Why does Java's HashMap produce output like this?

public class CHashMap {
    static HashMap<Integer, Student> student = new HashMap<Integer, Student>();

    public static void main(String[] args) {
        student.put (1111, new Student ("Kim Ye-si", "3rd grade", "English"));
        student.put (2222, new Student ("Advanced Example", "Grade 1", "English"));
        student.put (3333, new Student ("Kim Ye-si", "Second Grade", "Com Engineering"));
        student.put (4444, new Student ("this example", "grade 4", "Chinese literature"));
        student.put (5555, new Student ("literary example", "fourth grade", "medical course"));

        printKey();
    }

    static public void printKey() {
        System.out.println("this is key----------");
        for(int key : student.keySet()) {
            System.out.println(key);
        }
    }
}