Java中Array的添加和删除项

Question

我一直想学习如何在数组中添加或删除元素。我知道在这些类型的东西上使用 Arraylist 包更容易，但我想学习如何在不使用它的情况下添加或删除元素。我曾尝试为此创建代码，但最终出现了一些错误。

我的代码的输出应该是这样的：

********************^

1 - Search Name
2 - Add Name
3 - Delete Name
Enter mode:2
Enter new name:professor x
Do you want to add a name again?(y/n):y
Enter new name:professor y
Do you want to add a name again?(y/n):n

********************^

1 - Search Name
2 - Add Name
3 - Delete Name
Enter mode: 1
Enter name:professor x
Match found!

但在搜索以前输入的记录时，我的代码给了我一个null pointer exception：

Exception in thread "main" java.lang.NullPointerException
at Practice.Practice1Main.sequentialSearch(Practice1Main.java:95)
at Practice.Practice1Main.mode(Practice1Main.java:18)
at Practice.Practice1Main.getMode(Practice1Main.java:79)
at Practice.Practice1Main.caseTwo(Practice1Main.java:63)
at Practice.Practice1Main.mode(Practice1Main.java:33)
at Practice.Practice1Main.getMode(Practice1Main.java:79)
at Practice.Practice1Main.main(Practice1Main.java:6)

我的代码：

package Practice;
import java.util.Scanner;
public class Practice1Main {
    public static void main(String[]args){
        getMode();
    }
    public static void mode(int mode){
        Scanner hold = new Scanner(System.in);
        int numInput = 2;
        String[] names = new String[numInput];
        String response = "";
        if(mode == 1){
            do{
                int result = sequentialSearch(names, getNames());
                if(result == -1){
                    System.out.print("No match found!\n");
                }else{
                    System.out.print("Match found!\n");
                }
                System.out.print("Do you want to search again?(y/n):");
                response = hold.nextLine();
                if(response.equalsIgnoreCase("n")){
                    getMode();
                }
            }while(response.equalsIgnoreCase("y"));
        }else if(mode == 2){
            caseTwo(response, names, numInput);
        }else if(mode == 3){
            // TODO: Part 3
        }else{
            System.out.print("ERROR 350\n");
        }
    }

    public static void caseTwo(String response, String[] names, int numInput){
        Scanner hold = new Scanner(System.in);
        do{
            int x = 0;
            System.out.print("Enter new name:");
            names[x] = hold.nextLine();
            System.out.print("Do you want to add a name again?(y/n):");
            response = hold.nextLine();
            if(response.equalsIgnoreCase("y")){
                x++;
                if(x == 2){
                    int add = numInput + 1;
                    names = new String[add];
                    System.out.print("Enter new name:");
                    names[add] = hold.nextLine();
                    System.out.print("Do you want to add a name again?(y/n):");
                    response = hold.nextLine();
                    if(response.equalsIgnoreCase("y")){
                        add++;
                    }
                }
            }else if(response.equalsIgnoreCase("n")){
                getMode();
            }else{
                System.out.print("CASE 2 ERROR!\n");
            }
        }while(response.equalsIgnoreCase("y"));
    }

    public static void getMode(){
        Scanner hold = new Scanner(System.in);
        int mode;
        System.out.print("********************\n");
        System.out.print("1 - Search Name\n");
        System.out.print("2 - Add Name\n");
        System.out.print("3 - Delete Name\n");
        System.out.print("Enter mode:");
        mode = hold.nextInt();
        mode(mode);
    }
    public static String getNames(){
        Scanner hold = new Scanner(System.in);
        String name;
        System.out.print("Enter name:");
        name = hold.nextLine();
        return name;
    }
    public static int sequentialSearch(String[]names,String name){
        int index = 0;
        int element = -1;
        boolean found = false;
        while(!found && index < names.length){
            if(names[index].equalsIgnoreCase(name)){
                found = true;
                element = index;
            }
            index++;
        }
        return element;
    }
}

Answer 1

任何时候输入mode() 方法，都会重新初始化names 数组。这就是为什么它只包含 null 值，因此是 NullPointerException。你应该让它成为类的一个字段。

此外，按照设计，它的长度始终为 2。

Answer 2

ArrayList 在功能上正是您正在寻找的，而我即将为您提供的替代方案将完全按照 ArrayList 所做的工作，但效率或吸引力要低得多。作为免责声明，我会说你应该学习使用 ArrayList，但无论如何我都会按照你的方式回答你的问题，同时你也会了解 ArrayList 的工作原理。

要调整数组大小，无论如何都必须创建一个新数组。最有效的方法可能是使用临时数组来存储您的数据，然后像这样重新实例化您的主数组变量

// Inside your adjustable array class. Let's call it "AdvArray"

// A field that's instantiated inside the constructor
int[] exampleArray = {1, 2, 3, 4, 5}; // create and fill an int array

public void addElement(int element){
    //Make temp array the size of your data and fill it
    int[] tempArray = new int[exampleArray.length];
    for(int i = 0; i<exampleArray.length; i++){
        tempArray[i] = exampleArray[i];
    }

    //reinstantiate exampleArray and fill with old data
    exampleArray = new int[exampleArray.length+1];
    for(int i = 0; i<tempArray.length; i++){
        exampleArray[i] = tempArray[i];
    }

    //Add new element to larger array
    exampleArray[exampleArray.length-1] = element;
}// ===================================================

// Example usage
exampleAdvArray.addElement(500);

此方法在末尾添加一个，但您可以通过创建一个带有要添加到的索引的溢出来遵循它的想法，然后将两个过程反转为一个 remove 方法。然后，您需要做的就是为任何给定索引处的元素添加一个 getter，或者甚至可能是一个返回“exampleArray”字段的 getArray 方法。

或者，您的代码如下所示：

ArrayList<int> exampleList = new ArrayList<int>();
for(int i=1; i<=5; i++){
    exampleList.add(i);
}
exampleList.add(500);