diff --git a/java/java-psi-impl/src/com/intellij/psi/impl/PsiSubstitutorImpl.java b/java/java-psi-impl/src/com/intellij/psi/impl/PsiSubstitutorImpl.java
index d83b7b1ae9ce..0de7124af8e0 100644
--- a/java/java-psi-impl/src/com/intellij/psi/impl/PsiSubstitutorImpl.java
+++ b/java/java-psi-impl/src/com/intellij/psi/impl/PsiSubstitutorImpl.java
@@ -24,6 +24,7 @@ import com.intellij.psi.impl.source.PsiClassReferenceType;
 import com.intellij.psi.util.PsiUtil;
 import com.intellij.psi.util.PsiUtilCore;
 import com.intellij.util.ArrayUtil;
+import com.intellij.util.containers.UnmodifiableTHashMap;
 import gnu.trove.THashMap;
 import gnu.trove.TObjectHashingStrategy;
 import org.jetbrains.annotations.NonNls;
@@ -58,21 +59,15 @@ public class PsiSubstitutorImpl implements PsiSubstitutor {
   private final SubstitutionVisitor mySimpleSubstitutionVisitor = new SubstitutionVisitor();
 
   private PsiSubstitutorImpl(@NotNull Map<PsiTypeParameter, PsiType> map) {
-    mySubstitutionMap = new THashMap<>(map, PSI_EQUIVALENCE);
-  }
-
-  private PsiSubstitutorImpl() {
-    mySubstitutionMap = new THashMap<>(2, PSI_EQUIVALENCE);
+    mySubstitutionMap = map;
   }
 
   PsiSubstitutorImpl(@NotNull PsiTypeParameter typeParameter, PsiType mapping) {
-    this();
-    mySubstitutionMap.put(typeParameter, mapping);
+    this(Collections.singletonMap(typeParameter, mapping));
   }
 
   PsiSubstitutorImpl(@NotNull PsiClass parentClass, PsiType[] mappings) {
-    this();
-    putAllInternal(parentClass, mappings);
+    this(putAllInternal(Collections.emptyMap(), parentClass, mappings));
   }
 
   @Override
@@ -304,45 +299,45 @@ public class PsiSubstitutorImpl implements PsiSubstitutor {
 
   @Override
   protected PsiSubstitutorImpl clone() {
-    return new PsiSubstitutorImpl(mySubstitutionMap);
+    throw new UnsupportedOperationException();
   }
 
   @NotNull
   @Override
   public PsiSubstitutor put(@NotNull PsiTypeParameter typeParameter, PsiType mapping) {
-    PsiSubstitutorImpl ret = clone();
     if (mapping != null && !mapping.isValid()) {
       LOG.error("Invalid type in substitutor: " + mapping + "; " + mapping.getClass());
     }
-    ret.mySubstitutionMap.put(typeParameter, mapping);
-    return ret;
+    return new PsiSubstitutorImpl(new UnmodifiableTHashMap<>(PSI_EQUIVALENCE, mySubstitutionMap, typeParameter, mapping));
   }
 
-  private void putAllInternal(@NotNull PsiClass parentClass, PsiType[] mappings) {
+  private static Map<PsiTypeParameter, PsiType> putAllInternal(Map<PsiTypeParameter, PsiType> originalMap, @NotNull PsiClass parentClass,
+                                                               PsiType[] mappings) {
     final PsiTypeParameter[] params = parentClass.getTypeParameters();
+    if (params.length == 0) return originalMap;
+    Map<PsiTypeParameter, PsiType> newMap = new THashMap<>(originalMap);
 
     for (int i = 0; i < params.length; i++) {
       PsiTypeParameter param = params[i];
       assert param != null;
       if (mappings != null && mappings.length > i) {
         PsiType mapping = mappings[i];
-        mySubstitutionMap.put(param, mapping);
+        newMap.put(param, mapping);
         if (mapping != null && !mapping.isValid()) {
           LOG.error("Invalid type in substitutor: " + mapping);
         }
       }
       else {
-        mySubstitutionMap.put(param, null);
+        newMap.put(param, null);
       }
     }
+    return new UnmodifiableTHashMap<>(PSI_EQUIVALENCE, newMap);
   }
 
   @NotNull
   @Override
   public PsiSubstitutor putAll(@NotNull PsiClass parentClass, PsiType[] mappings) {
-    PsiSubstitutorImpl substitutor = clone();
-    substitutor.putAllInternal(parentClass, mappings);
-    return substitutor;
+    return new PsiSubstitutorImpl(putAllInternal(mySubstitutionMap, parentClass, mappings));
   }
 
   @NotNull
@@ -350,9 +345,9 @@ public class PsiSubstitutorImpl implements PsiSubstitutor {
   public PsiSubstitutor putAll(@NotNull PsiSubstitutor another) {
     if (another instanceof EmptySubstitutorImpl) return this;
     final PsiSubstitutorImpl anotherImpl = (PsiSubstitutorImpl)another;
-    PsiSubstitutorImpl substitutor = clone();
-    substitutor.mySubstitutionMap.putAll(anotherImpl.mySubstitutionMap);
-    return substitutor;
+    Map<PsiTypeParameter, PsiType> newMap = new THashMap<>(mySubstitutionMap, PSI_EQUIVALENCE);
+    newMap.putAll(anotherImpl.mySubstitutionMap);
+    return createSubstitutor(newMap);
   }
 
   @Override
@@ -389,7 +384,7 @@ public class PsiSubstitutorImpl implements PsiSubstitutor {
   @NotNull
   public static PsiSubstitutor createSubstitutor(@Nullable Map<PsiTypeParameter, PsiType> map) {
     if (map == null || map.isEmpty()) return EMPTY;
-    return new PsiSubstitutorImpl(map);
+    return new PsiSubstitutorImpl(new UnmodifiableTHashMap<>(map));
   }
 
   @Override
@@ -412,6 +407,6 @@ public class PsiSubstitutorImpl implements PsiSubstitutor {
   @Override
   @NotNull
   public Map<PsiTypeParameter, PsiType> getSubstitutionMap() {
-    return Collections.unmodifiableMap(mySubstitutionMap);
+    return mySubstitutionMap;
   }
 }
diff --git a/platform/util/src/com/intellij/util/containers/UnmodifiableTHashMap.java b/platform/util/src/com/intellij/util/containers/UnmodifiableTHashMap.java
new file mode 100644
index 000000000000..40f58c53884e
--- /dev/null
+++ b/platform/util/src/com/intellij/util/containers/UnmodifiableTHashMap.java
@@ -0,0 +1,86 @@
+// Copyright 2000-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license that can be found in the LICENSE file.
+package com.intellij.util.containers;
+
+import gnu.trove.THashMap;
+import gnu.trove.TObjectFunction;
+import gnu.trove.TObjectHashingStrategy;
+import gnu.trove.TObjectObjectProcedure;
+import org.jetbrains.annotations.NotNull;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Map;
+import java.util.Set;
+
+public class UnmodifiableTHashMap<K,V> extends THashMap<K,V> {
+  public UnmodifiableTHashMap(@NotNull Map<? extends K, ? extends V> map) {
+    //noinspection unchecked
+    this(CANONICAL, map);
+  }
+
+  public UnmodifiableTHashMap(@NotNull TObjectHashingStrategy<K> strategy,
+                              @NotNull Map<? extends K, ? extends V> map) {
+    super(map.size(), strategy);
+    for (Map.Entry<? extends K, ? extends V> e : map.entrySet()) {
+      super.put(e.getKey(), e.getValue());
+    }
+  }
+  public UnmodifiableTHashMap(@NotNull TObjectHashingStrategy<K> strategy,
+                              @NotNull Map<? extends K, ? extends V> map, K additionalKey, V additionalValue) {
+    this(strategy, map);
+    super.put(additionalKey, additionalValue);
+  }
+
+  @Override
+  public V put(K key, V value) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public boolean retainEntries(TObjectObjectProcedure<K, V> procedure) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public void transformValues(TObjectFunction<V, V> function) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public void clear() {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public V remove(Object key) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  protected void removeAt(int index) {
+    throw new UnsupportedOperationException();
+  }
+
+  @Override
+  public void putAll(Map<? extends K, ? extends V> map) {
+    throw new UnsupportedOperationException();
+  }
+
+  @NotNull
+  @Override
+  public Collection<V> values() {
+    return Collections.unmodifiableCollection(super.values());
+  }
+
+  @NotNull
+  @Override
+  public Set<K> keySet() {
+    return Collections.unmodifiableSet(super.keySet());
+  }
+
+  @NotNull
+  @Override
+  public Set<Map.Entry<K, V>> entrySet() {
+    return Collections.unmodifiableSet(super.entrySet());
+  }
+}