Simple way to implement find(select) and collect block closures in java

Languages like ruby, groovy, smalltalk have language constructs for blocks and closures. Java 8 now has feature to evaluate lambda expressions. But here is one simple and straight way to implement the same:

In groovy :

[1, 2, 3, 10, 20, 30].findAll { num -> num > 5 }
=> yields a new list [10, 20, 30]

[ 1, 2, 3, 4 ].collect{ num -> num * num}
=> yields a new list [1, 4, 9, 16]

The above code for those more familiar with the smalltalk syntax will be:

#(1 2 3 10 20 30) select: [ :num | num > 5 ]
==> displays [10, 20, 30]

#( 1 2 3 4 ) collect: [:i | i * i ].
==> displays [1, 4, 9, 16]

In java, the same could be implemented with a set of abstract classes as a collection utility that mandate implementation of method which returns the boolean condition to add or transform the element under iteration :

Find.java

abstract class Find extends ArrayList{ 

    /** let the default constructor worry about iteration **/

    public Find(List list){    

        Iterator it = list.iterator();

        while(it.hasNext()){

            Object element = it.next();        

            if(all(element))           

                this.add(element);         

        }

    }

     

    /** This defines the boolean condition for find/select **/

    public abstract Boolean all(Object obj);

     

}

FindDemo.java

// find all elements in the list that are greater than 5

List list = Arrays.asList(1, 2, 3, 10, 20, 30);

selected = new Find(list) {

    public Boolean all(Object num) {               

        return (int) num > 5;

    }

};

// => yields a new list [10, 20, 30]

Collect.java

abstract class Collect extends ArrayList{  

    /** let the default constructor worry about iteration **/

    public Collect(List list){

        Iterator it = list.iterator();

        while(it.hasNext()){

            Object element = it.next();

            Object transformed = transform(element);

            this.add(transformed);

        }

    }

    /** This defines the transformation logic for collect **/

    public abstract Object transform(Object obj);

}

CollectDemo.java

// collect the squares of all elements in the list

List list = Arrays.asList(1, 2, 3, 4);

collected = new Collect(list) {    

    public Object transform(Object num) {

            return (int) num * (int) num;

    }

};

// => yields a new list [1, 4, 9, 16]

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