array duplication check

public class Main {
    
    public void main(String[]args) { 
        StudentInfo student=new StudentInfo(); 
        student.addStudents(newStudents("Taro Sato", "111", 21); 
        student.addStudents(newStudents("Taro Sato", "111", 21); 
        student.addStudents(new Student("Akari Kato", "112", 20); 
        student.addStudents(newStudents("Taizo Hayashi", "113", 23); 
        student.addStudents(new Student("Hikari Watanabe", "114", 22); 
                
        double average=StudentInfo.getAverage(); 
        System.out.println("average age:" + average); 
        
    }
}

public class Student {

    String name="; // Name    
    String studentID=";// Student ID    
    age;// age 
    public Object id;

    public Student(String name, String id, intage) {        
        this.name = name;        
        This.studentID=id;        
        This.age=age;
    }
    
    public String getName() {
        return name;
    }
    public String getId() {
        return studentID;
    }
    public int getage() {
        return;
    }
}

public class StudentInfo{
    
    public static final int MAX_COUNT = 100; // Maximum number of Student handled;
    static Student [ ] students = new Student [MAX_COUNT ]; // Student to manage
    static int studentCount = 0; // Number of Students stored in the array

    public String id="; // Initialization
    public String name="";
    public age = 0;

    // Argument: Student/Student Add Student
    // ·Return value: Returns true when added and false when failed.
    // ·Purpose: Add Student to the array in StudentInfo. If the value of the argument is null,
    //         If you have exceeded the maximum number that you can handle in an array, or if you have already registered a Student with the same information,
    //         return false without registering

    public boolean addStudents (Student student) {
        // Returns false if argument specifies a class other than Student
        if(student instance of Student!=true)
                return false;
        // When the maximum number of arrays is exceeded
        if(studentCount>MAX_COUNT||student==null){
                return false;
            }
        // Check if each element in the array is the same and return true if they are all the same
            Student target=(Student) student;
            // Duplicate check with for statement
            for(inti=0;i<=studentCount;i++){
            
            if(this.name.equals(target.name)&(this.id.equals(target.id))&this.age==target.age)
                return false;
            }
                Students [studentCount] = student; /// student [studentCount] replaces student
                studentCount++;
                System.out.println(studentCount); 
            
                return true;                
            }
    
    // · Argument: None
    // ·Return value: Average Student value registered in the array
    // Objective: Calculate and return the average age of Student registered in the StudentInfo array.
    //         Returns 0 if no student is registered.

    public static double getAverage() {
        double average = 0.0;
            for (inti=0;i<studentCount;i++)
                average+=(double)students[i].getage();
                System.out.println (average/studentCount); 
            return average/studentCount;
    }