How to properly annotate java.util.function.Predicate.not

[Jon Schewe]

I'm filtering null and empty strings out of a stream and having some issues with the annotations on Predicate.not. Is there a way to properly annotate Predicate.not to handle this situation with checker?

When I compile the following code I get

```

/home/jpschewe/projects/checker-bugs/src/main/java/net/mtu/eggplant/checker/PredicateNot.java:17: error: [argument] incompatible argument for parameter arg0 of filter.

        .filter(Predicate.not(PredicateNot::isBlank)) //

                             ^

  found   : @Initialized @NonNull Predicate<@Initialized @NonNull String>

  required: @Initialized @NonNull Predicate<? extends @Initialized @Nullable Object super @Initialized @Nullable String>

```

```java

package net.mtu.eggplant.checker;

import java.util.*;

import java.util.function.*;

import java.util.stream.*;

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

public class PredicateNot {

  public static void function(final @Nullable String @Nullable [] headerRow) {

    final Collection<String> headerNames;

    if (null == headerRow) {

      headerNames = Collections.emptyList();

    } else {

      headerNames = Arrays.asList(headerRow).stream() //

        .filter(Predicate.not(PredicateNot::isBlank)) //

        .collect(Collectors.toList());

    }

  }

  // from apache commons lang3 StringUtils

  @EnsuresNonNullIf(expression="#1", result=false)

  public static boolean isBlank(final @Nullable CharSequence cs) {

    if(null == cs) {

      return true;

    }

    

    final int strLen = length(cs);

    if (strLen == 0) {

      return true;

    }

    for (int i = 0; i < strLen; i++) {

      if (!Character.isWhitespace(cs.charAt(i))) {

        return false;

      }

    }

    return true;

  }

  // from apache commons lang3 StringUtils

  public static int length(final @Nullable CharSequence cs) {

    return cs == null ? 0 : cs.length();

  }

  

}

```

[Suzanne Millstein]

Hi Jon,

This code requires Java 8 type argument inference, so the Checker Framework incorrectly issues an error here.  We are working on fixing this as tracked by https://github.com/typetools/checker-framework/issues/979, but it's a huge undertaking.  In the meantime, you can add an explicit type argument to get this code to pass:

```

.filter(Predicate.<@Nullable CharSequence>not(PredicateNot::isBlank)) //

```

-Suzanne

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