Working with References

1
Working with References
2
References

• Every object variable is a reference to an object
• Also true when an object is passed as an argument
  
• When the object is used, the reference is
  followed to find the actual object

3
null References

• A variable that does not currently point to
  anything is called a null reference
• e.g. String name // name is null
• We can check if a variable contains a null
  reference using the null identifier
• e.g. if(name ! null)
• Java doesnt like null references
• but they can be hard to find for instance
  variables

4
null References

• Look at StudentReferenceTest.java
• Note
• Compiler catches null reference in the local
  variable
• null references are never allowed in local
  variables
• but the compiler can not catch the null
  reference in the instance variables
• it doesnt know where the reference has been

5
null References

• Three situations can arise
• local variable null compile error
• null reference printed/passed this can be
  missed initially
• null reference used to call a method this gets
  a null pointer exception
• null objects dont have defined methods

6
The Picture

• Student s1 new Student(300012345, uid)
• s1.setFirstName(Sam)

Student ----------- studentNumber
300012345 firstName Sam lastName null ...
s1
7
Aliases

• Student s1 new Student(300012345, uid)
• s1.setFirstName(Sam)
• Student s2 s1

Student ----------- studentNumber
300012345 firstName Sam lastName null ...
s1
s2
8
Aliases

• s2.setFirstName(Pat)
• System.out.println(s1.getFirstName())
• \\prints Pat

Student ----------- studentNumber
300012345 firstName Pat lastName null ...
s1
s2
9
Copying

• If we really do want to copy an object, it has to
  be done manually
• Create a new instance, and copy the relevant data
  over
• Not an issue if there are no methods that change
  the object i.e. for immutable objects
• e.g. String objects

10
Immutable Object Reference

• For strings
• String s1 "Sam"
• String s2 s1
• s2 "Pat"
• System.out.println(s1)
• System.out.println(s2)

11
Changeable Object Reference

• For students
• Student s1 new Student(11111111,"u1")
• s1.setFirstName("Sam")
• Student s2 s1
• s2.setFirstName("Pat")
• System.out.println(s1.getFirstName())
• System.out.println(s2.getFirstName())

12
The clone Method

• Many classes contain a clone() method
• this returns a copy of the object
• i.e. a new object with the relevant info copied
• e.g.
• public Student clone()
• Student s new Student(studentNumber, userid)
• // copy the rest of the relevant data
• return s

13
Equality and References

• Compare references not objects
• Student s1 new Student(300012345,uid)
• Student s2 new Student(300012345,uid)
• Student s3 s1
• \\now s1s3 and s1!s2

Student ----------- 300012345 uid
Student ----------- 300012345 uid
s1
s2
s3
14
The equals Method

• Many classes define an equals method
• equal depends on the class
• For students
• public boolean equals(Student s)
• return studentNumber s.studentNumber

15
The compareTo Method

• Used for more general comparison
• a.compareTo(b) should return
• a negative int if a
• 0 if ab
• a positive int if ab
• Used by the built-in sorts
• one call to compareTo gives all the info needed
  about the relative order
• For students not 100 clear
• studentNumber?
• But not all objects are sortable

16
The this Reference

• It is often convenient/necessary to explicitly
  refer to members of the current object
• e.g. studentNumber s.studentNumber
• This can be confusing same variable name
• The special identifier this refers to the object
  that the code is defining
• e.g. this.studentNumber s.studentNumber
• This is more clear

17
Using this in Constructors

• In constructors, we need to pass a formal
  parameter to fill a data member
• public Student(long stunum, )
• studentNumber stunum
• Using this can clarify the situation
• public Student(long studentNumber, )
• this.studentNumber studentNumber