implement visitor to collect all struct names

src/main/java/de/hsrm/compiler/Klang/GetStructNames.java

@@ -0,0 +1,37 @@
+package de.hsrm.compiler.Klang;
+
+import java.util.Set;
+
+import de.hsrm.compiler.Klang.helper.Helper;
+
+public class GetStructNames extends KlangBaseVisitor<Void> {
+    private Set<String> structNames;
+
+    public GetStructNames(Set<String> structNames) {
+        this.structNames = structNames;
+    }
+
+    @Override
+    public Void visitProgram(KlangParser.ProgramContext ctx) {
+        for (int i = 0; i < ctx.structDef().size(); i++) {
+            this.visit(ctx.structDef(i));
+        }
+
+        return null;
+    }
+
+    @Override
+    public Void visitStructDef(KlangParser.StructDefContext ctx) {
+        String name = ctx.structName.getText();
+        int line = ctx.start.getLine();
+        int col = ctx.start.getCharPositionInLine();
+
+        if (this.structNames.contains(name)) {
+            String error = "Struct " + name + " defined multiple times.";
+            throw new Error(Helper.getErrorPrefix(line, col) + error);
+        }
+
+        this.structNames.add(name);
+        return null;
+    }
+}