checker/tests/nullness/ReadyReadLine.java

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

public 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)
    return true;
  }
}