How to write a piece of java code that will consume lots of memories and the memories can be released if needed

Question

I wrote such a piece of code:

public class TestClass {
    private static List<String> stringList = new ArrayList<>();
    long num;

    public void testHighLoad() {

        stringList.clear();
        for (int i = 0; i < num; i++) {         
            String string = getRandomString(10000);
            stringList.add(string);
        }
    }

    public void setNum (int num) {
        this.num = num;
    }
 }

And I can call it like this to make it cost high memories:

testClass.setNum(500000);
testClass.testHighLoad();

But then if I call it in this way, it cannot release the memory:

testClass.setNum(0);
testClass.testHighLoad();