Constructors and Visibility
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Happy Valentine’s Day
(Don’t expect your human relationships to be like your computer relationship.)
Q3 Review: Functions
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Q3 Review: Functions
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Q3 Review: Function Terminology
What do we call the combination of the function name and list of argument types that is used by Java to determine which function to call?
(Everyone got credit for this because it wasn’t written on a slide, although it is now.)
Review: this
class Dimensions {
int width;
int height;
int area() {
return this.width * this.height;
}
}Instance methods can refer to their instance variables using the this
keyword.
this refers to the instance that is executing the method.
New Examples
> Click or hit Control-Enter to run Example.main above
Constructors
class Course {
String name;
// This is a constructor.
Course(String setName) {
this.name = setName;
}
}When we create a new object, a special method called a constructor is run.
Constructors: Syntax
public class Course {
String name;
// The constructor must be named Course
// It's not declared to return anything...
Course(String setName) {
this.name = setName;
// And doesn't explicitly return anything, but always returns a new Course
}
}-
Constructors can and do perform class-specific initialization.
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Constructors always shared the same same name as the class they create:
Coursein the example above. -
Constructors don’t declare or explicitly return anything—but always return a new instance of their class.
Constructors: Overloading
public class Course {
String name;
// This constructor sets the name
Course(String setName) {
this.name = setName;
}
// This constructor initializes the name to a blank string
Course() {
this.name = "";
}
}Like other functions, constructors can be overloaded.
The Default Constructor
class Course {
String name;
}
// This is the same as
class Course {
String name;
Course() {
}
}If you don’t define a constructor, it’s equivalent of an empty constructor that doesn’t set any fields.
Constructors Can’t Fail
class Course {
String name;
Course(String setName) {
// What do I do if the name is invalid?
this.name = setName;
}
}Constructors must return a new instance of their class.
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So we don’t have a good way to reject invalid inputs in constructors—yet.
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We’ll come back to this when we talk about static methods…
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and when we discuss exceptions.
> Click or hit Control-Enter to run Example.main above
Questions about Constructors?
Access Modifiers
public class Person {
public String name;
private int age;
private void printName() {
System.out.println(this.name);
}
public int getAge() {
return this.age;
}
}Java provides ways to protect instance variables and methods. We refer to these as access modifiers:
Variable Access Modifiers
public class Person {
// Anybody can modify name
public String name;
// age can only be changed by this class's methods
private int age;
}
Person me = new Person();
me.name = "Geoffrey"; // This works
System.out.println(me.age); // This does not work-
public: the variable can be read or written by anyone -
private: the variable can only read or written by methods defined on that class
> Click or hit Control-Enter to run Example.main above
Function Access Modifiers
public class Person {
public void printIt() {
System.out.println("It");
}
private void printYou() {
System.out.println("You");
}
}
Person me = new Person();
me.printIt(); // This works
me.printYou(); // This does not work-
public: the method can be called by anyone -
private: the method can only be called by other methods on that class
> Click or hit Control-Enter to run Example.main above
Announcements
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TC 8 will be out today and due Sunday by midnight. It’s the first set of object-related Turing’s Craft exercises.
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MP3 is out and due a week from Friday. It’s your introduction to object-oriented programming, so you’ll be learning what you need to complete it in lecture and lab this week.
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My office hours continue today at 11AM in the lounge outside of Siebel 0226.