System.gc

Java runs in a virtual machine. Memory is allocated, and freed, without any programmer intervention. This makes programs easier to develop.

Rarely, a programmer may want to force a garbage collection. This is usually a bad idea. But it can sometimes help, even in just debugging a program.

An example program

This program mainly is used to test the Runtime.getRuntime, freeMemory, and gc() methods. So it is not a useful program in other ways.

public class Program {

    public static void main(String[] args) {

        long total = Runtime.getRuntime().freeMemory();

        // Allocate an array and ensure it is used by the program.
        int[] array = new int[1000000];
        array[0] = 1;
        if (array[0] == 3) {
            return;
        }
        long total2 = Runtime.getRuntime().freeMemory();

        // Collect the garbage.
        System.gc();

        long total3 = Runtime.getRuntime().freeMemory();

        // Display our memory sizes.
        System.out.println("BEFORE:" + total);
        System.out.println("DURING:" + total2);
        System.out.println("AFTER:" + total3);
        System.out.println("CHANGE AFTER GC: " + (total2 - total3));
    }
}
BEFORE:125535480
DURING:121535464
AFTER:122580096
CHANGE AFTER GC: -1044632

Notes, System.gc

The gc() method is invoked in the above program. It causes the free memory to increase by about 1 MB. This is because the int array is freed.

Notes, avoiding GC

Basically, calling System.gc is a bad idea. To optimize a program to run more efficiently, you can try to avoid object allocations in the first place.

A final note

The freeMemory() method is useful in real programs and benchmarks. It can help inform us how much memory we are using (and wasting).