Lecture2 Defining and Using Classes

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we wrote like before→

public class Dog {  
    public static void makenoise(){  
        System.out.println("Bark!");  
    }  
    public static void main(String[] args) {  
        makenoise();  
    }  
}

then we define another class:

public class DogLauncher {  
    public static void main(String[] args) {  
        Dog.makenoise();  
    }  
}

Why Dog.makenoise()?
dog actually doesn't live in the DogLauncher Class!
- go to the dog class and to go to the makenoise function

every method should be in a class

public class Dog {  
    public int weightInPounds;  
    public void makenoise(){  
        if (weightInPounds<10){  
            System.out.println("yip!");  
        }  
        else (weightInPounds<30){  
            System.out.println("rooooo!");  
        }  
        else {  
            System.out.println("bark!");  
        }  
    }  
    public static void main(String[] args) {  
        makenoise();  
    }  
}

pay attention to the usage of else

but warning! non-static field cannot be referenced from a static context so delete the word static

but since makenoise() is non-static, it can't be quoted in the static function main

why? our original codes are Dog.makenoise() Dog are too general

so we need to make a new object like:

Dog d = new Dog()

and codes like:

d.weightInPounds = 20;
d.makeNoise();

so the whole script looks like:

in Dog.java, we wrote:

public class Dog {  
    public int weightInPounds;  
    public Dog(int w){  
        weightInPounds = w;  
    }
    // when we wrote a method which is not static, we need to create a instance
    public void makenoise(){  
        if (weightInPounds<10){  
            System.out.println("yip!");  
        }  
        else if (weightInPounds<30){  
            System.out.println("bark!");  
        }  
        else {  
            System.out.println("roooooooooooo!");  
        }  
    }  
}

and in DogLauncher.java, which contains the main function:

public class DogLauncher {  
    public static void main(String[] args) {  
        Dog d = new Dog();  // Declaration of a Dog variable
        d.weightInPounds = 1;  
        d.makenoise();  
    }  
}

argument can also be filled when creating a new instance

public class DogLauncher {  
    public static void main(String[] args) {  
        Dog d = new Dog(1);  
        d.makenoise();  
    }  
}

to sum up, static method -> how do dogs make noise? non-static method -> how does a specific dog make noise? e.g. how does Maya make noise

some method doesn't need to be intantiated. for example, math

x=Math.round(5,6)

public class Dog {  
    public int weightInPounds;  
    public static String binomen = "Cans";  
    //this name is the general name of every dog  
    public Dog(int w){  
        weightInPounds = w;  
    }  
    public void makenoise(){  
        if (weightInPounds<10){  
            System.out.println("yip!");  
        }  
        else if (weightInPounds<30){  
            System.out.println("bark!");  
        }  
        else {  
            System.out.println("roooooooooooo!");  
        }  
    }  
    public static Dog maxDog(Dog d1, Dog d2) {  
        if (d1.weightInPounds>d2.weightInPounds){  
            return d1;  
        }  
        else{  
            return d2;  
        }  
    }  
    public Dog MaxDog(Dog d2){  
        if (weightInPounds>d2.weightInPounds){  
            return this;  
        }else{  
            return d2;  
        }  
    }  
}

public class DogLauncher {  
    public static void main(String[] args) {  
        Dog chester = new Dog(17);  
        Dog yusuf = new Dog(2);  
        Dog larger=Dog.maxDog(chester, yusuf);  
        larger.makenoise();  
        System.out.println(Dog.binomen);  //chester.binoman is also ok, just bit ugly
    }  
}  
// why? dog actually doesn't live in the DogLogic Class

Interactive Debugging

debugging button - set breaking point - step over allows us to go on - debugger will desplay the states of the arguments

use step into to see what is in the function inside
resume program -> run the program until it hits the break point

Lecture2 Defining and Using Classes

Interactive Debugging

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