Reorder the String

The following code does works but producing as
Bob Mary Geoff
82 65 23

What can I change to reorder the String to make it looks like
Bob 82 \n
Mary 65 \n
Geoff 23

Furthermore, I want to set a range for each grade, for example, grade over 85 is"HD", 85-75 is "D", 74 -65 is "C", 64 -55 is "P", 54- 0 is "F".
So instead of giving Bob, Mary and Geoff a actual grade, it gives them an interval. For example: Bob D, Mary C, Geoff F.

public class Student {
String[] names;
int[] grades;
public Student(int numStudents) {
this.names = new String[numStudents];
this.grades = new int[numStudents];
}
public void addStudent(String name, int grade) {
int i = 0;
while (i < this.names.length && names[i] != null) {
i++;
}
if (i < names.length) {
names[i] = name;
grades[i] = grade;
}
}
public String toString() {
String result = "";
for (int i=0; i < this.names.length; i ++) {
result += this.names[i] + " ";
}
result += "\n";
for (int i = 0; i< this.grades.length; i++) {
result += this.grades[i] + " ";
}
return result;
}
public static void main(String[] args) {
Student classRoom  = new Student(3);
classRoom.addStudent("Bob", 82);
classRoom.addStudent("Mary", 65);
classRoom.addStudent("Geoff", 23);
System.out.println(classRoom);
}

You loop through all the names, add those to the string and then do the same with the grades.

If you loop through both of them, you can add them to the string one after another:

	public String toString() {
String result = "";
for (int i=0; i < this.names.length; i ++) {
result += this.names[i] + " "+this.grades[i]+"\n";
}
return result;
}

Also, do not use string concadation with loops. Use StringBuilder instead:

	public String toString() {
StringBuilder result = new StringBuilder();
for (int i=0; i < this.names.length; i ++) {
result.append(this.names[i]).append(" ").append(this.grades[i]).append("\n");
}
return result;
}

This is a performance improvement as you do not need to create a new string in every iteration.

If you want to set the grades according to criterias, you can use if-else as demonstrated in e.g. the answer of nylanderDev (don't forget to also upvote it if you want to upvote my answer):

public String toString() {
StringBuilder result = new StringBuilder();
for (int i=0; i < this.names.length; i ++) {
String grade;
if (this.grades[i] > 85) {
grade= "HD";
} else if (this.grades[i] >= 75) {
grade= "D";
} else if (this.grades[i] >= 65) {
grade= "C";
} else if (this.grades[i] >= 55) {
grade= "P";
} else {
grade="F";
}
result.append(this.names[i]).append(" ").append(grade).append("\n");
}
return result.toString();
}

The second part of your question can be solved as follows:

Define a new method geLetterGrade:

String geLetterGrade(int grade) {
if (55 <= grade && grade <= 64)
return "P";
...
}

And then include it in the toString code:

            result += this.names[i] + " "+getLetterGrade(this.grades[i])+"\n";

Here is how you can produce a string from a grade by checking what range it's in.

String gradeToString(int grade) {
if (grade > 85) {
return "HD";
} else if (grade >= 75) {
return "D";
} else if (grade >= 65) {
return "C";
} else if (grade >= 55) {
return "P";
} else return "F";
}

Change toString implementation as following:

public String toString() {
String result = "";
for (int i = 0; i < names.length && i < grades.length; i++) {
result += names[i] + " " + grades[i] + "\n";    
}
return result;
}

Also I'd suggest using HashMap or a simple map if you want to implement this, that way you can add a Grade field as well, with a new method that can decide upon the grade based on values provided

> What can I change to reorder the String to make it looks like Bob 82
> \n Mary 65 \n Geoff 23

Change toString implementation to include name and grade in the same line followed by a line break as shown below:

public String toString() {
String result = "";
for (int i = 0; i < names.length && i < grades.length; i++) {
result += names[i] + " " + grades[i] + "\n";    
}
return result;
}

> Furthermore, I want to set a range for each grade, for example, grade
> over 85 is"HD", 85-75 is "D", 74 -65 is "C", 64 -55 is "P", 54- 0 is
> "F". So instead of giving Bob, Mary and Geoff a actual grade, it gives
> them an interval. For example: Bob D, Mary C, Geoff F.

Change the implementation of toString to put name and the calulated grade in the same line followed by a line break as shown below follows:

public String toString() {
String result = "";
String grade;
for (int i = 0; i < names.length && i < grades.length; i++) {
grade = grades[i] > 85 ? "HD"
: (grades[i] >= 75 ? "D" : (grades[i] >= 65 ? "C" : (grades[i] >= 55 ? "P" : "F")));
result += names[i] + " " + grade + "\n";
}
return result;
}

This should be your final version of toString in order to meet your requirements.