hardcode Map#get as having unknown nullity (IDEA-176626)

until we have a better solution

java/java-analysis-impl/src/com/intellij/codeInspection/dataFlow/DfaPsiUtil.java

@@ -25,6 +25,7 @@ import com.intellij.lang.java.JavaLanguage;
 import com.intellij.openapi.util.Ref;
 import com.intellij.psi.*;
 import com.intellij.psi.search.LocalSearchScope;
+import com.intellij.psi.search.searches.DeepestSuperMethodsSearch;
 import com.intellij.psi.search.searches.ReferencesSearch;
 import com.intellij.psi.tree.IElementType;
 import com.intellij.psi.util.*;
@@ -103,6 +104,10 @@ public class DfaPsiUtil {
       return Nullness.NOT_NULL;
     }
 
+    if (owner instanceof PsiMethod && isMapGet((PsiMethod)owner)) {
+      return Nullness.UNKNOWN;
+    }
+
     Nullness fromType = getTypeNullability(resultType);
     if (fromType != Nullness.UNKNOWN) return fromType;
 
@@ -120,6 +125,12 @@ public class DfaPsiUtil {
     return Nullness.UNKNOWN;
   }
 
+  private static boolean isMapGet(@NotNull PsiMethod method) {
+    if (!"get".equals(method.getName())) return false;
+    PsiMethod superMethod = DeepestSuperMethodsSearch.search(method).findFirst();
+    return "java.util.Map.get".equals(PsiUtil.getMemberQualifiedName(superMethod != null ? superMethod : method));
+  }
+
   @NotNull
   public static Nullness inferParameterNullability(@NotNull PsiParameter parameter) {
     PsiElement parent = parameter.getParent();

java/java-tests/testData/inspection/dataFlow/fixture/MapGetWithNotNullKeys.java

@@ -0,0 +1,17 @@
+import foo.*;
+import java.util.*;
+
+class Test {
+
+  public void main(Map<@NotNull String, @NotNull Integer> map, HashMap<@NotNull String, @NotNull Integer> hashMap) {
+    Integer value = map.get("y");
+    if (value == null) {
+      System.out.println("it's not contained");
+    }
+
+    value = hashMap.get("y");
+    if (value == null) {
+      System.out.println("it's not contained");
+    }
+  }
+}

java/java-tests/testSrc/com/intellij/java/codeInspection/DataFlowInspection8Test.java

@@ -197,6 +197,8 @@ public class DataFlowInspection8Test extends DataFlowInspectionTestCase {
   public void testStreamInlining() { doTest(); }
   public void testStreamComparatorInlining() { doTest(); }
   public void testStreamKnownSource() { doTest(); }
+  
+  public void testMapGetWithNotNullKeys() { doTestWithCustomAnnotations(); }
 
   public void testMethodVsExpressionTypeAnnotationConflict() {
     setupAmbiguousAnnotations("withTypeUse", myFixture);