Add Hashcode methods

src/main/java/de/dhbwstuttgart/parser/scope/GatherNames.java

@@ -43,7 +43,7 @@ public class GatherNames {
 	      }
 	      else{
 	        if(typeDecl.classDeclaration().normalClassDeclaration() != null){
-	          if(pkgName != ""){
+	          if(!pkgName.isEmpty()){
 	            nameString = pkgName + "." + typeDecl.classDeclaration().normalClassDeclaration().Identifier().toString();
 	          }
 	          else{

src/main/java/de/dhbwstuttgart/syntaxtree/type/TypePlaceholder.java

@@ -64,6 +64,10 @@ public class TypePlaceholder extends RefTypeOrTPHOrWildcardOrGeneric
         }
     }
 
+    @Override
+    public int hashCode() {
+        return this.getName().hashCode();
+    }
 
     public String toString()
     {

src/main/java/de/dhbwstuttgart/typeinference/unify/model/UnifyType.java

@@ -114,6 +114,7 @@ public abstract class UnifyType {
 
 	@Override
 	public boolean equals(Object obj) {
+		if(obj == null)return false;
 		return this.toString().equals(obj.toString());
 	}
 }