#멋쟁이사자처럼 #부트캠프 #백엔드 #JAVA

Practice: Constructors of TV

public class TV {
    private boolean power = false;
    private int channel;
    private int volume;

    public TV(boolean power, int channel, int volume) {
        this.power = power;
        this.channel = channel;
        this.volume = volume;
    }

    public TV(boolean power, int channel) {
        this(power, channel, 0);
    }

    public TV(boolean power) {
        this(power, 0, 0);
    }
}

Practice: Person

/*
Console:
public Person(String name, int age, String phoneNumber) {}
public Person(String name, int age) {}
public Person(String name) {}
 */
public class Person {
    private String name;
    private int age;
    private String phoneNumber;

    public Person(String name, int age, String phoneNumber) {
        System.out.println("public Person(String name, int age, String phoneNumber) {}");
        this.name = name;
        this.age = age;
        this.phoneNumber = phoneNumber;
    }

    public Person(String name, int age) {
        this(name, age, "");
        System.out.println("public Person(String name, int age) {}");
    }

    public Person(String name) {
        this(name, 0);
        System.out.println("public Person(String name) {}");
    }

    public static void main(String[] args) {
        Person park = new Person("Park");
    }
}

Overriding

Practice: Parent and Child

class Parent {
    int i = 5;

    public int getI() {
        return i;
    }
}

class Child extends Parent {
    int i = 10;
}

public class Main {
    public static void main(String[] args) {
        Child child = new Child();
        System.out.println(child.getI()); // 5
    }
}

Implicit Type Casting (or Type Promotion)

class Parent {
    int i = 5;

    public int getI() {
        return i;
    }
}

class Child extends Parent {
    int i = 10;

    public void cry() {
        System.out.println("I want this!");
    }
}

public class Main {
    public static void main(String[] args) {
        Child child = new Child();
        child.heyHey();
    }
}

// ...

public class Main {
    public static void main(String[] args) {
        Parent child = new Child();
        // ❌ Cannot resolve method 'heyHey' in 'Child'
        child.heyHey();
    }
}

Explicit Type Casting

Casting 'p' to 'Child' will produce ClassCastException for any non-null value. It doesn't cause a compile time error because p can also contain a Child type instance.

class Parent {
    int i = 5;

    public int getI() {
        return i;
    }
}

class Child extends Parent {
    int i = 10;

    public void cry() {
        System.out.println("I want this!");
    }
}

public class Main {
    public static void main(String[] args) {
        Parent p = null;
        Child c = null;
        c = (Child) p;
    }
}

// ...

public class Main {
    public static void main(String[] args) {
        Parent p = null;
        Child c = null;

        // ❌ Exception in thread "main" java.lang.ClassCastException:
        // class org.example.Parent cannot be cast to class org.example
        // Child (org.example.Parent and org.example.Child are in
        // unnamed module of loader 'app')
        p = new Parent();
        c = (Child) p;
    }
}

Meanwhile... the following example works fine: