LiquidJava Tutorial

Made by Ricardo Costa and Catarina Gamboa

What are Liquid Types?

Programs go wrong. Developers make mistakes. What if we could catch more bugs before the program even runs? Traditional type systems help us achieve this. For example, strongly typed programming languages, like Java, don't allow us to assign a string to an integer variable. But what if we could have stronger guarantees? What if we want an integer variable to always be positive? Or within a certain range?

Liquid types make this possible. They extend traditional type systems by allowing us to refine types with logical predicates. This means that we can specify more precise properties about our programs. This allows us to catch errors earlier in the development process. These include division by zero, array out-of-bounds access and protocol violations.

Let's look at an example. Consider the following Java code:

public class Example {

    public static int divide(int a, int b) {
        return a / b;
    }

    public static void example() {
        int result = divide(2, 0); // division by zero exception at runtime!
    }
}

Wouldn't it be great to specify that the parameter b of the divide method should never be zero, in the same way that it specifies that it should never be a String?

Introducing LiquidJava

LiquidJava is an implementation of a liquid type checker for Java. It allows developers to add specifications to their code using Java annotations.

Let's see how we can improve the previous code using LiquidJava:

public class Example {

    public static int divide(int a, @Refinement("b != 0") int b) {
        return a / b;
    }

    public static void example() {
        int result = divide(2, 0); // compile error! b cannot be zero!
    }
}

Notice that the method divide now requires the parameter b to be non-zero. If we try to call divide with a zero value for b, LiquidJava will raise a type error at compile time, preventing the division by zero exception.

Getting Started with LiquidJava

To follow along with this tutorial, make sure you have the following installed:

• Visual Studio Code, for editing the code and using the LiquidJava extension
• Java Extension Pack by Red Hat, which provides Java support in VS Code
• LiquidJava Extension, which provides the LiquidJava typechecker with real-time error reporting and syntax highlighting for the refinements.

Don't forget to star LiquidJava and its extension on GitHub! ⭐

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Tutorial

1. Basic Refinements

First of all, let's explore how basic refinements work in LiquidJava.

Open RefinementsExample.java

In the main method, you can find four variables, positive, nonzero, percentage and direction, with three of them containing comments with the refinements that should be used in each one. Notice that _ can also be used as a placeholder for the variable name in the refinement expression, as shown in the refinement for the percentage variable.

Now, one by one, uncomment the @Refinement annotations and observe each error reported. Remember that only one can be shown at a time. Then, change each value to satisfy the corresponding refinement — change the value of positive to a positive integer, the value of nonzero to any non-zero integer, and the value of percentage to an integer between 0 and 100.

Then, add the refinement for the direction variable, which should ensure its value is always either -1 or 1 (you should use the || operator for this). After that, change its value to satisfy the refinement.

As demonstrated previously, we can also refine method parameters and return values.

Open MethodRefinementExample.java.

Here, the method divide is refined to ensure that the parameter b is never zero.

Try changing the value of the second argument in the divide method call to zero and observe the error reported by LiquidJava.

But wait, we can also refine the return value of the method! Let's introduce a bug.

Change the implementation of the divide method to return a - b instead of a / b.

No error! Let's fix this.

Above of the method signature, add the following refinement to specify that the return value should be exactly equal to the division of a by b:

@Refinement("_ == a / b")

Now we get an error! LiquidJava is telling us that the implementation of the method does not satisfy the refinement specified for the return value. This way, we can catch errors not only in the inputs of the methods, but also in their outputs.

Exercise

Open Bank.java.

This class simulates a simple bank account with two methods: deposit and withdraw. In the main method, we simulate a wrong usage of the deposit and withdraw methods of a bank account, since it tries to withdraw more money than the current balance. Let's make use of LiquidJava refinements to ensure the correct usage of these methods.

Replace the "true" refinements with the appropriate ones to ensure the correct behavior of both methods.

• The balance parameter of both methods should be non-negative.
• The amount parameter of the deposit method should be greater than zero.
• The amount parameter of the withdraw method should be greater than zero and less than or equal to the balance.
• The return value of the deposit method should be equal to the sum of balance and amount.
• The return value of the withdraw method should be equal to the difference between balance and amount.

With the correct refinements in place, LiquidJava will report an error in the withdraw method call, since it tries to withdraw more money than it was deposited.

Modify the withdraw method call to withdraw 10 or less to fix the error.

However, notice that we are repeating the same refinement twice in the balance parameter of both methods. For this, we can use a refinement aliases to define commonly used refinements and avoid repetition.

Add the following lines of code above the class definition:

import liquidjava.specification.RefinementAlias;

@RefinementAlias("NonNegative(int v) { v >= 0 }")

Then, replace all occurrences of @Refinement("_ >= 0") with @Refinement("NonNegative(_)").

The refinements are now easier to understand, while still providing the same guarantees!

2. State Refinements

Let's explore how to use state refinements to specify and verify properties about the state of an object.

Open LightBulb.java.

Here, we specify that this object can only be in two states: on or off. Then, in the constructor, we specify that the initial state is off, through the @StateRefinement annotation. This annotation allows us to specify in which state the object should be before the method is called (from), and in which state it will be after the method execution (to). In the constructor, since it's the first method to be called, we can only specify the to state.

This object has two methods, turnOn and turnOff. From the state refinements, we can see that the method turnOn can only be called when the object is in state off transiting to state on. Similarly, the method turnOff can only be called when the object is in state on, transiting to state off. This means that we cannot call the same method twice in a row, since it would violate the protocol established by the state refinements. The following DFA illustrates this:

Uncomment line 22 to observe the error.

Exercise

Open MediaPlayer.java. Your task is to replace the "true" refinements with the appropriate ones to ensure the correct behavior of the play, pause, resume and stop methods, using the stopped, playing, and paused states.

For example, we want to ensure that the pause method can only be called when the player is playing, and that the stop method can only be called when the player is not stopped (you can either use the ! or the || operator for this). The state transitions are represented by the following DFA:

If you get stuck, here are some hints:

• Follow the diagram carefully
• For each edge in the diagram, identify the method that causes that transition and the source and target states
• If a method is allowed from multiple source states, use the || operator to combine them
• Don't forget the (this) after each state name, since states are always associated with an object instance

With the correct implementation, LiquidJava will report an error in line 30, since we are trying to resume playback when the player is stopped.

3. External Refinements

To demonstrate the state refinements in a real world scenario, let's learn about external refinements. LiquidJava allows us to refine external classes, such as classes from the Java standard library. For instance, we can refine the Socket class to ensure its correct usage.

Open SocketRefinements.java.

Here, we refine the Socket class through state refinements, with the possible states being unconnected, bound, connected, and closed. Then, for each method, we specify the allowed state transitions. This way, we can ensure that, for example, the connect method can only be called after the bind method, and that the close method can only be called once. The following DFA details the allowed state transitions:

Open SocketExample.java.

Here, we see a simple usage of the Socket class. If you comment out the line 9 containing with the bind method call, LiquidJava will report an error in the connect method call, since it violates the state refinement specified for the Socket class! Notice that when using the Socket class, we don't need to deal with any refinement annotations, since they are already specified in the external refinement interface.

Exercise

Let's refine another external class.

Open ReentrantLockRefinements.java. Your task is to replace the "true" refinements with the appropriate ones to refine the ReentrantLock class.

We want to ensure that the lock method can only be called in the unlocked state, and that the unlock method can only be called in the locked state. These transitions are represented by the following DFA:

With the correct implementation, LiquidJava will report an error in line 10 of ReentrantLockExample.java, since we are trying to unlock a lock that is not locked.

4. Ghost Variables

Finally, LiquidJava also offers a way to model objects using ghost variables through the @Ghost annotation, which are used to track additional information about the program's state when states aren't enough. These can be, for instance, counters (integers) or flags (booleans), to model more complex protocols.

Open ArrayListRefinements.java.

Here, we define the refinements for the ArrayList class, using a ghost variable size to keep track of the number of elements in the list. Using the size ghost variable in state refinements, we can prevent out-of-bounds access.

In the constructor, we specify that after it is called, the ghost variable size will be equal to 0. This is optional since its default value is already zero, but it helps us understand this example. Then, in the add method, we specify that it can be called in any state (since we don't specify a from state), and that after it is called, the size ghost variable will be incremented by one — the new size will be equal to the old size plus one (old is a special keyword that refers to the previous state of the object, so calling size(old(this)) gets the value of size before the method was called). Finally, in the get method, we specify that the index parameter must be non-negative and less than the current size of the list, therefore preventing out-of-bounds errors.

Open ArrayListExample.java.

Here, we can see a simple usage of the refined ArrayList class. If you uncomment line 11, LiquidJava will report an error, since we are trying to access an index that is out of bounds!

Exercise

Let's do the same but for the Stack class.

Open StackRefinements.java. Your task is to refine the Stack class by replacing the "true" refinements with the appropriate ones to ensure the correct behavior of the push, pop and peek methods, using the count ghost variable to keep track of the number of elements in the stack, and not allow incorrect uses of these methods — popping or peeking from an empty stack.

If you get stuck, here are some hints:

• You may find it useful to look at the previous example for reference
• The predicates must be boolean expressions
• You should use the old keyword to refer to the previous state of the object
• You should use the count ghost variable in all refinements

With the correct implementation, LiquidJava will report an error in line 11 of StackExample.java, since we are trying to pop an element of the stack when it is empty.

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Quiz

Test your knowledge about LiquidJava by taking a simple quiz available here. Think you can get a perfect score?

If you encounter any problems or have any questions, feel free to send an email to:

• Ricardo Costa
• Catarina Gamboa

References

• LiquidJava Website
• LiquidJava Repository
• LiquidJava VS Code Extension Repository
• LiquidJava Examples Repository
• LiquidJava External Libraries Repository