Main.java

import java.util.*;
import java.util.stream.*;

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

/**
 * export CHECKERFRAMEWORK=${HOME}/lib/checker-framework-2.1.9
 * alias javach='$CHECKERFRAMEWORK/checker/bin/javac' 
 * export PATH=~/bin/jdk1.8.0_51/bin:$PATH
 * javach -processor org.checkerframework.checker.nullness.NullnessChecker Main.java -J-Duser.language=en
 */
public class Main {

    Object[] omitNullsOriginal(Object[] args) {
        int size = 0;
        for (Object s : args) {
            if (s != null) {
                size++;
            }
        }

        Object[] ret = new Object[size];
        for (int i = 0, j = 0; i < args.length; i++) {
            if (args[i] != null) {
                ret[j++] = args[i];
            }
        }

        return ret;
    }

    @NonNull Object @NonNull [] omitNulls1(@Nullable Object @NonNull [] args) {
        int size = 0;
        for (@Nullable Object s : args) {
            if (s != null) {
                size++;
            }
        }

        @NonNull Object @NonNull [] ret = new Object[size];
        for (int i = 0, j = 0; i < args.length; i++) {
            if (args[i] != null) {
                ret[j++] = args[i];
            }
        }

        return ret;
    }


    // trying to use PolyNull
    @NonNull Object @PolyNull [] omitNulls2(@Nullable Object @PolyNull [] args) {
        if (args == null) {
            return null;
        }

        int size = 0;
        for (@Nullable Object s : args) {
            if (s != null) {
                size++;
            }
        }

        @NonNull Object @NonNull [] ret = new Object[size];
        for (int i = 0, j = 0; i < args.length; i++) {
            if (args[i] != null) {
                ret[j++] = args[i];
            }
        }

        return ret;
    }

    // trying to use generics and type inference
    <T> @NonNull T @PolyNull [] omitNulls3(@Nullable T @PolyNull [] args) {
        if (args == null) {
            return null;
        }

        int size = 0;
        for (@Nullable Object s : args) {
            if (s != null) {
                size++;
            }
        }

        @NonNull T @NonNull [] ret = newArray(size);
        for (int i = 0, j = 0; i < args.length; i++) {
            if (args[i] != null) {
                ret[j++] = args[i];
            }
        }

        return ret;
    }

    @SuppressWarnings("unchecked")
    <T> T[] newArray(int size) {
        return (T[])new Object[size];
    }

    // trying to use Stream API
    <T> @NonNull T @PolyNull [] omitNulls4(@Nullable T @PolyNull [] args) {
        if (args == null) {
            return null;
        }

        /*
        return Arrays.stream(args)
            .filter(o -> o != null)
            .toArray(this::newArray); // error:
Main.java:105: warning: [methodref.inference.unimplemented] This version of the Checker Framework does not type-check method references with implicit type arguments.
            .toArray(this::newArray); // error
                     ^
Main.java:105: error: [return.type.incompatible] incompatible types in return.
            .toArray(this::newArray); // error
                    ^
  found   : T extends @Initialized @Nullable Object @Initialized @NonNull []
  required: T extends @Initialized @NonNull Object @Initialized @PolyNull []
1 error
1 warning
         */

        /* return Arrays.stream(args)
            .filter(o -> o != null)
            .<@NonNull T>map(o -> (@NonNull T)o)
            .toArray(this::newArray); // still error:
Main.java:112: warning: [methodref.inference.unimplemented] This version of the Checker Framework does not type-check method references with implicit type arguments.
            .toArray(this::newArray);
                     ^
Main.java:112: error: [return.type.incompatible] incompatible types in return.
            .toArray(this::newArray);
                    ^
  found   : T extends @Initialized @Nullable Object @Initialized @NonNull []
  required: T extends @Initialized @NonNull Object @Initialized @PolyNull []
Main.java:111: warning: [cast.unsafe] "T extends @Initialized @Nullable Object" may not be casted to the type "T extends @Initialized @NonNull Object"
            .<@NonNull T>map(o -> (@NonNull T)o)
                                  ^
1 error
2 warnings
         */

        /* return Arrays.stream(args)
            .filter(o -> o != null)
            .<@NonNull T>map(o -> (@NonNull T)o)
            .collect(Collectors.toList()).toArray(newArray(0)); // error
Main.java:142: error: [return.type.incompatible] incompatible types in return.
             .collect(Collectors.toList()).toArray(newArray(0));
                                                  ^
  found   : ?[ extends @UnknownKeyFor Object super @KeyForBottom Void] @UnknownKeyFor []
  required: T[ extends @UnknownKeyFor Object super @KeyForBottom Void] @UnknownKeyFor []
Main.java:142: error: [return.type.incompatible] incompatible types in return.
             .collect(Collectors.toList()).toArray(newArray(0));
                                                  ^
  found   : @Initialized @Nullable ?[ extends @Initialized @Nullable Object super @Initialized @Nullable Void] @Initialized @NonNull []
  required: T[ extends @Initialized @NonNull Object super @Initialized @NonNull Void] @Initialized @PolyNull []
Main.java:141: warning: [cast.unsafe] "T extends @Initialized @Nullable Object" may not be casted to the type "T extends @Initialized @NonNull Object"
             .<@NonNull T>map(o -> (@NonNull T)o)
                                   ^
2 errors
1 warning
         */

        /*
        @NonNull T @NonNull [] ret = newArray(0);
        return Arrays.stream(args)
            .filter(o -> o != null)
            .<@NonNull T>map(o -> (@NonNull T)o)
            .collect(Collectors.toList()).toArray(ret); // error
Main.java:166: error: [return.type.incompatible] incompatible types in return.
            .collect(Collectors.toList()).toArray(ret);
                                                 ^
  found   : T[ extends @Initialized @Nullable Object super @Initialized @Nullable Void] @Initialized @NonNull []
  required: T[ extends @Initialized @NonNull Object super @Initialized @NonNull Void] @Initialized @PolyNull []
Main.java:165: warning: [cast.unsafe] "T extends @Initialized @Nullable Object" may not be casted to the type "T extends @Initialized @NonNull Object"
            .<@NonNull T>map(o -> (@NonNull T)o)
                                  ^
1 error
1 warning
        */

        return newArray(0); // I've give up \(;_;)/
    }

}