Implement two enums for conversion between strings and tokens.

src/org/nwapw/abacus/tree/OperatorAssociativity.java

@@ -0,0 +1,5 @@
+package org.nwapw.abacus.tree;
+
+public enum OperatorAssociativity {
+    LEFT, RIGHT
+}

src/org/nwapw/abacus/tree/TokenType.java

@@ -0,0 +1,13 @@
+package org.nwapw.abacus.tree;
+
+public enum TokenType {
+
+    ANY(0), OP(1), NUM(2), WORD(3), OPEN_PARENTH(4), CLOSE_PARENTH(5);
+
+    public final int priority;
+
+    TokenType(int priority){
+        this.priority = priority;
+    }
+
+}