Java封装、继承、多态

1.封装核心理念【合理隐藏合理暴露】package com.popo.fengzhuang; public class Student { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name name; } public int getAge() { return age; } // Added age validation public void setAge(int age) { if (age 0) { throw new IllegalArgumentException(Age cannot be negative.); } this.age age; } }2.继承子类能继承父类的非私有成员子类对象由子类和父类共同完成package com.popo.jicheng; public class People { private String name; private int age; public String getName() { return name; } public void setName(String name) { this.name name; } public int getAge() { return age; } public void setAge(int age) { this.age age; } }package com.popo.jicheng; public class Student extends People { private String studentId; public String getStudentId() { return studentId; } public void setStudentId(String studentId) { this.studentId studentId; } }package com.popo.jicheng; public class extendDemo1 { public static void main(String[] args) { // 子类对象由子类和父类共同完成所以也有name和age属性 Student student new Student(); student.setName(张三); student.setAge(18); student.setStudentId(001); System.out.println(student.getName() student.getAge() student.getStudentId()); } }3.权限修饰符4.多态把子类对象赋值给父类变量存在方法重写package com.popo.duotai; public class People { String name People; public void run(){ System.out.println(People is running); } }package com.popo.duotai; public class Student extends People { String name Student; Override public void run() { System.out.println(Student is running); } }package com.popo.duotai; public class Teacher extends People { String name Teacher; Override public void run() { System.out.println(Teacher is running); } }package com.popo.duotai; public class DuotaiDemo1 { public static void main(String[] args) { // 只讨论对象和行为的多态不考虑变量的多态 People p1 new Teacher(); // 编译看左边运行看右边 p1.run(); // 输出: Teacher is running System.out.println(p1.name); // 输出: People People p2 new Student(); p2.run(); // 输出: Student is running System.out.println(p2.name); // 输出: People } }如果要使用子类独有的方法可以使用强制类型转换子类型 变量 子类型new 父对象使用instanceof可以判断父对象是否与子类型对应package com.popo.duotai; public class DuotaiDemo2 { public static void main(String[] args) { People p1 new Teacher(); People p2 new Student(); goRun(p1); // 输出: Teacher is running goRun(p2); // 输出: Student is running } public static void goRun(People p) { if (p instanceof Teacher) { Teacher t (Teacher) p; t.run(); } else if (p instanceof Student) { Student s (Student) p; s.run(); } else { p.run(); } } }