变量在构造函数中定义但抛出 NullPointerexception

Question

我是 Java 的新手，我正在练习 Strings 但是当我运行这会抛出 NullPointeException

我已经定义了一个构造函数，其值为 String s1,stopCodon,startCodon。
它采用 s1 的值但不采用 stopCodon,startCodon
当我将值放入实例变量时它工作正常请稍微解释一下，以便对我有所帮助...

/* package codechef; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Codechef
{   
        String s1;
        String startCodon;
        String stopCodon;
    public static void main (String[] args) throws java.lang.Exception
    {
        // your code goes here
        //String s1;
        //String startCodon;
        //String stopCodon;
        Codechef test1 = new Codechef();
        test1.testSimpleGene();
    }
     public void testSimpleGene(){
           System.out.println("Gene Strand is = " + s1); 
           System.out.println("Gene1 is = " + findSimpleGene(s1));
           System.out.println("Gene2 is = " + findSimpleGene(s1,startCodon,stopCodon));
     }
     private String findSimpleGene(String dna,String x,String y){
           String dnaResult  = "";
           int startIndex = dna.toUpperCase().indexOf(x);
           if (startIndex == -1){
               return "";            
            }
           int stopIndex = dna.toUpperCase().indexOf(y,startIndex+3);
           if (stopIndex == -1){
               return "";            
            } 
           //System.out.println(startIndex +" Part2 "+ (stopIndex));   
            if((stopIndex - startIndex)%3 == 0){
            dnaResult = dna.substring(startIndex,stopIndex+3);
           }
           return dnaResult;
        }
        public String findSimpleGene(String dna){
           String dnaResult  = "";
           int startIndex = dna.indexOf(startCodon);
           if (startIndex == -1){
               return "";            
            }
            int stopIndex = dna.indexOf(stopCodon,startIndex+3);
           if (stopIndex == -1){
               return "";            
            } 
          //System.out.println(startIndex +" "+ (stopIndex));   
            if((stopIndex - startIndex)%3 == 0){
            dnaResult = dna.substring(startIndex,stopIndex+3);
        }
           return dnaResult;
        }
        Codechef(){
        String s1 = "taaatg";
        String startCodon = "TAA";
        String stopCodon  = "ATG";
        }
    }

错误

    at Codechef.findSimpleGene(Main.java:45)
    at Codechef.testSimpleGene(Main.java:24)
    at Codechef.main(Main.java:20)```

Answer 1

将构造函数更改为如下所示。

public Codechef(){
    s1 = "taaatg";
    startCodon = "TAA";
    stopCodon  = "ATG";
}

通过在每行的开头使用单词 String，您实际上是在构造函数中声明了局部变量。您不希望这样，因为您希望使用对象中的字段，而不是其他变量。

Answer 2

如评论中所述，您正在构造函数中定义新的局部变量，这些变量恰好与 class 字段变量同名。

在您的构造函数中，您应该分配 class 字段的值（s1、startCodon、stopCodon）：

Codechef(){
     s1 = "taaatg";
     startCodon = "TAA";
     stopCodon  = "ATG";
}

但是，如果存在与字段变量（对象的属性）同名的局部变量，则可以引用该字段变量关键字 this:

Codechef(){
     String s1 = "local variable";

     this.s1 = "taaatg";
     startCodon = "TAA";
     stopCodon  = "ATG";
}

变量在构造函数中定义但抛出 NullPointerexception

Variable is defined in constructor but throwing NullPointerexception

java

string

constructor

nullpointerexception