Day019 Method overriding and introduction of related classes

One, method rewriting

1. The concept of method overriding

(1) The subclass rewrites the method inherited from the parent class according to the requirements

(2) When rewriting, you can use the super. method to retain the method of the parent class

(3) The constructor cannot be overridden

2. Method rewrite rules

(1) The method name is the same

(2) The parameter list is the same

(3) The return value type is the same or its subclass

(4) The access authority cannot be stricter than the parent class

(5) The static method of the parent class cannot be overridden as a non-static method by the child class, and the non-static method of the parent class cannot be overridden as a static method by the child class

(6) A subclass can define a static method with the same name as the parent class, so as to hide the static method of the parent class in the subclass (Note: super cannot be used in static methods)

(7) The private method of the parent class cannot be overridden by the child class

(8) Cannot throw more exceptions than the parent class method

2. The difference between method overriding and method overloading

Second, Object class

Object class: is the parent class of all classes

Methods of the Object class that are often overridden by subclasses

1. equals() method of Object class

(1) Compare whether the two objects are the same object, if yes, return true

(2) operator

1 Simple data type, compare the value directly.Such as 1==2

②Reference type, compare whether the two are the same object

Note:

(1) The equals() method of the Object class is no different from ==

(2) When there are special requirements, such as when the same attributes are considered to be the same object, equals() needs to be rewritten

(3) Java.lang.String rewrites the equals() method, changing the judgment of the equals() method into judging its value

2. Exercise

package cn.bdqn.demo04;import java.util.Objects;public class Student {private String name;private int idCard;private int age;public Student() {}public Student(String name, int idCard, int age) {this.name = name;this.idCard = idCard;this.age = age;}public String getName() {return name;}public void setName(String name) {this.name = name;}public int getIdCard() {return idCard;}public void setIdCard(int idCard) {this.idCard = idCard;}public int getAge() {return age;}public void setAge(int age) {this.age = age;}@Overridepublic boolean equals(Object o) {if (this == o) return true;if (o == null || getClass() != o.getClass()) return false;Student student = (Student) o;return idCard == student.idCard && age == student.age && Objects.equals(name, student.name);}}

package cn.bdqn.demo04;import java.sql.SQLOutput;public class Test {public static void main(String[] args) {Student stu1=new Student("Zhang San",1234,22);Student stu2=new Student("xx",1234,22);System.out.println(stu1.equals(stu2));}}