[Java] Static variable

せいてきへんすう

従業員が入社するたびに発行される従業員番号、学生の学号など、複数のインスタンスで共有する変数が必要な場合は、Static変数を使用します.

// serialNum은 static변수!

Student student1 = new Student();
student1.serial = 100; 
// (X) 이렇게 인스턴스 이름을 사용해 참조하지 않고 아래와 같이 class이름으로 참조한다.

Student.serialNum = 100; // (O)

	public static int getSerialNum() {
		employeeName = "Lee"; // 이렇게 인스턴스 변수 사용이 안됨 (오류가 발생함)
		return serialNum;
	}

👨🏻‍💻 Example Code

Employee.java

package ch17;

public class Employee {

	private static int serialNum = 1000;
	
	private int employeeId;
	private String employeeName;
	private String department;
	
	public Employee() {
		serialNum++;
		employeeId = serialNum;
	}
	
	public static int getSerialNum() {	 
		return serialNum;
	}
	
	public int getEmployeeId() {
		return employeeId;
	}
	public void setEmployeeId(int employeeId) {
		this.employeeId = employeeId;
	}
	public String getEmployeeName() {
		return employeeName;
	}
	public void setEmployeeName(String employeeName) {
		this.employeeName = employeeName;
	}
	public String getDepartment() {
		return department;
	}
	public void setDepartment(String department) {
		this.department = department;
	}
}

EmployeeTest.java

package ch17;

public class EmployeeTest {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		
		Employee employee1 = new Employee();
		employee1.setEmployeeName("이순신");
		
		System.out.println(Employee.getSerialNum());
		
		Employee employee2 = new Employee();
		employee2.setEmployeeName("김유신");

		System.out.println(employee1.getEmployeeName() + "님의 사번은 " + employee1.getEmployeeId());
		System.out.println(employee2.getEmployeeName() + "님의 사번은 " + employee2.getEmployeeId());
	}

}

変数のメモリ容量と有効範囲

Reference