Methods and Classes#

• Every method (a.k.a. function) is associated with some class.
• To run a class, we must define a main method.
• Not all classes have a main method!

[!TIP] Shortcut to Run

• Use control + shift + . as a shortcut to run.

Creating Instances When writing methods and classes, we often want to simulate real-world characteristics. For example, each dog’s bark can be different, and each student’s reaction to a bell can vary.

A Not-So-Good Approach We could create a separate class for every single dog, but this would become redundant quickly.

Solution: Instance Variables#

The real solution is to create an entity (instance) that can represent specific characteristics. For example, the sound a dog makes can depend on its weight.

package lec2_intro2;
public class Dog {
    public int weightInPounds;

    public void makeNoise() {
        if (weightInPounds < 10) {
            System.out.println("yipyipyip!");
        } else if (weightInPounds < 30) {
            System.out.println("bark!");
        } else {
            System.out.println("aroooooooo!");
        }
    }
}

DogLauncher.java with Instance

package lec2_intro2;
public class DogLauncher {
    public static void main(String[] args) {
        Dog d = new Dog();
        d.weightInPounds = 20;
        d.makeNoise();
    }
}

[!TIP] Object Instantiation Classes can contain not just functions (methods), but also data. For example, we can add a size variable to each Dog.

[!TIP] Class Instantiation

• We create a single Dog class and then create instances of this Dog.
• These instances are also called ‘objects’.
• The class provides a blueprint that all Dog objects will follow.
• Cannot add new instance variables to a Dog; they must all obey the blueprint exactly.

package lec2_intro2;
public class Dog {
    public int weightInPounds;

    // Instance variable. Can have as many of these as you want.

    public Dog(int w) {
        weightInPounds = w;
    } // Constructor (similar to a method but not a method). Determines how to instantiate the class.

    public void makeNoise() {
        if (weightInPounds < 10) {
            System.out.println("yipyipyip!");
        } else if (weightInPounds < 30) {
            System.out.println("bark!");
        } else {
            System.out.println("aroooooooo!");
        }
    }

    // Non-static method, a.k.a Instance Method. Idea: if the method is going to be invoked by an instance of the class, then it should be non-static.
}

Static vs. Non-Static Methods#

In the same class, we can have both static and non-static methods.

package lec2_intro2;
public class Dog {
    public int weightInPounds;

    // Instance variable. Can have as many of these as you want.

    public Dog(int w) {
        weightInPounds = w;
    } // Constructor (similar to a method but not a method). Determines how to instantiate the class.

    public void makeNoise() {
        if (weightInPounds < 10) {
            System.out.println("yipyipyip!");
        } else if (weightInPounds < 30) {
            System.out.println("bark!");
        } else {
            System.out.println("aroooooooo!");
        }
    }

    // Non-static method, a.k.a Instance Method. Idea: if the method is going to be invoked by an instance of the class, then it should be non-static.

    public static Dog maxDoge(Dog d1, Dog d2) {
        if (d1.weightInPounds > d2.weightInPounds) {
            return d1;
        } else {
            return d2;
        }
    }
}

Instance Methods#

package lec2_intro2;
public class Dog {
    // ... (previous code)

    public Dog maxDog(Dog d2) {
        if (weightInPounds > d2.weightInPounds) {
            return this;
        } else {
            return d2;
        }
    }
}

DogLauncher.java with Static and Instance Methods#

package lec2_intro2;
public class DogLauncher {
    public static void main(String[] args) {
        Dog chester = new Dog(17);
        Dog yusuf = new Dog(150);

        Dog larger = chester.maxDoge(yusuf);
        larger.makeNoise();
    }
}

Static Variables#

Static variables are common to the entire class, whereas instance variables are specific to each object.

Interactive Debugging#

So far (e.g., in CS61A), you might have added print statements to find bugs in your code. Today, we’ll use IntelliJ’s built-in, interactive debugging tool to find bugs in some code. Debugging is more of an art than a science.

Example: Using IntelliJ’s Debugger#

package lec2_intro2;
public class DogLauncher {
    public static void main(String[] args) {
        Dog chester = new Dog(17);
        Dog yusuf = new Dog(150);

        Dog larger = Dog.maxDoge(chester, yusuf);
        larger.makeNoise();
    }
}

NOTE

Class Attributes vs. Instance Attributes

• static defines class attributes.
• Without static, it defines instance attributes.