java - Incorrect GPA method computation -

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my method keeps returning 0. can me find why gpa not compute?

public static double getgpa(string a) {     double value = 0;     double sum = 0;     for(int = 0; i<a.length(); i++)     {         string grade = a.substring(i,i+1);         if(grade == "a") value = 4;         if(grade == "b") value = 3;         if(grade == "c") value = 2;         if(grade == "d") value = 1;         if(grade == "f") value = 0;         sum += value;     }     return sum/(a.length()+1); } 

you should using .equals(), not ==.

you've got couple of lesser mistakes:

1. you're dividing string length plus one.
2. you're not handling invalid inputs sensibly. error or ignore.

here's corrected copy:

public static double getgpa(string a) {     long total = 0;     long count = 0;     for(int = 0; i<a.length(); i++)     {         string grade = a.substring(i,i+1);         count++;         if(grade.equals("a")) total += 4;         else if(grade.equals("b")) total += 3;         else if(grade.equals("c")) total += 2;         else if(grade.equals("d")) total += 1;         else if(grade.equals("f")) total += 0;         else count--;  /* ignore invalid characters, e.g. whitespace. */     }     return total / (double) count; }