AssertIfClient.java

import org.checkerframework.checker.nullness.qual.*;
import org.checkerframework.dataflow.qual.*;

public class AssertIfClient {

    @RequiresNonNull("#1.rpcResponse()")
    void rpcResponseNonNull(Proxy proxy) {
        @NonNull Object response = proxy.rpcResponse();
    }

    void rpcResponseNullable(Proxy proxy) {
        @Nullable Object response = proxy.rpcResponse();
    }

    void rpcResponseTypestate() {
        Proxy proxy = new Proxy();
        //:: error: (assignment.type.incompatible)
        @NonNull Object response1 = proxy.rpcResponse();
        //:: error: (contracts.precondition.not.satisfied)
        rpcResponseNonNull(proxy);
        rpcResponseNullable(proxy);

        proxy.issueRpc();
        @NonNull Object response2 = proxy.rpcResponse();
        @NonNull Object response3 = proxy.rpcResponse();
        rpcResponseNonNull(proxy);
        rpcResponseNullable(proxy);
    }
}

class Proxy {

    // the RPC response, or null if not yet received
    @MonotonicNonNull Object response = null;

    @SuppressWarnings("contracts.postcondition.not.satisfied")
    @EnsuresNonNull({"response", "rpcResponse()"})
    void issueRpc() {
        response = new Object();
    }

    // If this method returns true,
    // then response is non-null and rpcResponse() returns non-null
    @SuppressWarnings("contracts.conditional.postcondition.not.satisfied")
    @EnsuresNonNullIf(
        expression = {"response", "rpcResponse()"},
        result = true
    )
    boolean rpcResponseReceived() {
        return response != null;
    }

    // Returns non-null if the response has been received, null otherwise;
    // but an @AssertNonNullIfNonNull annotation would states the converse,
    // that if the result is non-null then the response hs been received.
    // See rpcResponseReceived.
    @Pure
    @Nullable Object rpcResponse() {
        return response;
    }
}