out.println("Enter the element " + i + " :");
            array[i] = scan.nextInt();
        }
        System.out.println("The elements are");
        for(i=0; i<=size-1; i++){
            System.out.print("[" + array[i] + "]");
        }
        int search = 0;
        System.out.println("\nEnter the element to search");
        search = scan.nextInt();
        int found = BSI.binarySearch(search);
        if (found > -1)
        {
            System.out.println("The element " + search +  " is found in location " + (found+1) );
        }
        else
        {
            System.out.println("Element " + search + " not found");
        }
    }
}

编辑（@Paul）： 由于您要在BinarySearchIterative中提供要执行二进制搜索的数组，我建议使用此方法而不是您的方法：

import java.util.Arrays;
import java.util.Scanner;

public class BinarySearchClass {
    public static int binarySearch(int search , int[] array){

        int maxIndex = size()-1;
        int minIndex = 0;

        int indexToLook = (int) Math.floor((minIndex + maxIndex)/2);
        while ((array[indexToLook] != search) && (maxIndex > minIndex)) {
            if(array[indexToLook] > search) {
                maxIndex = indexToLook - 1;
            }
            else{
                minIndex = indexToLook + 1; 
            }
            indexToLook = (int) Math.floor((minIndex+maxIndex/2));
        }
        if(array[indexToLook] == search)
            return indexToLook;

        return -1;
    }

    public static int binarySearch( int search, int minIndex, int maxIndex , int[] arr){
        if (minIndex == maxIndex){
            if(array[minIndex] == search) 
                return minIndex;
            return -1;
        }
        int indexToLook = (int) Math.floor((minIndex + maxIndex)/2);
        if(array[indexToLook] == search){
            return indexToLook;
        }
        if(array[indexToLook] < search)
            return binarySearch( search, indexToLook+1, maxIndex );
        return binarySearch( search, minIndex, indexToLook-1);
    }
}

class BinarySearchIterative:

import java.util.Scanner;

public class BinarySearchIterative   {

    public static void main(String[] args){
        ...
        int found = BinarySearchClass.binarySearch(search ,  array);
        ...
    }
}

请注意，输入数组必须排序才能执行二进制搜索，而代码无法确保这一点。您的代码存在更一般的设计问题。 BinarySearchClass不应存储任何数组本身，只提供实用程序方法来搜索数组（使用static方法，如我的例子）。这将是编程的一般方法：模块化。每个类应仅为特定目的提供功能，或者应仅保存表示单个数据集的数据，并提供处理此数据的功能。看看一般的编码风格指南（关于这个主题有大量的书籍，网络材料等）。