add some useful classes

tool/src/org/antlr/v4/misc/FrequencySet.java

@@ -0,0 +1,22 @@
+package org.antlr.v4.misc;
+
+import java.util.Hashtable;
+
+/** Count how many of each key we have; not thread safe */
+public class FrequencySet<T> extends Hashtable<T, MutableInt> {
+	public int count(T key) {
+		MutableInt value = get(key);
+		if (value == null) return 0;
+		return value.v;
+	}
+	public void add(T key) {
+		MutableInt value = get(key);
+		if (value == null) {
+			value = new MutableInt(1);
+			put(key, value);
+		}
+		else {
+			value.v++;
+		}
+	}
+}

tool/src/org/antlr/v4/misc/MutableInt.java

@@ -0,0 +1,17 @@
+package org.antlr.v4.misc;
+
+public class MutableInt extends Number implements Comparable<Number> {
+	public int v;
+
+	public MutableInt(int v) { this.v = v; }
+	@Override public int compareTo(Number o) { return v; }
+	@Override public int intValue() { return v; }
+	@Override public long longValue() { return v; }
+	@Override public float floatValue() { return v; }
+	@Override public double doubleValue() { return v; }
+
+	@Override
+	public String toString() {
+		return String.valueOf(v);
+	}
+}

tool/src/org/antlr/v4/misc/Pair.java

@@ -0,0 +1,11 @@
+package org.antlr.v4.misc;
+
+public class Pair<A,B> {
+	public A a;
+	public B b;
+
+	public Pair(A a, B b) {
+		this.a = a;
+		this.b = b;
+	}
+}