Java是一種面向對象的編程語言，在面向對象的編程中，有兩個重要的概念——繼承和組合。

繼承是指一個類可以擴展或繼承一個或多個已有的類，從而擁有已有類的方法和屬性。

// 父類
public class Animal {
private int age;
private String name;
public Animal(int age, String name) {
this.age = age;
this.name = name;
}
public void eat() {
System.out.println("Animal eat food");
}
}
// 子類
public class Dog extends Animal {
public Dog(int age, String name) {
super(age, name);
}
public void bark() {
System.out.println("Dog bark");
}
}
// 使用繼承
Dog dog = new Dog(2, "Jack");
dog.eat();  // Animal eat food
dog.bark(); // Dog bark

組合則是將不同的類組合在一起使用，而不是擴展已有類。組合可以將不同的類的特點進行組合，從而形成一個新的類。

// 車輪類
public class Wheel {
private int size;
public Wheel(int size) {
this.size = size;
}
public int getSize() {
return this.size;
}
}
// 車身類
public class Body {
private String color;
public Body(String color) {
this.color = color;
}
public String getColor() {
return this.color;
}
}
// 汽車類
public class Car {
private Wheel wheel;
private Body body;
public Car(Wheel wheel, Body body) {
this.wheel = wheel;
this.body = body;
}
public void getInfo() {
System.out.println("This car has a " + wheel.getSize() + " inch wheel, and its color is " + body.getColor());
}
}
// 使用組合
Wheel wheel = new Wheel(18);
Body body = new Body("Red");
Car car = new Car(wheel, body);
car.getInfo();  // This car has a 18 inch wheel, and its color is Red