Contracts: check in a demo of how to implement a simple contract in Java.

src/contracts/JavaCommercialPaper.java

@@ -0,0 +1,106 @@
+package contracts;
+
+import core.*;
+import core.serialization.*;
+import org.jetbrains.annotations.*;
+
+import java.security.*;
+import java.time.*;
+
+import static core.ContractsDSLKt.*;
+import static kotlin.CollectionsKt.*;
+
+/**
+ * This is a Java version of the CommercialPaper contract (chosen because it's simple). This demonstrates how the
+ * use of Kotlin for implementation of the framework does not impose the same language choice on contract developers.
+ *
+ * NOTE: For illustration only. Not unit tested.
+ */
+public class JavaCommercialPaper implements Contract {
+    public static class State implements SerializeableWithKryo {
+        private InstitutionReference issuance;
+        private PublicKey owner;
+        private Amount faceValue;
+        private Instant maturityDate;
+
+        public State() {}  // For serialization
+
+        public State(InstitutionReference issuance, PublicKey owner, Amount faceValue, Instant maturityDate) {
+            this.issuance = issuance;
+            this.owner = owner;
+            this.faceValue = faceValue;
+            this.maturityDate = maturityDate;
+        }
+
+        public InstitutionReference getIssuance() {
+            return issuance;
+        }
+
+        public PublicKey getOwner() {
+            return owner;
+        }
+
+        public Amount getFaceValue() {
+            return faceValue;
+        }
+
+        public Instant getMaturityDate() {
+            return maturityDate;
+        }
+    }
+
+    public static class Commands implements core.Command {
+        public static class Move {
+            @Override
+            public boolean equals(Object obj) {
+                return obj instanceof Move;
+            }
+        }
+
+        public static class Redeem {
+            @Override
+            public boolean equals(Object obj) {
+                return obj instanceof Redeem;
+            }
+        }
+    }
+
+    @Override
+    public void verify(@NotNull TransactionForVerification tx) {
+        // There are two possible things that can be done with CP. The first is trading it. The second is redeeming it
+        // for cash on or after the maturity date.
+
+        // Find the command that instructs us what to do and check there's exactly one.
+        AuthenticatedObject<Command> cmd = requireSingleCommand(tx.getArgs(), Commands.class);
+        // For now do not allow multiple pieces of CP to trade in a single transaction. Study this more!
+        State input = single(filterIsInstance(tx.getInStates(), State.class));
+
+        if (!cmd.getSigners().contains(input.getOwner()))
+            throw new IllegalStateException("Failed requirement: the transaction is signed by the owner of the CP");
+
+        if (cmd.getValue() instanceof JavaCommercialPaper.Commands.Move) {
+            // Check the output CP state is the same as the input state, ignoring the owner field.
+            State output = single(filterIsInstance(tx.getOutStates(), State.class));
+            if (!output.getFaceValue().equals(input.getFaceValue()) ||
+                    !output.getIssuance().equals(input.getIssuance()) ||
+                    !output.getMaturityDate().equals(input.getMaturityDate()))
+                throw new IllegalStateException("Failed requirement: the output state is the same as the input state except for owner");
+        } else if (cmd.getValue() instanceof JavaCommercialPaper.Commands.Redeem) {
+            Amount received = CashKt.sumCashOrNull(tx.getInStates());
+            if (received == null)
+                throw new IllegalStateException("Failed requirement: no cash being redeemed");
+            if (input.getMaturityDate().isAfter(tx.getTime()))
+                throw new IllegalStateException("Failed requirement: the paper must have matured");
+            if (!input.getFaceValue().equals(received))
+                throw new IllegalStateException("Failed requirement: the received amount equals the face value");
+            if (!filterIsInstance(tx.getOutStates(), State.class).isEmpty())
+                throw new IllegalStateException("Failed requirement: the paper must be destroyed");
+        }
+    }
+
+    @NotNull
+    @Override
+    public String getLegalContractReference() {
+        return "https://en.wikipedia.org/wiki/Commercial_paper";
+    }
+}

src/core/ContractsDSL.kt

@@ -105,3 +105,6 @@ inline fun <reified T : Command> List<AuthenticatedObject<Command>>.requireSingl
 } catch (e: NoSuchElementException) {
     throw IllegalStateException("Required ${T::class.qualifiedName} command")   // Better error message.
 }
+
+// For Java
+fun List<AuthenticatedObject<Command>>.requireSingleCommand(klass: Class<out Command>) = filter { klass.isInstance(it) }.single()