java - reference to println is ambiguous error -

i have code basic program when type system.out.println, thes error: "reference println ambiguous" , occured randomly use work , did not have erroe , cant figure error occurs, how fix ,than you.

    package blue.light;     //name is: slash     import java.util.*;      public class bluelight {          public static void main(string[] args) throws interruptedexception {     scanner scan = new scanner(system.in);      string name;     int age;     string a;     string yes;     string no;     double num1;     double num2;     double ans;          system.out.println("hello, name slash");     system.out.println("what name?");     name = scan.nextline();     system.out.println("hello " + name);     thread.sleep(1000);     system.out.println("how old you?");     age = scan.nextint();     if(age < 18)             {                 system.out.println("ur young af");             }     else if (age == 21)     {         system.out.println("shots, shots, shots");      }     else if (age == 69)     {         system.out.println("ohhh yea");     }     system.out.println("you calculate thing?");     = scan.nextline();     if(a==("yes")){         system.out.println("what u do?(multiply,or add)");              = scan.nextline();                     if(a == "add"){             system.out.println("enter number 1");             num1 = scan.nextdouble();             system.out.println("enter number 2");             num2 = scan.nextdouble();                     ans = num1+num2;          }       }  }     } 

first, nextint() doesn't consume new line after age (so nextline empty). compare object values call .equals() because == tests reference identity (and string i'd use string.equalsignorecase(string)). finally, i'd declare variables minimum visibility in java; close first use possible. like,

public static void main(string[] args) throws interruptedexception {     scanner scan = new scanner(system.in);      system.out.println("hello, name slash");     system.out.println("what name?");     string name = scan.nextline();     system.out.println("hello " + name);     thread.sleep(1000);     system.out.println("how old you?");     int age = scan.nextint();     if (age < 18) {         system.out.println("ur young af");     } else if (age == 21) {         system.out.println("shots, shots, shots");     } else if (age == 69) {         system.out.println("ohhh yea");     }     system.out.println("you calculate thing?");     scan.nextline(); // <-- nextint leaves trailing new line.     string = scan.nextline();     if (a.equalsignorecase("yes")) { // <-- compare string equality in                                         // java         system.out.println("what u do?(multiply,or add)");         = scan.nextline();         if (a.equalsignorecase("add")) {// <-- compare string equality                                         // in java             system.out.println("enter number 1");             double num1 = scan.nextdouble();             system.out.println("enter number 2");             double num2 = scan.nextdouble();             double ans = num1 + num2;             system.out.println(ans); // <-- display answer         }     } }