A Different Builder Pattern Example in Java

Builder design pattern is often used to separate the construction of a complex object from its various representations. I will show you a nicer way of writing a Builder pattern, rather than the example shown in the wiki.

First of all, I created a POJO called Vehicle, which has some simple properties about its model, its make, etc. Then, nested within this POJO, I created a Builder that holds some default properties which can be used to build a vehicle. The builder can be created by a static method newBuilder(), and we can override the vehicle properties by chaining the with* methods, and finally return the constructed vehicle using the build() method.

public class Vehicle {

    private String model;
    private String make;
    private int numberOfWheels;
    private double height;
    private double width;
    private double length;
    private int numberOfDoors;

    private Vehicle(String model, String make, int numberOfWheels, double height, double width, double length, int numberOfDoors) {
        this.model = model;
        this.make = make;
        this.numberOfWheels = numberOfWheels;
        this.height = height;
        this.width = width;
        this.length = length;
        this.numberOfDoors = numberOfDoors;
    }

    public static class Builder {

        private String model = "M5";
        private String make = "BMW";
        private int numberOfWheels = 4;
        private double height = 1.45;
        private double width = 1.9;
        private double length = 4.62;
        private int numberOfDoors = 5;

        public static Builder newBuilder() {
            return new Builder();
        }

        public Builder withModel(String model) {
            this.model = model;
            return this;
        }

        public Builder withMake(String make) {
            this.make = make;
            return this;
        }

        public Vehicle build() {
            return new Vehicle(model, make, numberOfWheels, height, width,length, numberOfDoors);
        }
    }
}

Let me use a unit test, written in Groovy, to demonstrate exactly how we use the builder. I created a test to show how we can build a default vehicle without providing any overrides to the default vehicle properties, and another test to show how we can override the default vehicle properties to create a different vehicle.

import org.junit.Test

class VehicleTest {

    @Test
    public void canBuildADefaultVehicle() {
        def defaultVehicle = Vehicle.Builder.newBuilder().build()
        assert defaultVehicle.model == "M5"
        assert defaultVehicle.make == "BMW"
        assert defaultVehicle.numberOfWheels == 4
        assert defaultVehicle.height == 1.45
        assert defaultVehicle.width == 1.9
        assert defaultVehicle.length == 4.62
        assert defaultVehicle.numberOfDoors == 5
    }

    @Test
    public void canBuildADifferentVehicle() {
        def defaultVehicle = Vehicle.Builder.newBuilder().withModel("M3").build()
        assert defaultVehicle.model == "M3"
        assert defaultVehicle.make == "BMW"
        assert defaultVehicle.numberOfWheels == 4
        assert defaultVehicle.height == 1.45
        assert defaultVehicle.width == 1.9
        assert defaultVehicle.length == 4.62
        assert defaultVehicle.numberOfDoors == 5
    }
}

Some says the chaining syntax makes the building block easier to read. :)

Solving Farmer-Wolf-Goat-Cabbage riddle with Groovy/Java

The Riddle - Farmer Wolf Goat Cabbage

A farmer is on the west bank of a river with a wolf, a goat and a cabbage in his care. Without his presence the wolf would eat the goat or the goat would eat the cabbage. The wolf is not interested in the cabbage. The farmer wishes to bring his three charges across the river. However the boat available to him can only carry one of the wolf, goat and cabbage besides himself. The puzzle is: is there a sequence of river crossings so that the farmer can transfer himself and the other three all intact to the east bank?

The Solution

It is a task that given an initial state, a final state, and a set of rules, one will start from the initial state and try to reach to the final state by passing through some intermediate states. Each move will transform from one state to the other, and there might be multiple valid moves from a given state. Such a collection of interconnected states can be represented by State Space Graph.
Let's use 'f', 'c', 'g', 'w' to denote the farmer, the cabbage, the goat, and the wolf, and use '|' to separate the river where left of the '|' denotes west bank and right of the '|' denotes east bank. Initially, they are all at the west bank of the river, which is represented as 'fcgw |' as shown below. We can solve the riddle by figuring out what the possible and valid moves are, using either Breadth-First Search or Depth-First Search, on a state space graph shown below

 

Depth First Search

I use DFS to find the first possible solution to the riddle, where it looks like this

or with a possibility of backtracking like this

Breadth First Search

I can build a complete state space graph using BFS, excluding the red states as shown below

Sample Source Code

Implemented in a mix of Groovy and Java, where Java is mainly used for the POJOs, the sample source code, including the unit test, demonstrates how to build a complete state space graph (which is also a directed graph) and how to find all possible solutions based on the state space graph.
DirectedGraph.java

import java.util.*;

public class DirectedGraph<T> implements Iterable<T> {

    // key is a Node, value is a set of Nodes connected by outgoing edges from the key
    private final Map<T, Set<T>> graph = new HashMap<T, Set<T>>();

    public boolean addNode(T node) {
        if (graph.containsKey(node)) {
            return false;
        }

        graph.put(node, new HashSet<T>());
        return true;
    }

    public void addNodes(Collection<T> nodes) {
        for (T node : nodes) {
            addNode(node);
        }
    }

    public void addEdge(T src, T dest) {
        validateSourceAndDestinationNodes(src, dest);

        // Add the edge by adding the dest node into the outgoing edges
        graph.get(src).add(dest);
    }

    public void removeEdge(T src, T dest) {
        validateSourceAndDestinationNodes(src, dest);

        graph.get(src).remove(dest);
    }

    public boolean edgeExists(T src, T dest) {
        validateSourceAndDestinationNodes(src, dest);

        return graph.get(src).contains(dest);
    }

    public Set<T> edgesFrom(T node) {
        // Check that the node exists.
        Set<T> edges = graph.get(node);
        if (edges == null)
            throw new NoSuchElementException("Source node does not exist.");

        return Collections.unmodifiableSet(edges);
    }

    public Iterator<T> iterator() {
        return graph.keySet().iterator();
    }

    public int size() {
        return graph.size();
    }

    public boolean isEmpty() {
        return graph.isEmpty();
    }

    private void validateSourceAndDestinationNodes(T src, T dest) {
        // Confirm both endpoints exist
        if (!graph.containsKey(src) || !graph.containsKey(dest))
            throw new NoSuchElementException("Both nodes must be in the graph.");
    }

}

BoatLocation.java

public enum BoatLocation {
    WestBank, EastBank
}

RiverRole.java, implemented Comparable interface in order to be used in a SortedSet

public class RiverRole implements Comparable<RiverRole> {

    public final String name;
    public final boolean canSailTheBoat;

    public RiverRole(String name, boolean canSailTheBoat) {
        this.name = name;
        this.canSailTheBoat = canSailTheBoat;
    }

    @Override
    public int compareTo(RiverRole o) {
        return this.name.compareTo(o.name);
    }

    @Override
    public String toString() {
        return name;
    }
}

RiverState.java

import java.util.Collection;
import java.util.SortedSet;
import java.util.TreeSet;

public class RiverState {

    private SortedSet<RiverRole> westBank;
    private SortedSet<RiverRole> eastBank;
    private BoatLocation boatLocation;

    public RiverState(Collection<RiverRole> westBank, Collection<RiverRole> eastBank, BoatLocation boatLocation) {
        this.westBank = new TreeSet<RiverRole>(westBank);
        this.eastBank = new TreeSet<RiverRole>(eastBank);
        this.boatLocation = boatLocation;
    }

    public SortedSet<RiverRole> getWestBank() {
        return new TreeSet<RiverRole>(westBank);
    }

    public SortedSet<RiverRole> getEastBank() {
        return new TreeSet<RiverRole>(eastBank);
    }

    public BoatLocation getBoatLocation() {
        return boatLocation;
    }

    @Override
    public String toString() {
        return "State{westBank=" + westBank + ", eastBank=" + eastBank + ", boatLocation=" + boatLocation + "}";
    }

    public boolean equals(Object o) {
        if (this == o) return true;
        if (!(o instanceof RiverState)) return false;

        RiverState riverState = (RiverState) o;

        return eastBank.equals(riverState.eastBank) &&
               westBank.equals(riverState.westBank) &&
               boatLocation.equals(riverState.boatLocation);
    }

    public int hashCode() {
        int result;
        result = westBank.hashCode();
        result = 31 * result + eastBank.hashCode();
        result = 31 * result + boatLocation.hashCode();
        return result;
    }
}

FarmerWolfGoatRiddle.groovy

import graph.DirectedGraph
import org.codehaus.groovy.runtime.MethodClosure

import static fun.rivercrossing.BoatLocation.EastBank
import static fun.rivercrossing.BoatLocation.WestBank

public class FarmerWolfGoatRiddle {

    /*  Farmer Wolf Goat Puzzle
        A farmer is on the east bank of a river with a wolf, goat and cabbage in his care. Without his presence the wolf would eat
        the goat or the goat would eat the cabbage. The wolf is not interested in the cabbage. The farmer wishes to bring his three
        charges across the river. However the boat available to him can only carry one of the wolf, goat and cabbage besides
        himself. The puzzle is: is there a sequence of river crossings so that the farmer can transfer himself and the other three all
        intact to the west bank?
    */

    RiverRole farmer = new RiverRole("farmer", true);
    RiverRole wolf = new RiverRole("wolf", false);
    RiverRole goat = new RiverRole("goat", false);
    RiverRole cabbage = new RiverRole("cabbage", false);

    RiverState initialState = new RiverState([farmer, wolf, goat, cabbage], [], BoatLocation.WestBank)
    RiverState finalState = new RiverState([], [farmer, wolf, goat, cabbage], EastBank)

    Closure<Boolean> areRolesHarmonisedOnRiverBank = { Collection<RiverRole> roles ->
        // describe clearly what the rules are
        if (!roles.contains(farmer)) {
            if (roles.contains(wolf) && roles.contains(goat)) return false
            if (roles.contains(goat) && roles.contains(cabbage)) return false
        }
        return true
    }


    public Collection<Deque<RiverState>> findAllPossibleSolutions() {
        DirectedGraph<RiverState> graph = buildCompleteGraph()

        Collection<Deque<RiverState>> solutions = []
        Deque<RiverState> currentSolution = new ArrayDeque<RiverState>()

        explore(graph, initialState, currentSolution, solutions)

        return solutions
    }

    // using Depth First Search (Stack Implementation) to identify a solution
    private void explore(DirectedGraph<RiverState> graph, RiverState currentNode, Deque<RiverState> currentSolution, Collection<Deque<RiverState>> solutions) {
        currentSolution.push(currentNode)
        graph.edgesFrom(currentNode).each { RiverState node ->
            if (node == finalState) {
                // add the current solution, including the final node, to the solutions
                currentSolution.push(node)
                solutions.add(new ArrayDeque<RiverState>(currentSolution))
                currentSolution.pop()
            } else {
                // recursively explore
                explore(graph, node, currentSolution, solutions)
            }
        }
        currentSolution.pop()
    }

    // using Breath First Search to build the complete graph
    private DirectedGraph<RiverState> buildCompleteGraph() {
        DirectedGraph<RiverState> graph = new DirectedGraph<RiverState>()
        graph.addNode(initialState)

        Set<RiverState> visitedNodes = new HashSet<RiverState>()

        // use a queue to keep the states to be visited in sequence
        Deque<RiverState> currentStates = new ArrayDeque<RiverState>()
        currentStates.add(initialState)

        while (!currentStates.isEmpty()) {
            if (currentStates.peek() != finalState) {
                def currentState = currentStates.removeFirst()
                if (!visitedNodes.contains(currentState)) {
                    visitedNodes.add(currentState)
                    def possibleStates = findNextPossibleStates(currentState)
                    def validStates = possibleStates.findAll { RiverState possibleState ->
                        // matched state should not be one of the visited states, and it should pass the validations
                        return !visitedNodes.contains(possibleState) && passRuleValidations(possibleState)
                    }
                    // add successors and edges
                    graph.addNodes(validStates)
                    validStates.each { graph.addEdge(currentState, it) }

                    // add to current states so that we can visit them next
                    currentStates.addAll(validStates)
                }
            } else {
                currentStates.removeFirst() // remove the final state from the currentStates queue
            }
        }

        return graph
    }

    private boolean passRuleValidations(RiverState state) {
        return areRolesHarmonisedOnRiverBank(state.eastBank) && areRolesHarmonisedOnRiverBank(state.westBank)
    }

    private List<RiverState> findNextPossibleStates(RiverState currentState) {
        // from the last visited state, find out who can sail the boat, and where is the sailor
        List<RiverState> possibleStates = []

        transitFrom(currentState, possibleStates)

        return possibleStates
    }

    private void transitFrom(RiverState currentState, List<RiverState> possibleStates) {
        // check where the boat is
        BoatLocation boatOrigin = currentState.boatLocation
        BoatLocation boatDestination

        // we use method closure to get a copy of west bank or east bank roles
        MethodClosure origin, destination

        // multiple assignments
        (origin, destination, boatDestination) = boatOrigin == WestBank ?
            [currentState.&getWestBank, currentState.&getEastBank, EastBank] :
            [currentState.&getEastBank, currentState.&getWestBank, WestBank]

        def possibleSailors = origin().findAll { RiverRole role -> role.canSailTheBoat }

        possibleSailors?.each { RiverRole sailor ->
            // pick 0 passengers onto the boat
            Collection<RiverRole> onBoat = [sailor]
            possibleStates.add(createNextPossibleState(origin, destination, onBoat, boatDestination))

            Collection<RiverRole> possiblePassengers = origin() as Collection<RiverRole>
            possiblePassengers.remove(sailor)

            // pick 1 passenger onto the boat
            possiblePassengers.each { RiverRole passenger ->
                onBoat = [passenger, sailor]
                possibleStates.add(createNextPossibleState(origin, destination, onBoat, boatDestination))
            }
        }
    }

    private RiverState createNextPossibleState(MethodClosure origin, MethodClosure destination, Collection<RiverRole> onBoat, BoatLocation boatDestination) {
        Collection<RiverRole> newDestination = destination() as Collection<RiverRole>
        Collection<RiverRole> newOrigin = origin() as Collection<RiverRole>

        // boat transits from origin to destination
        newOrigin.removeAll(onBoat)
        newDestination.addAll(onBoat)

        return boatDestination == WestBank ? new RiverState(newDestination, newOrigin, boatDestination) : new RiverState(newOrigin, newDestination, boatDestination)
    }


}

FarmerWolfGoatRiddleTest.groovy

import org.junit.Test

import static fun.rivercrossing.BoatLocation.EastBank
import static fun.rivercrossing.BoatLocation.WestBank


class FarmerWolfGoatRiddleTest {

    FarmerWolfGoatRiddle riddle = new FarmerWolfGoatRiddle()

    @Test
    public void findAllPossibleSolutions() {
        def solutions = riddle.findAllPossibleSolutions()
        // the ordering of solutions will be random
        assert solutions.size() == 2
        assert solutions[0].size() == 8
        assert solutions[1].size() == 8

        def assertAllSolutions = { Deque<RiverState> solution1, Deque<RiverState> solution2 ->
            assertFirstSolution(solution1)
            assertSecondSolution(solution2)
        }

        // assert with the correct expectations regardless of which is the first solution
        if (solutions[0].contains(new RiverState([riddle.wolf], [riddle.farmer, riddle.goat, riddle.cabbage], EastBank))) {
            assertAllSolutions(solutions[0], solutions[1])
        } else {
            assertAllSolutions(solutions[1], solutions[0])
        }
    }

    private void assertFirstSolution(Deque<RiverState> solution) {
        assert solution.poll() == new RiverState([riddle.farmer, riddle.wolf, riddle.goat, riddle.cabbage], [], WestBank)
        assert solution.poll() == new RiverState([riddle.wolf, riddle.cabbage], [riddle.farmer, riddle.goat], EastBank)
        assert solution.poll() == new RiverState([riddle.farmer, riddle.wolf, riddle.cabbage], [riddle.goat], WestBank)
        assert solution.poll() == new RiverState([riddle.wolf], [riddle.farmer, riddle.goat, riddle.cabbage], EastBank)
        assert solution.poll() == new RiverState([riddle.farmer, riddle.wolf, riddle.goat], [riddle.cabbage], WestBank)
        assert solution.poll() == new RiverState([riddle.goat], [riddle.farmer, riddle.wolf, riddle.cabbage], EastBank)
        assert solution.poll() == new RiverState([riddle.farmer, riddle.goat], [riddle.wolf, riddle.cabbage], WestBank)
        assert solution.poll() == new RiverState([], [riddle.farmer, riddle.wolf, riddle.goat, riddle.cabbage], EastBank)
        assert solution.poll() == null
    }

    private void assertSecondSolution(Deque<RiverState> solution) {
        assert solution.poll() == new RiverState([riddle.farmer, riddle.wolf, riddle.goat, riddle.cabbage], [], WestBank)
        assert solution.poll() == new RiverState([riddle.wolf, riddle.cabbage], [riddle.farmer, riddle.goat], EastBank)
        assert solution.poll() == new RiverState([riddle.farmer, riddle.wolf, riddle.cabbage], [riddle.goat], WestBank)
        assert solution.poll() == new RiverState([riddle.cabbage], [riddle.farmer, riddle.goat, riddle.wolf], EastBank)
        assert solution.poll() == new RiverState([riddle.farmer, riddle.cabbage, riddle.goat], [riddle.wolf], WestBank)
        assert solution.poll() == new RiverState([riddle.goat], [riddle.farmer, riddle.wolf, riddle.cabbage], EastBank)
        assert solution.poll() == new RiverState([riddle.farmer, riddle.goat], [riddle.wolf, riddle.cabbage], WestBank)
        assert solution.poll() == new RiverState([], [riddle.farmer, riddle.wolf, riddle.goat, riddle.cabbage], EastBank)
        assert solution.poll() == null
    }
}

An efficient Stack implementation with getMin() and getMax() methods

When we use stack to keep a collection of numbers, we might want to know what are the minimum and maximum numbers in the stack. 

Suppose we are pushing {72,12,55,78,22,34} into a stack, this is what happens

Action	Stack Elements (Top ... Bottom)	Min	Max
push(72)	72	72	72
push(12)	12,72	12	72
push(55)	55,12,72	12	72
push(78)	78,55,12,72	12	78
push(22)	22,78,55,12,72	12	78
push(34)	34,22,78,55,12,72	12	78

If we pop the stack above 3 times, the max should change to 72; if we pop 2 more times, the min should change to 72. It sounds like we need a data structure that keeps the current min and max when we push or pop the stack. Indeed, a stack is a perfect choice here. When pushing a number to the stack, we also push the current min and max values into the min and max stacks respectively. Similarly, when poping out a number from the stack, we also pop out from the min and max stacks, such that the top of the min and max stacks will always have the min and max values of the current stack respectively.

The complexity of the getMin() and getMax() methods are O(1), regardless of how many items are in the stack, same as the push(item) and pop() methods.

Here is an implementation of the Stack with getMin() and getMax() methods

import java.util.Stack;
import java.util.concurrent.locks.ReentrantReadWriteLock;

public class SmartStack<E extends Comparable<E>> {

    private ReentrantReadWriteLock readWriteLock = new ReentrantReadWriteLock();
    private ReentrantReadWriteLock.ReadLock readLock = readWriteLock.readLock();
    private ReentrantReadWriteLock.WriteLock writeLock = readWriteLock.writeLock();

    private Stack<E> mins = new Stack<E>();
    private Stack<E> maxs = new Stack<E>();
    private Stack<E> delegate = new Stack<E>();

    public E push(E item) {
        writeLock.lock();
        try {
            // stores the current min to the mins stack
            if (mins.isEmpty() || (!mins.isEmpty() && item.compareTo(mins.peek()) < 0)) {
                mins.push(item);
            } else {
                mins.push(mins.peek());
            }

            // stores the current max to the maxs stack
            if (maxs.isEmpty() || (!maxs.isEmpty() && item.compareTo(maxs.peek()) > 0)) {
                maxs.push(item);
            } else {
                maxs.push(maxs.peek());
            }

            return delegate.push(item);
        } finally {
            writeLock.unlock();
        }
    }

    public E pop() {
        writeLock.lock();
        try {
            E item = delegate.pop();
            // pop both mins and maxs stacks, such that peeking on them will tell us the current min or max
            mins.pop();
            maxs.pop();
            return item;
        } finally {
            writeLock.unlock();
        }
    }

    public E peek() {
        readLock.lock();
        try {
            return delegate.peek();
        } finally {
            readLock.unlock();
        }
    }

    public E getMin() {
        readLock.lock();
        try {
            return mins.peek();
        } finally {
            readLock.unlock();
        }
    }

    public E getMax() {
        readLock.lock();
        try {
            return maxs.peek();
        } finally {
            readLock.unlock();
        }
    }

    public boolean empty() {
        readLock.lock();
        try {
            return delegate.empty();
        } finally {
            readLock.unlock();
        }
    }

    public int search(E item) {
        readLock.lock();
        try {
            return delegate.search(item);
        } finally {
            readLock.unlock();
        }
    }
    
}

Here is the unit test of the above class written in Groovy

import org.junit.Before
import org.junit.Test

class SmartStackTest {

    SmartStack stack

    @Before
    public void before() {
        stack = new SmartStack()
        stack.push(72)
        stack.push(12)
        stack.push(55)
        stack.push(78)
        stack.push(22)
        stack.push(34)
    }

    @Test
    public void canGetCurrentMinValue() {
        assert stack.min == 12

        5.times { stack.pop() }

        assert stack.min == 72
    }

    @Test
    public void canGetCurrentMaxValue() {
        assert stack.max == 78

        4.times { stack.pop() }

        assert stack.max == 72
    }

    @Test
    public void canPeek() {
        assert stack.peek() == 34
    }

    @Test
    public void isEmpty() {
        assert !stack.empty()
    }

    @Test
    public void canSearch() {
        // the 1-based position from the top of the stack where the object is located
        assert stack.search(34) == 1
        assert stack.search(22) == 2
        assert stack.search(78) == 3
        assert stack.search(55) == 4
        assert stack.search(12) == 5
        assert stack.search(72) == 6
        // not in the stack should return -1
        assert stack.search(11112) == -1
    }
    
}