Does it compile? questions in java II

Q11.java

class A {
    private int x;

    public A() {
        this(7);
    }

    public A(int x) {
        setX(x);
    }

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public void setXFrom(A other) {
        setX(other.x);
    }
}

class B {
    private A a1;
    private A a2;

    public B(A a1, A a2) {
        this.a1 = a1;
        this.a2 = new A(a2.getX());
    }

    public int getFirst() {
        return a1.getX();
    }

    public int getSecond() {
        return a2.getX();
    }
}

public class Main {
    public static void main(String[] args) {
        A a1 = new A();
        A a2 = new A(5);
        B b = new B(a1, a2);
        System.out.println(b.getFirst());
        System.out.println(b.getSecond());
        System.out.println("----");
        a1.setXFrom(a2);
        a2.setX(17);
        System.out.println(b.getFirst());
        System.out.println(b.getSecond());
    }
}

Q12.java

class A {
    private int x;
    
    public A(int x) {
        this.x = x;
    }

    public static int square() {
        return x * x;
    }
} 
 
public class Main { 
    public static void main(String[] args) { 
        A a = new A(3); 
        System.out.println(a.square());
    } 
}

Q13.java

class Util {
    public static double pi() { 
        return 3.14; 
    }
} 
 
public class Main { 
    public static void main(String[] args) { 
        Util u = new Util();    
        System.out.println(u.pi());       
    } 
}

Q14.java

class A {            
    private static int x = 3;                                                                                      
    private int y = 2;                                                                                             
                                                                                                                   
    public A() {                                                                                                   
        x++;                                                                                                       
        y++;                                                                                                       
    }                                                                                                              
                                                                                                                   
    public static int multiple(A a) {                                                                              
        return x * a.y;                                                                                            
    }                                                                                                              
}                                                                                                                  
                                                                                                                   
public class Main {                                                                                                
    public static void main(String[] args) {                                                                       
        A a1 = new A();                                                                                            
        A a2 = new A();                                                                                            
        System.out.println(A.multiple(a2));                                                                            }                                                                                                              
}                                                                                                                  

Q15.java

interface Printable {
    void print();
}

interface Cool {
}

class BasePrintable implements Printable {
    public void print() {
        System.out.println(toString());
    }

    public String toString() {
        if (this instanceof Cool) {
            return getClass().getName() + ": I'm cool";  
        } else {
            return getClass().getName() + ": lame...";
        }
    }
}

class A extends BasePrintable implements Cool {    
    public String toString() {
        return super.toString() + " and I know it!";
    }
}

class B extends BasePrintable {
}

public class Main {
    public static void main(String[] args) {
        A a = new A();
        B b = new B();
        a.print();
        b.print();
    }
}

Q16.java

interface I {        
    void bla();                                                                                                    
}                                                                                                                  
                                                                                                                   
abstract class A implements I {                                                                                    
    public abstract void blat();                                                                                   
}                                                                                                                  
                                                                                                                   
class B extends A {                                                                                                
    public void blat() {                                                                                           
        System.out.println("Hello");                                                                               
    }                                                                                                              
}                                                                                                                  
                                                                                                                   
public class Main {                                                                                                
    public static void main(String[] args) {                                                                       
        B b = new B();                                                                                             
        b.blat();                                                                                                  
    }                                                                                                              
}                                                                                                                  

Q17.java

abstract class A {                                                                                                 
    public int calcStuff() {                                                                                       
        return 7 * smart() * stupid();                  
    }    

    protected abstract int smart();         

    private int stupid() {         
        return 10 * 20 - 30;
    }
}                                                                                                                  
                                                                                                                   
class B extends A {                                                                                                
    protected int smart() {         
        return super.stupid() + 8;         
    }
}                                                                                                                  
                                                                                                                   
public class Main {                                                                                                
    public static void main(String[] args) {                                                                       
        B b = new B();                                                                                             
        System.out.println(b.calcStuff());                                                                         
    }                                                                                                              
}

Q18.java

abstract class Message {
    public abstract String getMessageType();
}

class SleepMessage extends Message {
    public String getMessageType() {
        return "Sleep";
    }

    public int getTime() {
         return 22;
    }
}

class EatMessage extends Message {
    public String getMessageType() {
         return "Eat";
    }

    public String whichMeal() {
        return "Breakfast";
    }
}

class MessageHandler {
    public void handleMessage(Message m) {
        if (m instanceof SleepMessage) {
            System.out.println("Sleeping until " + ((SleepMessage)m).getTime());
        } else if (m instanceof EatMessage) {
            System.out.println("Eating " + ((EatMessage)m).whichMeal());
        } else {
            System.out.println("Illegal message type: " + m.getMessageType());
        }
    }
}

public class Main {
     public static void main(String[] args) {
         Message[] m = new Message[2];
         m[0] = new SleepMessage();
         m[1] = new EatMessage();
         MessageHandler mh = new MessageHandler();
         for (int i = 0; i < 2; i++) {
              mh.handleMessage(m[i]);
         }
     }
}

TODO.txt

- casting
- collections
- exceptions
- misc.