java.Lang.NullPointerException

Question

Hi every one!

What is " java.lang.NullPointerException " and how do I fix it?

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

String test = "2x + 3 = 8";
Solve obj = new Solve(test);

obj.del_space();
System.out.println(obj.find_a());
}
}
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here is the solve class but it's not complete yet:
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class Solve {
private String equation;
private String nsstr;//no space string
private char[] arr;
private char[] nsarr;//no space array
private int a;
private int b;
private int c;

Solve(String str) {
equation = str;
arr = equation.toCharArray();
}

public String del_space() {
this.nsstr = equation.replace(" ","");
return nsstr;
}

public Double find_a() {
int IX = nsstr.indexOf('x');
String astr = "";
for(int i = IX ; i >= 0 ; i--) {
if(IX==0||nsarr[IX-1]=='+'||nsarr[IX-1]=='=') {
return 1.0;
}
else if(nsarr[IX-1]=='-') {
return -1.0;
}
else {
if(nsarr[i]=='=') {
AddFuncs obj = new AddFuncs();
astr = obj.rev(astr);
return Double.parseDouble(astr);
}
if(i == IX) {
continue;
}
astr += nsarr[i];
if(nsarr[i]=='+' || nsarr[i]=='-') {
AddFuncs obj = new AddFuncs();
astr = obj.rev(astr);
return Double.parseDouble(astr);
}
}
}
return -1.0;
}

Answer 1

you forgot to assign value for nsarr

Answer 2

Thanks!