Playing with closures in Java7. Thanks to Prof. Grossman.

1.Func.java

package com.fs;

/**
 * This interface is responsible for holding the closures when it comes to map.
 * It uses two generic types. One for the argument and one for the return type.
 * @param <B> Generic type 
 * @param <A> Generic type
 */
public interface Func<B,A> {
    /**
     * Function prototype m takes an argument of type A and returns a type B.
     * A map operation can produce a different type.
     * @param x of type A
     * @return type B
     */
     B m(A x); 
}

2.Pred.java

package com.fs;


/**
 * This interface is responsible for holding the closures when it comes to filter.
 * @param <A> Generic type 
 */
public interface Pred<A> {
    /**
     * Function prototype m takes an argument of type A and returns a boolean.
     * A filter operation checks every element if it fits a predicate.
     * @param x of type A
     * @return boolean
     */
    boolean m(A x);
}

3.CList.java

package com.impl.list;

import com.fs.*;
import java.util.Stack;

public class CList<T> {
    T head;
    CList<T> tail;
    
    public CList(T x, CList<T> xs){
        head = x;
        tail = xs;
    }
    
    @Override
    public String toString(){
        StringBuilder sb = new StringBuilder();
        CList<T> xs = this;
        while(xs != null){
            sb.append(xs.head.toString()).append(" ");
            xs=xs.tail;
        }
        return sb.toString();
    }
    
    static <A,B> CList<B> map(Func<B,A> f, CList<A> xs){
        if(xs==null){
            return null;
        }
        return new CList<>(f.m(xs.head),map(f,xs.tail));
    }
    
    static <A,B> CList<B> maploop(Func<B,A> f, CList<A> xs){
        Stack<CList<A>> stack = new Stack<>();
        while(xs != null){
            stack.push(xs);
            xs = xs.tail;
        }
        CList<B> result = null;
        while(!stack.empty()){
            xs = stack.pop();
            result = new CList<>(f.m(xs.head),result);
        }
        return result;
    }
    
    static <A> CList<A> filter(Pred<A> f, CList<A> xs){
        if(xs == null){
            return null;
        }
        if(f.m(xs.head)){
            return new CList<>(xs.head, filter(f,xs.tail));
        }
        return filter(f,xs.tail);
    }
    
    static <A> int length(CList<A> xs){
        int ans =0;
        while(xs!= null){
            ++ans;
            xs=xs.tail;
        }
        return ans;
    }
}

4.CListClient.java

package com.impl.list;

import com.fs.Func;
import com.fs.Pred;

public class CListClient {
    public static CList<Integer> doubleAll(CList<Integer> xs){
        Func<Integer, Integer> df = new Func<Integer, Integer>() {
            @Override
            public Integer m(Integer x) {
                return x * 2;
            }
        };
        
        return CList.map(df, xs);
    }
    
    public static int countNs(CList<Integer> xs,final int n){
        Pred<Integer> pf = new Pred<Integer>() {
            @Override
            public boolean m(Integer x) {
                return x==n;
            }
        };
        return CList.length(CList.filter(pf, xs));
    }
    
    public static CList<Integer> doubleAllloop(CList<Integer> xs){
        Func<Integer, Integer> df = new Func<Integer, Integer>() {
            @Override
            public Integer m(Integer x) {
                return x * 2;
            }
        };
        
        return CList.maploop(df, xs);
    }
}

5.ListTester.java

package basic;


import com.impl.list.CList;
import com.impl.list.CListClient;
import org.junit.Test;


public class ListTester {
    
    CList<Integer> intlist_1 = new CList<>(new Integer(1),null);
    CList<Integer> intlist_2 = new CList<>(new Integer(2),intlist_1);
    CList<Integer> intlist_3 = new CList<>(new Integer(3),intlist_2);
    CList<Integer> intlist_4 = new CList<>(new Integer(4),intlist_3);
    CList<Integer> intlist_5 = new CList<>(new Integer(4),intlist_4);
    CList<Integer> intlist = new CList<>(new Integer(5),intlist_5);
            
    @Test
    public void test_doubleAll(){
        CList<Integer> doubled = CListClient.doubleAll(intlist);
        System.out.println("doubled : " + doubled.toString());
        CList<Integer> doubledloop = CListClient.doubleAllloop(intlist);
        System.out.println("doubledloop : " + doubledloop.toString());
    }
    
    @Test
    public void test_CountNs(){
        int count3s = CListClient.countNs(intlist, 3);
        System.out.println("count3s : " + count3s);
    }
}