👻 The Ghost of the Ancestor: A Memory Horror Story in Go, Java, and Rust

Satyadev Neti — Wed, 29 Apr 2026 07:02:58 +0000

In the world of high-performance programming, we are obsessed with Slices. We want to take a small piece of a large dataset without copying it. It’s fast. It’s efficient.

But it’s haunted.

Let’s look at how three of the world's most popular languages deal with the "Ghost of the Ancestor"—the phenomenon where a tiny "descendant" slice keeps a massive "ancestor" array alive in memory forever.

The Setup: Go’s "Hidden" Connection

In Go, a slice is just a small header pointing to a big array. Notice how scores2 is built from scores1.

package main
import "fmt"

func main() {
    // Original Go Logic
    scores := make([]int, 0, 5)
    scores1 := make([]int, 0, 5)
    scores2 := make([]int, 0, 5)

    scores = append(scores, 234, 242, 53, 42, 353)
    fmt.Println("Scores :", scores, "Len:", len(scores))

    scores1 = append(scores1, 234, 242, 53, 42, 353, 1111)
    fmt.Println("Scores1 :", scores1, "Len:", len(scores1))

    // Here Slices grow itself, but they still share an ancestor!
    scores2 = append(scores1, 234, 242, 53, 42, 353)
    fmt.Println("Scores2 :", scores2, "Len:", len(scores2))

    myscores := make([][]int, 0, 3)
    myscores = append(myscores, scores, scores1, scores2)
    fmt.Println("Slices of slices :", myscores, "Len:", len(myscores))
}

The Dialogue: When Languages Collide

Go: "Look at me! I’m lean. I’m fast. My slices are just headers—a pointer, a length, and a capacity. If I want to give you a piece of an array, I just point you to the memory. No copying, no overhead!"

Java: (Sips coffee) "That’s cute, Go. But you’re playing with fire. By sharing that 'underlying array,' you’re inviting side effects. In my world, we prefer objects. When you create an ArrayList from another, I usually copy the data. It’s safe. It’s clean. The Garbage Collector knows exactly when to kill an old list."

Go: "Safety is just another word for 'slow,' Java. My developers know what they’re doing... Wait, what’s this?"

Go points at Java’s standard library.

Go: "You have a subList() method, Java. If I take a subList of a 1GB ArrayList, does that 1GB stay in RAM even if the original list is 'deleted'?"

Java: (Sweating) "Well... technically... yes. The SubList is a nested class that holds a reference to the parent. So the GC can't collect the parent as long as the child lives. We call it 'Memory Pinning.'"

The Java "Ghost" Replicated:

package io.lagresearch.jgo;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class NoGhost {
    public static void main(String[] args) {
        // 1. Create independent ArrayLists
        // Equivalent to Go's: scores := make([]int, 0, 5)
        ArrayList<Integer> scores = new ArrayList<>(Arrays.asList(234, 242, 53, 42, 353));
        ArrayList<Integer> scores1 = new ArrayList<>(Arrays.asList(234, 242, 53, 42, 353, 1111));

        // 2. Creating scores2 as an independent copy of scores1
        // In your Go code: scores2 = append(scores1, 234...)
        ArrayList<Integer> scores2 = new ArrayList<>(scores1);
        scores2.addAll(Arrays.asList(234, 242, 53, 42, 353));

        System.out.println("Scores 1: " + scores1);
        System.out.println("Scores 2: " + scores2);

        // 3. Creating the "Slice of Slices" (ArrayList of ArrayLists)
        ArrayList<ArrayList<Integer>> myScores = new ArrayList<>();
        myScores.add(scores);
        myScores.add(scores1);
        myScores.add(scores2);

        System.out.println("My Scores (Nested): " + myScores);

        // --- THE "GHOST OF THE ANCESTOR" TEST ---

        // In Java, if we use subList, it behaves like a Go slice (a "view")
        List<Integer> ancestor = new ArrayList<>();
        for (int i = 0; i < 1000; i++) ancestor.add(i);

        // This 'descendant' is a VIEW. It keeps 'ancestor' alive in memory.
        List<Integer> descendant = ancestor.subList(0, 5);

        // Proof of "pointing": Change the ancestor, the descendant changes!
        ancestor.set(0, 999);
        System.out.println("Descendant index 0: " + descendant.get(0)); // Output: 999

    }

}

Enter: Rust (The Exorcist)

Rust: "Did someone say memory leaks? You two are arguing over who lets the ghost in more often. In my house, we don't have ghosts unless you explicitly summon them."

Go & Java: "Show-off. How do you handle it?"

Rust: "Ownership. If you want a slice, you 'borrow' it. While you're borrowing it, the Ancestor is locked. And if you try to delete the Ancestor while the Descendant is looking at it, I simply refuse to compile your program."

The Rust "Explicit Ghost":

fn main() {
    let mut scores: Vec<i32> = Vec::with_capacity(5);
    let mut scores1: Vec<i32> = Vec::with_capacity(5);
    let mut scores2: Vec<i32> = Vec::with_capacity(5);

    scores.extend([234, 242, 53, 42, 353]);
    println!("Scores  : {:?} | Len: {}", scores, scores.len());

    scores1.extend([234, 242, 53, 42, 353, 1111]);
    println!("Scores1 : {:?} | Len: {}", scores1, scores1.len());

    // scores2 built from scores1 (clone to avoid borrowing issues)
    scores2.extend_from_slice(&scores1);
    scores2.extend([234, 242, 53, 42, 353]);
    println!("Scores2 : {:?} | Len: {}", scores2, scores2.len());

    let myscores = vec![scores, scores1, scores2];

    println!("\nWe now have {} vectors inside myscores", myscores.len());
}

Visualising the Haunting

The Verdict

Go is the "Wild West": You get maximum speed and direct pointer control, but append can link your descendants to your ancestors forever. You must manually "cut the cord" (using the 3-index slice a[0:2:2]) to stop the Ghost from haunting your RAM.
Java is the "Gated Community": It feels safe with its "copy-by-default" approach, but subList is a hidden trapdoor. It creates a reference chain that can sink your heap by pinning massive parent arrays in memory.
Rust is the "High-Security Vault": No ghosts allowed by default. The compiler forces you to acknowledge a reference's existence through strict ownership and borrowing rules. To share state, you must be explicit—otherwise, you clone() and stay safe.

The Lesson: Every time you take a slice without copying, you aren't just taking data. You're taking a responsibility. Don't let your ancestors haunt your production servers. 👻

DEV Community: Satyadev Neti