为什么即使输出相同，s1不等于s3？

Question

/* Student class with a constructor that initialises the name, gender, and degreeProgramme. */

public class Student {

    public static void main(String[] args){
        Student s1 = new Student("a", "b", "c", "d");
        System.out.println(s1.toString());

        s1.setName("Mary Jones");
        s1.setGender("female");
        s1.setStudentID("0564");;
        s1.setDegreeProgramme("History");


        Student s2 = new Student("Mary jones", "female", "0564", "History");
        Student s3 = new Student("Mary Jones", "female", "0564", "History");
        System.out.println(s1);
        System.out.println(s3);
        System.out.println(s2);
        System.out.println(s1.equals(s2));
        System.out.println(s1.equals(s3));
    }

    private String name; /* instance variable */
    private String gender; /* instance variable */
    private String studentID; /* instance variable */
    private String degreeProgramme; /* instance variable */

    /* Student constructor that receives 4 parameters */
    public Student(String name, String gender, String studentID, String degreeProgramme){

        this.name = name; /* assigns name to instance variable name */
        this.gender = gender; /* assigns gender to instance variable gender */
        this.studentID = studentID; /* assigns studentID to instance variable studentID */
        this.degreeProgramme = degreeProgramme; /* assigns degreeProgramme to instance variable degreeProgramme */


    }
    /* method that returns the name of the student */
    public String getName(){
        return name;
    }

    /* method that returns the gender of the student */
    public String getGender(){
        return gender;
    }
    /* method that returns the degree programme of the student */
    public String getDegreeProgramme(){
        return degreeProgramme;
    }

    /* method that returns the student ID */
    public String getStudentID(){
        return studentID;
    }

    /* method that sets the name of the student */
    public void setName(String name){
        this.name = name;
    }
    /* method that sets the student ID */
    public void setStudentID(String studentID){
        this.studentID = studentID;
    }

    /* method that sets the gender of the student */
    public void setGender(String Gender){
        this.gender = Gender;
    }

    /* method that sets the degree programme of the student */
    public void setDegreeProgramme(String degreeProgramme){
        this.degreeProgramme = degreeProgramme;
    }

    /* method that returns the name, gender, and degree programme of the student */
    public String toString(){
        String studentInfo = "["+name+ ", " +gender+ ", ID: " +studentID+ ", " +degreeProgramme+"]" ;
        return studentInfo;
    }


} /* end class Student */

Answer 1

当前，您正在使用从类java.lang.Object继承的equals方法，该方法实现如下：

public boolean equals(Object obj) {
    return this == obj;
}

它检查this和obj是否引用了同一个实例。在您的示例中，s1和s3是Student的不同实例，因此s1.equals(s3)返回false。

您可以实现自己的equals方法来实现您的目的，例如：

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof Student)) return false;

    Student student = (Student) o;

    if (name != null ? !name.equals(student.name) : student.name != null) return false;
    if (gender != null ? !gender.equals(student.gender) : student.gender != null) return false;
    if (studentID != null ? !studentID.equals(student.studentID) : student.studentID != null) return false;
    return !(degreeProgramme != null ? !degreeProgramme.equals(student.degreeProgramme) : student.degreeProgramme != null);
}