NullPointerException when using Predicate during static field initialization

Question

Could someone explain what is causing the NPE in the following example code. It seems to be something to do with how static fields (in particular the Predicate) are initialized maybe, but I can't quite figure out what is going on.

import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;

public class Foo {
    public static List<Integer> stuff = doStuff();
    private static Predicate<Integer> bar = i -> i == 42;

    public static int howBigIsStuff(){
        return stuff.size();
    }

    private static List<Integer> doStuff(){
        // java.lang.NullPointerException 
        //    at java.util.Objects.requireNonNull(Objects.java:203)
        List<Integer> foo = Arrays.asList(1,2,42,42).stream()
                .filter(bar)
                .collect(Collectors.toList());
        return foo;
    }
}


import java.util.Arrays;
import java.util.List;
import java.util.function.Predicate;
import java.util.stream.Collectors;
import org.junit.Test;

public class FooTest {

    private static Predicate<Integer> bar = i -> i == 42;

    @Test
    public void test() {
        // This is fine
        List<Integer> foo = Arrays.asList(1,2,42,42).stream()
                .filter(bar)
                .collect(Collectors.toList());
        System.out.println(foo);    // [42, 42]

        Foo.howBigIsStuff();
    }
}

Where is the stacktrace? That will probably help the most. – Orin Aug 22 '16 at 14:20 — Orin, Aug 22 '16 at 14:20

score 2 · Accepted Answer · answered Aug 22 '16 at 14:23

2

See JLS-12.4.2. The order of static fields initialization matters. Fields are initialized in the order in which they appear in the source.

So at the moment doStuff() is called bar is equals to default value null.

answered Aug 22 '16 at 14:23

vsminkov

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score 1 · Answer 2 · answered Aug 22 '16 at 14:23

You are calling to bar before it has been initialized, try this:

public class Foo {
    private static Predicate<Integer> bar = i -> i == 42;
    public static List<Integer> stuff = doStuff();

    public static int howBigIsStuff() {
        return stuff.size();
    }

    private static List<Integer> doStuff() {
        // java.lang.NullPointerException 
        //    at java.util.Objects.requireNonNull(Objects.java:203)
        List<Integer> foo = Arrays.asList(1, 2, 42, 42).stream()
                .filter(bar)
                .collect(Collectors.toList());
        return foo;
    }
}

score 0 · Answer 3 · answered Aug 22 '16 at 14:24

Sequence of initialization is:

Execution of method doStuff (here bar is not yet initialized)
Assignement of stuff variable
Assignement of bar variable

Here is how you can solve it:

public static List<Integer> stuff;
private static Predicate<Integer> bar;

static  {
    bar = i -> i == 42;
    stuff = doStuff();
}

NullPointerException when using Predicate during static field initialization

3 Answers3