this Keyword: Revision Notes (Computer Science A 3.9)

These notes recap Topic 3.9. For worked examples, read the study guide.

Recap

In an instance method or constructor, this is a special variable holding a reference to the current object: the object whose method or constructor is being run.
this.name = name; stores the parameter name in the instance variable name, solving the hiding problem of Topic 3.8.
this can be passed as an argument so that one object can give another a reference to itself, as in t.setCaptain(this).
this == other tests whether two references refer to the same object. this.age > other.age compares the values held by the two objects.
Class methods do not have a this reference.

Key relationships

Expression	Meaning
`this`	The object before the dot in the call, or the object being constructed
`this.age`	The `age` instance variable of the current object
`age` (with a parameter called `age`)	The parameter
`age` (with no hiding)	The instance variable, the same as `this.age`
`t.setCaptain(this)`	Passes a reference to the current object
`this == other`	`true` only if both are the same object

Worked reminder

With Pet a = new Pet("Rex", 4), Pet b = new Pet("Tia", 7) and Pet c = a: a.sameObjectAs(c) is true, a.sameObjectAs(b) is false, a.olderThan(b) is false (4 > 7) and b.olderThan(a) is true (7 > 4), because this is a in the first calls and b in the last. After p.becomeCaptainOf(team), the team’s captain is the same object as p: team.getCaptain() == p is true.

Using this in class-design questions

In a constructor that takes parameters named like the instance variables, begin each assignment with this. on the left.
When another method needs to know which object is calling it, pass this.
When a method compares two objects of the same class, write the current object as this and the other as the parameter.
Never use this inside a method declared static; pass an object in as a parameter instead.

Assumptions

this exists only in instance methods and constructors.
A reference-typed instance variable that is never set holds null.
Without a hiding parameter, this. is optional.

Compile errors to recognise

Message	Cause
`non-static variable this cannot be referenced from a static context`	`this` used in a class method

Mistakes to avoid

Writing name = name; in a constructor instead of this.name = name;.
Writing name = this.name; (reversed), which leaves the instance variable unchanged.
Using this in a static method.
Thinking this is a fixed object rather than the object before the dot.
Using == to compare contents.

Quick self-check

What does this.score = score; do when score is also a parameter? Answer: stores the parameter in the instance variable.
In x.show(), what does this refer to inside show? Answer: the object x refers to.
Can this be used in a static method? Answer: No.
A method contains other.setBuddy(this);. What does other receive? Answer: a reference to the object running the method.