Writing a class: fields, constructor, methods

Make your own type

• So far you used classes like String and Math that Java gave you.
• Now you will write your own class. A class is a plan for an object.
• An object holds data (its own values) and methods (things it can do).

Instance variables

• Data that each object keeps is stored in instance variables (also called fields).
• You declare them inside the class, but outside any method.
• Each object gets its own copy of these variables.

public class Dog {
    String name;   // instance variable
    int age;       // instance variable
}

A constructor

• A constructor sets up a new object. It runs when you write new.
• It has the same name as the class and no return type (not even void).
• Use the word this to mean "this object's own variable".

public class Dog {
    String name;
    int age;

    // Constructor: runs when we make a new Dog
    public Dog(String name, int age) {
        this.name = name;   // this.name is the field; name is the parameter
        this.age = age;
    }
}

Instance methods

• An instance method is an action an object can do.
• It can read and change the object's own instance variables.
• You call it on an object with a dot: myDog.speak().

public class Dog {
    String name;
    int age;

    public Dog(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // Instance method: returns a sentence about this dog
    public String describe() {
        return this.name + " is " + this.age;
    }
}

Make and use an object

• Use new ClassName(...) to build an object. This calls the constructor.
• Store it in a variable whose type is the class name.
• Then call its methods with a dot.

public class Main {
    public static void main(String[] args) {
        Dog d = new Dog("Rex", 4);
        System.out.println(d.describe());   // Rex is 4
    }
}

class Dog {
    String name;
    int age;

    public Dog(String name, int age) {
        this.name = name;
        this.age = age;
    }

    public String describe() {
        return this.name + " is " + this.age;
    }
}

Why this?

• A parameter and a field can have the same name (like name).
• Inside the constructor, name means the parameter; this.name means the field.
• this always means the object the method was called on.

Common mistakes

• A constructor has no return type and matches the class name.
• Use this.field to tell a field from a parameter of the same name.

Now you try

• Each task gives you a class skeleton with a TODO. Fill it in so it compiles and works.
• A hidden Harness builds your object and checks the results.
• Press Run to compile, then Check answer.