Review java - OO

Class & Object

A class only exists at compile time;

An object only exists at runtime.

Data Encapsulation

Data Encapsulation/information hiding: where the internal state and operation are hidden from others.

The more information Class A knows about Class B, the greater the possibility that changing Class A will adversely affect Class B. In an ideal world, making internal changes to Class A should have no, or very little, effect on other classes.

Access control allows for encapsulation

Access modifiers

Instance variable & Local variable

Instance variable is declared inside a class but not inside a method.

Local variable is declared within a method.

Getter & Setter

If a instance variable is private, constructor should use the setter of that variable.

Wrong code

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public class Flower {
    private String color;
    private int height;

    public Flower(){
    }

    public Flower(String c, int h){
        this.color = c;
        this.height = h;
    }

    public void setColor(String c){
        this.color = c;
    }

    public String getColor(){
        return this.color;
    }

    public void setHeight(int h){
        if(h < 0 || h > 10){
            System.out.println("ERROR");
        }
        else{
            this.height = h;
        }
    }

    public int getColor(){
        return this.color;
    }
}

Test code:

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public class FlowerTest(){
    public static void main(String[] args) {
        Flower f1 = new Flower();
        f1.setColor = "red";
        f1.setHeight = 11; // wrong
        f1.setHeight = 10; // OK

        Flower f2 = new Flower("red",11) // Should print error. BUT it pass.
    }
}

Change constructor into:

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...
    public Flower(String c, int h){
        this.setColor(c);
        this.setHeight(h);
    }
...

Method overloading

Whenever two or more methods have the same name but different input parameters.

Write a java class

1. Instance variable

2. Constructors

3. Accessors(getter)

4. Mutator(setter)

5. Service Methods

6. toString() method

   With the toString() method, the object can be printed with a user-defined format.

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   public string toString(){ return "< user-defined format"; }

7. A test class