Java 8 Incompatible Types

Question

Here's the simple code

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class SimpleTest {

    public static void main(String[] args) {
    final ArrayList<Map<String, Object>> maps = newArrayList(
        createMap("1", "a", Collections.EMPTY_MAP, Collections.EMPTY_MAP),
        createMap("2", "b", Collections.EMPTY_MAP, Collections.EMPTY_MAP),
        createMap("3", "c", Collections.EMPTY_MAP, Collections.EMPTY_MAP)
    ); 

    System.out.println(" maps = " + maps);
    }

    public static Map<String, Object> createMap(String value1, String value2, Map<String, Object> object1, Map<String, Object> object2) {
       Map<String, Object> map = new HashMap<>();
       map.put("value1", value1);
       map.put("value1", value1);
       map.put("object1", object1);
       map.put("object2", object2);
       return map;
    }    

    public static <E> ArrayList<E> newArrayList(E... elements) {
    ArrayList<E> list = new ArrayList<E>(elements.length);
    Collections.addAll(list, elements);
    return list;
    }
}

When JAVA_HOME points to JDK 8 and I use javac -source 1.7 SimpleTest.java I get

SimpleTest.java:9: error: incompatible types: ArrayList<Map> cannot be converted to ArrayList<Map<String,Object>>
        final ArrayList<Map<String, Object>> maps = newArrayList(
                                                                ^

When I use -source 1.8 or no -source option everything works ok. Now, when JAVA_HOME points to JDK 7 the code always compiles whether I use -source 1.7 or not.

Initially this question was about a piece of software where POM file has a <source> and <target> set to 1.7 and the build was failing on JDK 8 but was ok on JDK 7.

Now for the question - what causes it to happen ? It seems to me as a major overlook of some sort. Why compiling on JDK 8 with source set to 1.7 fails ?

Answer 1

You can simplify the code to a self-contained example which doesn’t need 3rd-party libraries:

public class Test2 {
    @SafeVarargs
    static <T> ArrayList<T> newArrayList(T... arg) {
        return new ArrayList<T>(Arrays.asList(arg));
    }
    private final List<Map<String, Object>> maps = newArrayList(
        createMap(null, Collections.EMPTY_MAP)
     );

    public static Map<String, Object> createMap(String id, Map<String,String> m) {
        return null;
    }
}

This can be compiled using javac from jdk1.7, but not with javac from jdk1.8 using -source 1.7.

The point here is that javac from jdk1.8 still is a different compiler than the one included in the previous version and the option -source 1.7 doesn’t tell it to mimic the old implementation’s behavior but to be compatible with the Java 7 specification. If the old compiler has a bug, the newer compiler doesn’t have to try to reproduce the bug.

Since the code uses Collections.EMPTY_MAP rather than Collections.<String,String>emptyMap(), the raw type Map will be passed to createMap, making it an unchecked invocation having the raw result type Map.

This is mandated by JLS §15.12.2.6:

The result type of the chosen method is determined as follows:

• If the chosen method is declared with a return type of void, then the result is void.

• Otherwise, if unchecked conversion was necessary for the method to be applicable, then the result type is the erasure (§4.6) of the method's declared return type.

…

It seems that this behavior has not been (fully) implemented in javac of jdk1.7. Interestingly, it will do it correctly when adding a type parameter like

public static <T> Map<String, Object> createMap(String id, Map<String,String> m)

making it a generic method. Then, jdk1.7 will produce the same error. But the cited rule applies to all method invocations.

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In contrast, the fact that compiling it with Java 8 compliance succeeds stems from the new target type inference, which has entirely different rules.