Immutability Lecture (Activity 1)

ImmutableDemo.java

import java.util.HashMap;

// This class is broken, follow the TODOs
public class ImmutableDemo {
    // TODO: How to prevent external actors from accessing the map directly?
    public HashMap<String,String> map;

    public ImmutableDemo() {
        this.map = new HashMap<String, String>();
        this.map.put("Important", "OKAY");
    }

    public boolean isOkay() {
        return this.map.get("Important").equals("OKAY");
    }

    public HashMap<String, String> getMap() {
        return map;
    }

    // TODO: How to fill out this method so that the test passes?
    public HashMap<String, String> getImmutableMap() {
        return map;
    }
}

ImmutableDemoTest.java

import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;

class ImmutableDemoTest {
    @Test
    void isOkayCantBeChanged_Wrong() {
        ImmutableDemo demo = new ImmutableDemo();
        assertTrue(demo.isOkay());

        // This is bad! The ImmutableDemo has been corrupted from the outside.
        // Hint: how can we get the same map out, but prevent changes from affecting it?
        demo.getMap().put("Important", "NOPE");
        assertFalse(demo.isOkay());
    }

    @Test
    void publicVariableChanged_Wrong() {
        ImmutableDemo demo = new ImmutableDemo();
        assertTrue(demo.isOkay());

        // This is bad! The ImmutableDemo has been corrupted from the outside.
        // Hint: once fixed, you will need to update this test to demo.getMap().clear();
        demo.map.clear();
        assertThrows(NullPointerException.class, demo::isOkay);
    }

    @Test
    void isOkayCantBeChanged_Fixed() {
        ImmutableDemo demo = new ImmutableDemo();
        assertTrue(demo.isOkay());

        demo.getImmutableMap().clear();
        assertTrue(demo.isOkay());
    }
}