ReadyReadLine.java

import org.checkerframework.checker.nullness.qual.EnsuresNonNullIf;
import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.dataflow.qual.Pure;

class ReadyReadLine {

    void m(MyBufferedReader buf) throws Exception {
        if (buf.ready()) {
            String line = buf.readLine();
            line.toString();
        }

        if (buf.readLine() != null) {
            //:: error: (dereference.of.nullable)
            buf.readLine().toString();
        }
    }
}

// this is a replication of the JDK BufferedReader (with only the relevant methods)
class MyBufferedReader {
    public @Nullable String readLine() throws Exception {
        return null;
    }

    @EnsuresNonNullIf(expression = "readLine()", result = true)
    @Pure
    public boolean ready() throws Exception {
        // don't bother with implementation.
        //:: error: (contracts.conditional.postcondition.not.satisfied)
        return true;
    }
}