Habit of reading source code

Reading code can be very enlightening but I have been following this practice only recently.One might come across nuggets that explain a particular code pattern clearly when one reads code.
Twitter has open-sourced its ‘Commons’ library and I came across the Curiously Recurring Generic Pattern and the Self-bounding pattern in it. These two are discussed in ‘Thinking in Java’ by Bruce Eckel. He also gives an exercise at the end of Page 709.

Exercise 34: Create a self-bounded generic type that contains an
abstract method that takes an argument of the generic type parameter and
produces a return value of the generic type parameter. In a non-abstract
method of the class, call the abstract method and return its result. Inherit
from the self-bounded type and test the resulting class

The code for this exercise could be

public abstract class SelfBound< T extends SelfBound<T>>{

    public abstract T get( T t );

    public T getValue( T t){
         return get( t );

public class SubSelfBound extends SelfBound{

    public SubSelfBound get(SubSelfBound subSelfBound) {
        return this;

    public static void  main( String[] argv ){

        SubSelfBound subSelfBound = new  SubSelfBound();


    private void test() {

        SubSelfBound subSelfBound = new  SubSelfBound();

        subSelfBound =  getValue( subSelfBound );//The sub-class's inherited method is forced
                                                 //to take only the derived type as parameter
                                                 //and not the base type. The parameter varies
                                                 //and matches the derived type( Covariant )
        SelfBound selfBound =  getValue( subSelfBound );//It also returns the super type.
                                                        //This is related to the covariant
                                                        //return types according to the book.


Even though I might not have the absolutely correct solution here the idea is clear and it is that the Curiously Recurring Generic Pattern and the Self-bounding pattern are similar and the Self-bounding pattern permits a type of covariant argument types.

I used to think only covariant return types are possible in J2SE 5 but there seems to be a way to implement covariant argument types also though the practical applications of this concept could stupify the developers because not many are
used to advanced generics.

I understood all of this by reading the Twitter Commons source code that has used self-bounding types like this.

public interface Unit<U extends Unit<U>>;{

  double multiplier();

If one is not used to thinking about Java generics deeply this type of code will be
hard to understand and implement.

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