What is Garbage Collection?

Direct Answer

Garbage collection is automatic heap-memory management. A collector traces reachability from GC roots such as active thread frames, static fields, and JNI references, then may reclaim objects that are no longer strongly reachable. It removes explicit free from ordinary Java code, but retained references can still cause memory leaks and collection timing is not guaranteed.

Code Example

public class OrderService {
    private OrderRepository orderRepo;

    public void processOrders() {
        // Object created on heap
        var tempOrders = orderRepo.findAll();
        
        for (var order : tempOrders) {
            processOrder(order);
        }
        
        // tempOrders goes out of scope here
        // If no other references exist, eligible for GC
    }

    public void createTemporaryReport() {
        var report = new StringBuilder();
        report.append("Sales: ").append(calculateSales());
        
        // report object eligible for GC after method returns
        // GC will reclaim this memory automatically
    }
}

Common Mistakes & Pitfalls

• Treating GC as universal resource cleanup: GC reclaims eligible managed objects; files, sockets, database connections, and other owned resources still need explicit closing.
• Assuming immediate reclamation: Removing one reference may make an object eligible, but only when no strong path from a GC root remains. Collection timing is not guaranteed.
• Ignoring runtime cost: Collector work uses CPU, and stop-the-world phases suspend Java application threads in that JVM. Measure allocation, pauses, and application latency instead of assuming a fixed effect.