Note Do not write accessor methods that return references to mutable objects

While looking at the Java Core Technology (9th edition Chinese version), I saw a warning on page 113, "Be careful not to write a variable that returns a reference the accessor method for the object. Not seen before, the original date object is a mutable object . On the concept of mutable objects , I looked at the net, I think the mutable Object is the object that can change the value in the class without passing the domain change method . .

1  PackageCom.csst.sort;2 3 Importjava.util.Date;4 ImportJava.util.GregorianCalendar;5 6  Public classEmployee {7     PrivateString name;8     Private Doublesalary;9     PrivateDate Hireday;Ten  One      PublicEmployee (String N,DoubleSintYearintMonthintDay ) { AName =N; -Salary =s; -GregorianCalendar Calendar =NewGregorianCalendar (Year, month-1, day); theHireday =(Date) calendar.gettime (); -     } -  -      Public Static voidMain (string[] args) { +  -Employee Employee =NewEmployee ("Lele", 600, 1994, 10, 24); +  ADate Date =Employee.gethireday (); at System.out.println (date); -  -Date.settime (2000500000); -  　　　　　　  - System.out.println (date); - System.out.println (Employee.gethireday ()); in     } -  to      Public voidRaisesalary (Doublebypercent) { +         Doubleraise = This. Salary * bypercent/100; -Salary = This. Salary +raise; the     } *  $      PublicString GetName () {Panax Notoginseng         returnname; -     } the  +      Public voidsetName (String name) { A          This. Name =name; the     } +  -      Public Doublegetsalary () { $         returnsalary; $     } -  -      Public voidSetsalary (Doublesalary) { the          This. Salary =salary; -     }Wuyi  the      PublicDate Gethireday () { -         returnHireday; Wu     } -  About      Public voidSethireday (Date hireday) { $          This. Hireday =Hireday; -     } -  -}

Final Result:

Mon OCT 1994 00:00:00 CST
Sat Jan 11:41:40 CST 1970
Sat Jan 11:41:40 CST 1970

Note that Hireday has changed to change its get method to

Public Date Gethireday () {return (Date) Hireday.clone ();}

The result is:

Mon OCT 1994 00:00:00 CST
Sat Jan 11:41:40 CST 1970
Mon OCT 1994 00:00:00 CST

Note Do not write accessor methods that return references to mutable objects