在数组列表中搜索特定对象并打印数组列表中的所有对象

Question

I have to implement a java application that demonstrates the use of array list of objects. I am using Students and I have a StudentTest class. I should be able to enter in details(which I have done). I'm just unsure how to search for a particular object in the array list by Student Number and also to print the array list. Any hints would be great thanks.

code so far:

package student;

public class Student {

    private String studentName;
    private String studentNo;
    private String email;
    private int year;

    public Student() {
        this.studentName = null;
        this.studentNo = null;
        this.email = null;
        this.year = -1;
    }

    public Student(String nName, String nNum, String nEmail, int nYr) {
        this.studentName = nName;
        this.studentNo = nNum;
        this.email = nEmail;
        this.year = nYr;
    }

    public void setStudentName(String newStudentName) {
        this.studentName = newStudentName;
    }

    public void setStudentNo(String newStudentNo) {
        this.studentNo = newStudentNo;
    }

    public void setEmail(String newEmail) {
        this.email = newEmail;
    }

    public void setYear(int newYear) {
        this.year = newYear;
    }

    public String getStudentName() {
        return studentName;
    }

    public String getStudentNo() {
        return studentNo;
    }

    public String getEmail() {
        return email;
    }

    public int getYear() {
        return year;
    }

}
package student;

import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class studentTest {

    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);


        List<Student> Students = new ArrayList();


        System.out.println("Enter number of students");
        int countStudents = input.nextInt();

        for (int i = 0; i < countStudents; i++) {
            Student student1 = new Student();
            System.out.println("Enter details for student: " + (i + 1));

            System.out.println("Enter name: ");
            student1.setStudentName(input.next());

            System.out.println("Enter Number: ");
            student1.setStudentNo(input.next());

            System.out.println("Enter email: ");
            student1.setEmail(input.next());

            System.out.println("Enter year: ");
            student1.setYear(input.nextInt());
            Students.add(student1);



        }
    }
}

Answer 1

You need to implement a equals() method in your Student class, that will return true if 2 students have same studentNo , else false .

@Override
public boolean equals(Object ob) {

    if (!(ob instanceof Student)) { 
        return false;
    }

    return ob.getStudentNo().equals(this.getStudentNo());
}

Now, you can just use contains method to check whether a student is in the List or not.

UPDATE : -

Also, you need to override hashcode method, whenever you are overriding equals method, to maintain the general contract between them. So override hashcode that would calculate the hashcode of your instances based only on the fields on which you are comparing your objects

So, you need to calculate hashcode using studentNo .

@Override
public int hashcode() {
    return this.getStudentNo().hashCode(); 
}

• HashCode calculating algorithm should be such that, if two instances are equal, their hashcode should also be equals
• It should return same value for same instance everytime, of course when the instance has not been modified.
• Two different instances can have the same hashcode

As suggested by @hyde in the comment : -

If this is just too much work for you, then you can just have a different method, that does the same task as equals method, say, checkEquals() , and ignore overriding equals() method. But in that case, you would have to do iteration on your List and for each list element, invoke this method with the student to compare.

for(Student student: studentList) {
    if (student.checkEquals(myStudent)) {
        System.out.println("True");
    }
}