聚合（has-A)示例

//Date类public class Date {private int month;private int day;private int year;public Date(int theMonth,int theDay,int theYear){month = checkMonth(theMonth);year = theYear;day = checkDay(theDay);System.out.println("Data object constructor for date"+ toDateString());}private int checkMonth(int testMonth){if (testMonth > 0 && testMonth <= 12)return testMonth;else{System.out.println("Invalid month("+ testMonth +")set to 1");return 1;}}private int checkDay(int testDay){int daysPerMonth[] = {0,31,28,31,30,31,30,31,31,30,31,30,31};if(testDay >0 && testDay <= daysPerMonth[month])return testDay;if(month ==2 && testDay ==29 && (year %400 ==0 ||(year %4 ==0 && year %100 != 0)))return testDay;System.out.println("Invalid day ("+ testDay +")set to 1");return 1;}public String toDateString(){return month + "/" + day + "/" + year;}}//Employee类中应用Date类public class Employee {private String firstName;private String lastName;private Date birthDate;private Date hireDate;public Employee(String first,String last,Date dateofBirth,Date dateofHire){firstName = first;lastName = last;birthDate = dateofBirth;hireDate = dateofHire;}public String toEmployeeString(){return lastName + "," + firstName + " Hired:" + hireDate.toDateString()+"Birthday:"+birthDate.toDateString();}}//程序入口import javax.swing.JOptionPane;public class EmployeeTest {public static void main(String[] args) {Date birth = new Date(7,24,1949);Date hire = new Date(3,12,1988);Employee employee = new Employee("Bob","Jones",birth,hire);JOptionPane.showMessageDialog(null,employee.toEmployeeString(),"Testing Class Employee",JOptionPane.INFORMATION_MESSAGE);System.exit(0);}}