Question

我想为我的项目寻求帮助。我是新手程序员，我没有收到此错误 "exception in thread main java.lang.nullpointerexception"

这是我的代码：

 public class Slumbook{

    public String codeName;

    public Slumbook(){
        //
    }

    public Slumbook(String codeName){
        this.codeName = codeName;
    }

    public String getCodeName(){return codeName;}

    public void setCodeName(String codeName){this.codeName = codeName;}
   }

Driver Program 代码为： -

import java.util.*;

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

        Scanner sc = new Scanner(System.in);
        Slumbook[] slum = new Slumbook[20];

        for(int n=0; n<10; n++){
            slum[n].setCodeName(sc.nextLine());
        }
    }
}

Answer 1

您已分配数组...但您未能分配数组中包含的任何对象。

建议：

import java.util.*;

public class SlumbookD { 

    public static void main(String[] args){
      Scanner sc = new Scanner(System.in);
      Slumbook[] slum = new Slumbook[20];
      for(int n=0; n<slum.length; n++){
          slum[n] = new Slumbook(sc.nextLine);
      }
    }

}

......或更好......

// You only need one class - with it's own main().  Not two classes...
public class Slumbook{

  public String codeName;

  public Slumbook(String codeName){
    this.codeName = codeName;
  }

  public String getCodeName(){
    return codeName;
  }

  public void setCodeName(String codeName){
    this.codeName = codeName;
  }

  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    Slumbook[] slum = new Slumbook[20];
    for(int n=0; n<slum.length; n++){
        slum[n] = new Slumbook(sc.nextLine);
    }
  }

}

Answer 2

您已创建空白数组但未初始化它，您必须将数组索引处的对象引用与相关联，然后您可以在该索引上调用任意数量的setter方法。

 Slumbook[] slum = new Slumbook[20];

    for(int n=0; n<10; n++){
        slum[n] = new Slumbook(); //  Instantiate Slumbook class and assign reference
        slum[n].setCodeName(sc.nextLine());
    }

Answer 3

您需要在使用之前初始化Array元素。

Slumbook[] slum = new Slumbook[20];

for(int n=0; n<10; n++){
    slum[n] = new Slumbook();
    slum[n].setCodeName(sc.nextLine());
}

您可以一次初始化数组的所有元素，以避免将来出现任何异常

Slumbook[] slum = new Slumbook[20];

for(int i = 0 ; i < slum.length ; i++)  //initializing all the elements
    slum[i] = new Slumbook();

for(int n=0; n<10; n++){
    slum[n].setCodeName(sc.nextLine());
}

请帮我“线程主java.lang.nullpointerexception中的java异常”

3 个答案: