Getters and Setters

Getters and Setters
> Click or hit Control-Enter to run Example.main above

This class did well on the midterm. We’ll discuss more on Monday, including how to interpret your result…

Fall 2018: 73%
Spring 2019: 79%
Fall 2019: 82%

public class Person {
  public String name;
  private int age;

  private void printName() {
    System.out.println(name);
  }
  public int getAge() {
    return age;
  }
}

Java provides ways to protect instance variables and methods. We refer to these as 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

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

(Java also provides protected and package private modifiers—but they don’t make sense until we discuss packages.)

public class Person {
  private int age;
  public void setAge(int setAge) {
    age = setAge;
  }
  public int getAge() {
    return age;
  }
}

In Java it’s common to have private instance variables with public methods that set or get their values: called setters and getters.

public class Person {
  private int name;
  private int firstName;
  private int lastName;
  public void setName(String setName) {
    String[] nameParts = setName.split(" ");
    name = setName;
    firstName = nameParts[0];
    lastName = nameParts[1];
  }
  public int getName() {
    return name;
  }
  public int getFirstName() {
    return firstName;
  }
  public int getLastName() {
    return lastName;
  }
}

Getters and setters allow an object to react to changes to its variables. It also allows us to create read- and write-only variables.

> Click or hit Control-Enter to run Example.main above

> Click or hit Control-Enter to run Example.main above

public class Course {
  public static int count = 0;
  public static void printName() {
    System.out.println("Name");
  }
}
public class Example {
  public static void main(String[] unused) {
    // We can call printName without creating an instance
    Course.printName();
    // We can increment count without creating an instance
    Course.count++;
  }
}

static methods and variables belong to the class, not to a specific instance.

public class Course {
  public static void printName() {
    System.out.println("Name");
  }
}
public class Example {
  public static void main(String[] unused) {
    // This works
    Course.printName();
    // This also works
    Course cs125 = new Course();
    cs125.printName();
  }
}

static methods are called directly on the class, rather than on an instance
…but they can be called on the instance as well.

public class Course {
  public String name;
  public static void printName() {
    // This doesn't work
    System.out.println(name);
  }
}

static methods can be called without an instance, and so can’t use this

public class Course {
  public static int count = 0;
  public void printCount() {
    System.out.println(count);
  }
}
public class Example {
  public static void main(String[] unused) {
    Course cs125 = new Course();
    Course cs225 = new Course();
    Course.count++;
    cs125.printCount();
    cs225.printCount();
  }
}

static variables are shared by all instances of a given class

> Click or hit Control-Enter to run Example.main above

public and private also work on static variables and methods

public: the (static) variable can be read or written by anyone
private: the (static) variable can only read or written by methods defined on that class
public: the (static) method can be called by anyone
private: the (static) method can only be called by other methods on that class

My office hours as usual today from 1–3PM in Siebel 2227.
I’ll be doing one "talk to your professor" interview today at 2PM in Siebel 2227.
Good luck finishing up the MP1 early deadline! It’s due on your deadline day this weekend.
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Congratulations!

Review: Access Modifiers

Review: Variable Access Modifiers

Review: Function Access Modifiers

Other Access Modifiers