封裝、繼承和多態性是Java面向對象編程中的三大特性。封裝是使用類將對象數據和方法封裝在一起的機制，保證了數據不被惡意修改或誤操作。繼承是指通過擴展現有類來創建新類的過程，子類可以獲得父類的屬性和方法。多態性是指同一方法在不同的對象上表現出不同的行為。

//封裝
public class Student {
private String name; //私有變量，外部無法直接訪問
private int age;
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setAge(int age) {
if (age< 0) { //數據校驗
this.age = 0;
} else {
this.age = age;
}
}
public int getAge() {
return age;
}
}
//繼承
public class Person {
protected String name; //受保護的變量，子類可以直接訪問
protected int age;
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}
public void setAge(int age) {
this.age = age;
}
public int getAge() {
return age;
}
}
public class Student extends Person {
private String school;
public void setSchool(String school) {
this.school = school;
}
public String getSchool() {
return school;
}
}
//多態性
public abstract class Animal {
public abstract void makeSound(); //抽象方法
}
public class Dog extends Animal {
public void makeSound() {
System.out.println("汪汪汪");
}
}
public class Cat extends Animal {
public void makeSound() {
System.out.println("喵喵喵");
}
}
public class AnimalTest {
public static void main(String[] args) {
Animal animal1 = new Dog();
Animal animal2 = new Cat();
animal1.makeSound();
animal2.makeSound();
}
}

Java的封裝、繼承和多態性讓程序更加模塊化和易于維護，是Java面向對象編程中的核心概念。