In dealing with death, this quote from Forster's novel has application/relvance to Salinger's novel:
“But it struck him that people are not really dead until they are felt to be dead. As long as there is some misunderstanding about them, they possess a sort of immortality.”
In Salinger's novel, Holden remains attached to his dead brother Allie and, to some extent, fixated by him. Holden keeps Allie's memory alive in his thinking, speaking to Allie sometimes. When Holden writes an essay for Stradlater, he chooses Allie's baseball glove as the subject of the essay. When Holden feels as if he is disappearing on the sidewalk, he calls on Allie to save him.
This connection certainly relates to the quote from Forster's novel. Holden's grief over his brother seems to keep him in a state of mourning or psychological suppression. Until Holden comes to terms with the loss of his brother, he will probably continue to fixate on the past; on innocence (and its inevitable loss).
We can take this dynamic to be one of the underlying themes of Holden's character and the (potential) source of much of his conflict and turmoil.