Question

Consider the interface Predicate defined as follows.

interface Predicate { 
  boolean eval(int j);
}

1. Recall that you can check if an integer “i” is even by using the expression “i%2 == 0”. Write a class IsEven that determines whether a number is even:

   class IsEven implements Predicate {
     public boolean eval(int j) {
   
   
   
   
   
   
      }
   } 
   

   For example, the following code

     Predicate p = new IsEven();
     if (  p.eval(2)) { System.out.println("2 is even"); }
     if (! p.eval(3)) { System.out.println("3 is not even");
   

   should produce the output

     2 is even
     3 is not even
   

2. Write a class Alternate that alternates between true and false, starting with true. You may add fields if necessary.

   class Alternate implements Predicate {
   
   
   
     boolean eval(int j){
   
   
   
   
   
   
   
     }
   }
   

   For example, the following code

     Predicate p = new Alternate();
     in = new DataInputStream(System.in);  
   
     for (int k=0; k<4; k++){
       int j = in.readInt();  // read a number from the user
       if (p.eval(j)) {
         System.out.println("true");
       } else {
         System.out.println("false");
       }
     } 
   

   should produce the following output, no matter what input is given:

     true
     false
     true
     false
   

3. Write a class Not that implements logical negation:

   class Not implements Predicate {
     Predicate _p;
   
     Not(Predicate p) { _p =p; }
     boolean eval(int j){
   
   
   
   
   
   
     }
   }
   

   For example,

     Predicate p = new IsEven();
     Predicate q = new Not(p);
     if (! q.eval(2)) { System.out.println("2 is even"); }
     if (  q.eval(3)) { System.out.println("3 is not even");
   

   should produce the same output as before, even though the negation operator (!) is moved with respect to question 1:

     2 is even
     3 is not even
   

   As another example, the code

     Predicate p = new Alternate();
     Predicate q = new Not(p);
     in = new DataInputStream(System.in);  
   
     for (int k=0; k<4; k++){
       int j = in.readInt();  // read a number from the user
       if (q.eval(j)) {
         System.out.println("true");
       } else {
         System.out.println("false");
       }
     }  
   

   should produce the following output, no matter what input the user gives:

     false
     true
     false
     true
   

   Notice that the outputs are just flipped, from true to false and from false to true, in comparison with question 2.

Answer 1

//Interface: Predicate.java
interface Predicate{
    boolean eval(int j);
}
----------------------------------------------------------------------
//Class: IsEven.java
class IsEven implements Predicate {
    @Override
    public boolean eval(int j) {
        if(j%2 == 0)
            return true;
        else
            return false;
    }
}
-------------------------------------------------------------
//Class: Alternate.java
class Alternate implements Predicate{

    static boolean flag;
    @Override
    public boolean eval(int j) {
        flag = !flag;
        return flag;
    }
}
----------------------------------------------------------------
//Class: Not.java
class Not implements Predicate{
    Predicate _p;

    Not(Predicate p){
        _p = p;
    }

    @Override
    public boolean eval(int j) {
        return !(_p.eval(j));
    }
}

//-------------------------------------------------------------------------------------------

//Class for executing input cases mentioned in question.

//Class: Main.java

import java.util.Scanner;

class Main{

    static void question1(){
        Predicate p = new IsEven();
        if (  p.eval(2)) {
            System.out.println("2 is even");
        }
        if (! p.eval(3)) {
            System.out.println("3 is not even");
        }
    }

    static void question2(){
        Predicate p = new Alternate();
        Scanner scanner = new Scanner(System.in);

        for (int k=0; k<4; k++){
            int j = scanner.nextInt();  // read a number from the user
            if (p.eval(j)) {
                System.out.println("true");
            } else {
                System.out.println("false");
            }
        }
    }

    static void question3(){
        Predicate p = new Alternate();
        Predicate q = new Not(p);
        Scanner scanner = new Scanner(System.in);

        for (int k=0; k<4; k++){
            int j = scanner.nextInt();  // read a number from the user
            if (q.eval(j)) {
                System.out.println("true");
            } else {
                System.out.println("false");
            }
        }
    }

    public static void main(String[] args) {
          question1();
        //question2();
        //question3();
    }
}