# OOP 概念 - 什么是 java 中的聚合？ > 原文： [https://beginnersbook.com/2013/05/aggregation/](https://beginnersbook.com/2013/05/aggregation/) 聚合是一种特殊的关联形式。它是[关联](https://beginnersbook.com/2013/05/association/)这两个类之间的关系，然而它是**方向**关联，这意味着它严格地是**单向关联。** 它代表 **HAS-A** 的关系。 ## Java 中的聚合示例 例如，考虑两个类`Student`类和`Address`类。每个学生都有一个地址，所以学生和地址之间的关系是一个 Has-A 关系。但如果你认为反之亦然那么它就没有任何意义，因为`Address`不一定需要`Student`。让我们在 java 程序中编写这个例子。 `Student Has-A Address` ```java class Address { int streetNum; String city; String state; String country; Address(int street, String c, String st, String coun) { this.streetNum=street; this.city =c; this.state = st; this.country = coun; } } class StudentClass { int rollNum; String studentName; //Creating HAS-A relationship with Address class Address studentAddr; StudentClass(int roll, String name, Address addr){ this.rollNum=roll; this.studentName=name; this.studentAddr = addr; } public static void main(String args[]){ Address ad = new Address(55, "Agra", "UP", "India"); StudentClass obj = new StudentClass(123, "Chaitanya", ad); System.out.println(obj.rollNum); System.out.println(obj.studentName); System.out.println(obj.studentAddr.streetNum); System.out.println(obj.studentAddr.city); System.out.println(obj.studentAddr.state); System.out.println(obj.studentAddr.country); } } ``` 输出： ```java 123 Chaitanya 55 Agra UP India ``` 上面的例子显示了`Student`和`Address`类之间的 **聚合** 。你可以看到在`Student`类中我已经声明了一个`Address`类型的属性来获取学生地址。它是 Java 中聚合的典型示例。 ## 为什么我们需要聚合？ **维护代码重用性**。要理解这一点，请再次使用相同的示例。假设还有另外两个类`College`和`Staff`以及上面两个类`Student`和`Address`。为了保持学生的地址，学院地址和工作人员的地址，我们不需要一次又一次地使用相同的代码。我们只需要在定义每个类时使用`Address`类的引用，如： ```java Student Has-A Address (Has-a relationship between student and address) College Has-A Address (Has-a relationship between college and address) Staff Has-A Address (Has-a relationship between staff and address) ``` 因此，我们可以通过使用聚合关系来提高代码的可重用性。 所以，如果我必须在一个程序中写这个，我会这样做： ```java class Address { int streetNum; String city; String state; String country; Address(int street, String c, String st, String coun) { this.streetNum=street; this.city =c; this.state = st; this.country = coun; } } class StudentClass { int rollNum; String studentName; //Creating HAS-A relationship with Address class Address studentAddr; StudentClass(int roll, String name, Address addr){ this.rollNum=roll; this.studentName=name; this.studentAddr = addr; } ... } class College { String collegeName; //Creating HAS-A relationship with Address class Address collegeAddr; College(String name, Address addr){ this.collegeName = name; this.collegeAddr = addr; } ... } class Staff { String employeeName; //Creating HAS-A relationship with Address class Address employeeAddr; Staff(String name, Address addr){ this.employeeName = name; this.employeeAddr = addr; } ... } ``` 正如您所看到的，我们没有在三个类中的任何一个中编写`Address`代码，我们只是使用`Address`类创建了 HAS-A 关系以使用`Address`代码。上面代码中的点点（...）部分可以用`public static void main`方法替换，其中的代码与我们在第一个例子中看到的类似。