diff --git a/src/main/java/edu/kit/typicalc/model/term/AbsTerm.java b/src/main/java/edu/kit/typicalc/model/term/AbsTerm.java
index 6abfe3e..f333519 100644
--- a/src/main/java/edu/kit/typicalc/model/term/AbsTerm.java
+++ b/src/main/java/edu/kit/typicalc/model/term/AbsTerm.java
@@ -12,7 +12,7 @@ import java.util.Set;
 /**
  * Representation of an abstraction term with its two sub-lambda terms.
  */
-public class AbsTerm implements LambdaTerm {
+public class AbsTerm extends LambdaTerm {
     private final VarTerm var;
     private final LambdaTerm body;
 
diff --git a/src/main/java/edu/kit/typicalc/model/term/AppTerm.java b/src/main/java/edu/kit/typicalc/model/term/AppTerm.java
index dd4285c..03f53ca 100644
--- a/src/main/java/edu/kit/typicalc/model/term/AppTerm.java
+++ b/src/main/java/edu/kit/typicalc/model/term/AppTerm.java
@@ -9,7 +9,7 @@ import java.util.*;
 /**
  * Representation of an application term consisting of a function and the parameter passed to it.
  */
-public class AppTerm implements LambdaTerm {
+public class AppTerm extends LambdaTerm {
     private final LambdaTerm left;
     private final LambdaTerm right;
 
diff --git a/src/main/java/edu/kit/typicalc/model/term/ConstTerm.java b/src/main/java/edu/kit/typicalc/model/term/ConstTerm.java
index 5597015..1bb35e1 100644
--- a/src/main/java/edu/kit/typicalc/model/term/ConstTerm.java
+++ b/src/main/java/edu/kit/typicalc/model/term/ConstTerm.java
@@ -12,7 +12,7 @@ import java.util.Set;
 /**
  * Abstract representation of a constant lambda term that has a predetermined type and a value of that type.
  */
-public abstract class ConstTerm implements LambdaTerm {
+public abstract class ConstTerm extends LambdaTerm {
 
     /**
      * Returns the named type of the constant
diff --git a/src/main/java/edu/kit/typicalc/model/term/LambdaTerm.java b/src/main/java/edu/kit/typicalc/model/term/LambdaTerm.java
index 36d5775..e386ce3 100644
--- a/src/main/java/edu/kit/typicalc/model/term/LambdaTerm.java
+++ b/src/main/java/edu/kit/typicalc/model/term/LambdaTerm.java
@@ -12,27 +12,27 @@ import java.util.Set;
  * Depending on the subclass used, a lambda term may contain several other lambda terms
  * and thus form a tree-like structure of lambda terms.
  */
-    public interface LambdaTerm {
+    public abstract class LambdaTerm {
     /**
      * Returns whether the lambda term contains a let expression
      *
      * @return whether the lambda term contains a let expression
      */
-    boolean hasLet();
+    public abstract boolean hasLet();
 
     /**
      * Returns a set of all free variables occurring in the lambda term.
      *
      * @return all free variables
      */
-    Set<VarTerm> getFreeVariables();
+    public abstract Set<VarTerm> getFreeVariables();
 
     /**
      * Calls exactly one method on the visitor depending on the lambda term type.
      *
      * @param termVisitor a visitor
      */
-    void accept(TermVisitor termVisitor);
+    public abstract void accept(TermVisitor termVisitor);
 
     /**
      * Uses exactly one method of the visitor and provides the arguments passed.
@@ -42,6 +42,6 @@ import java.util.Set;
      * @param type            a type
      * @return the result returned by the visitor
      */
-    InferenceStep accept(TermVisitorTree termVisitorTree,
+    public abstract InferenceStep accept(TermVisitorTree termVisitorTree,
                                          Map<VarTerm, TypeAbstraction> assumptions, Type type);
 }
diff --git a/src/main/java/edu/kit/typicalc/model/term/LetTerm.java b/src/main/java/edu/kit/typicalc/model/term/LetTerm.java
index 105492e..3da544c 100644
--- a/src/main/java/edu/kit/typicalc/model/term/LetTerm.java
+++ b/src/main/java/edu/kit/typicalc/model/term/LetTerm.java
@@ -13,7 +13,7 @@ import java.util.Set;
  * Representation of a let term with its variable, the lambda term assigned
  * to this variable and the lambda term the variable is used in.
  */
-public class LetTerm implements LambdaTerm {
+public class LetTerm extends LambdaTerm {
     private final VarTerm variable;
     private final LambdaTerm definition;
     private final LambdaTerm body;
diff --git a/src/main/java/edu/kit/typicalc/model/term/VarTerm.java b/src/main/java/edu/kit/typicalc/model/term/VarTerm.java
index 303f368..8d1da7a 100644
--- a/src/main/java/edu/kit/typicalc/model/term/VarTerm.java
+++ b/src/main/java/edu/kit/typicalc/model/term/VarTerm.java
@@ -9,7 +9,7 @@ import java.util.*;
 /**
  * Representation of a variable term with its name.
  */
-public class VarTerm implements LambdaTerm {
+public class VarTerm extends LambdaTerm {
     private final String name;
 
     /**
diff --git a/src/main/java/edu/kit/typicalc/model/type/FunctionType.java b/src/main/java/edu/kit/typicalc/model/type/FunctionType.java
index 2081d9d..bfa8500 100644
--- a/src/main/java/edu/kit/typicalc/model/type/FunctionType.java
+++ b/src/main/java/edu/kit/typicalc/model/type/FunctionType.java
@@ -10,7 +10,7 @@ import java.util.Set;
 /**
  * Models the type of an abstraction/function.
  */
-public class FunctionType implements Type {
+public class FunctionType extends Type {
 
     private final Type parameter;
     private final Type output;
diff --git a/src/main/java/edu/kit/typicalc/model/type/NamedType.java b/src/main/java/edu/kit/typicalc/model/type/NamedType.java
index 6fd7220..e814c1d 100644
--- a/src/main/java/edu/kit/typicalc/model/type/NamedType.java
+++ b/src/main/java/edu/kit/typicalc/model/type/NamedType.java
@@ -10,7 +10,7 @@ import java.util.Set;
 /**
  * Models a simple named type.
  */
-public class NamedType implements Type {
+public class NamedType extends Type {
     /**
      * boolean type
      */
diff --git a/src/main/java/edu/kit/typicalc/model/type/Type.java b/src/main/java/edu/kit/typicalc/model/type/Type.java
index b501e8b..f87a578 100644
--- a/src/main/java/edu/kit/typicalc/model/type/Type.java
+++ b/src/main/java/edu/kit/typicalc/model/type/Type.java
@@ -9,21 +9,21 @@ import java.util.Set;
 /**
  * Models the type of a lambda term.
  */
-public interface Type {
+public abstract class Type {
     /**
      * Checks whether some type occurs in this type.
      *
      * @param x the type to look for
      * @return whether the specified type occurs in this type
      */
-    boolean contains(Type x);
+    public abstract boolean contains(Type x);
 
     /**
      * Returns a set of all free type variables occurring in the type.
      *
      * @return all free type variables
      */
-    Set<TypeVariable> getFreeTypeVariables();
+    public abstract Set<TypeVariable> getFreeTypeVariables();
 
     /**
      * Substitutes a type Variable for a different type.
@@ -32,7 +32,7 @@ public interface Type {
      * @param b the type to insert
      * @return a Type that is created by replacing a with b
      */
-    Type substitute(TypeVariable a, Type b);
+    public abstract Type substitute(TypeVariable a, Type b);
 
     /**
      * Applies the given substitution to the type.
@@ -40,7 +40,7 @@ public interface Type {
      * @param substitution the substitution to apply to the type
      * @return the substituted type
      */
-    default Type substitute(Substitution substitution) {
+    public Type substitute(Substitution substitution) {
         return substitute(substitution.getVariable(), substitution.getType());
     }
 
@@ -49,7 +49,7 @@ public interface Type {
      *
      * @param typeVisitor the visitor that wants to visit this
      */
-    void accept(TypeVisitor typeVisitor);
+    public abstract void accept(TypeVisitor typeVisitor);
 
     /**
      * Computes the necessary constraints (and substitution) to unify this type with
@@ -58,7 +58,7 @@ public interface Type {
      * @param type the other type
      * @return unification steps necessary, or an error if that is impossible
      */
-    Result<UnificationActions, UnificationError> constrainEqualTo(Type type);
+    public abstract Result<UnificationActions, UnificationError> constrainEqualTo(Type type);
 
     /**
      * Computes the necessary constraints (and substitution) to unify this type with a
@@ -67,7 +67,7 @@ public interface Type {
      * @param type the function type
      * @return unification steps necessary, or an error if that is impossible
      */
-    Result<UnificationActions, UnificationError> constrainEqualToFunction(FunctionType type);
+    public abstract Result<UnificationActions, UnificationError> constrainEqualToFunction(FunctionType type);
 
     /**
      * Computes the necessary constraints (and substitution) to unify this type with a
@@ -76,7 +76,7 @@ public interface Type {
      * @param type the named type
      * @return unification steps necessary, or an error if that is impossible
      */
-    Result<UnificationActions, UnificationError> constrainEqualToNamedType(NamedType type);
+    public abstract Result<UnificationActions, UnificationError> constrainEqualToNamedType(NamedType type);
 
     /**
      * Computes the necessary constraints (and substitution) to unify this type with a
@@ -85,5 +85,5 @@ public interface Type {
      * @param type the type variable
      * @return the unification steps necessary, or an error if that is impossible
      */
-    Result<UnificationActions, UnificationError> constrainEqualToVariable(TypeVariable type);
+    public abstract Result<UnificationActions, UnificationError> constrainEqualToVariable(TypeVariable type);
 }
diff --git a/src/main/java/edu/kit/typicalc/model/type/TypeVariable.java b/src/main/java/edu/kit/typicalc/model/type/TypeVariable.java
index bf68f9a..dedf1ae 100644
--- a/src/main/java/edu/kit/typicalc/model/type/TypeVariable.java
+++ b/src/main/java/edu/kit/typicalc/model/type/TypeVariable.java
@@ -8,7 +8,7 @@ import java.util.*;
 /**
  * Models a type variable
  */
-public class TypeVariable implements Type, Comparable<TypeVariable> {
+public class TypeVariable extends Type implements Comparable<TypeVariable> {
 
     private final TypeVariableKind kind;
     private final int index;