In regards to these two important concepts in Barabbas, I will begin with the explanation of foreshadowing and end with the description of symbolism.  There are two important aspects of foreshadowing that we must discuss.  The first is the most specific.  Barabbas says the following about the day Jesus died on the Cross:

[I] had not wanted to come up here at all, for everything was unclean, full of contagion; if a man set foot in this potent and accursed place part of him would surely remain, and he could be forced back there never to leave it again.

This conveys a sure sense of foreshadowing: this feeling that Barabbas has on that fateful Good Friday.  He feels the place is "unclean."  He feels the place is full of "contagion."  He feels it is an "accursed place."  He feels that he will be "forced back there."  Still Barabbas CHOOSES to go to Golgotha and watch the events.  This foreshadows the depravity that follows in his own life which leads to his downfall.

In a more general sense, then, we could say that the beginning of the book foreshadows the end of the book.  Barabbas lacks the love and restoration and salvation that Christianity deems so important. 

No, the man said, looking past him with his empty gaze, the realm of the dead isn't anything. But to those who have been there, nothing else is anything either.

This depravity foreshadows his downfall at the end where Barabbas embraces the same dividing factors that he acknowledges at the beginning.  The city is engulfed in flames in the end.  Barabbas bears some of the blame, but renounces Christ when apprehended.  In fact, the hope at the end comes when Barabbas finally makes a surrender "to thee."  Now there is less division and more of a unity with Christian ideals.  This is the only hope for Barabbas.

Symbols are even more prominent in Barabbas than foreshadowing!  First, there are scars as symbols.  Many of the people Barabbas comes in contact with are scarred in some way.  Why?  Because all humanity is scarred!  The literal scars are symbols for the soul staring vacantly outward and wondering about existence.  The healing of the scars is one word:  love.  The symbol of the slave's badge is also important.  It symbolizes how the person is always slave to the earth; however, once that badge is marked with Christ,  happiness and freedom and hope are possible. Thus Christian symbols abound, focusing on hope and Christ, Christianity, and love.