added interpreter sample

interpreter/src/main/java/com/iluwatar/NumberExpression.java

@@ -0,0 +1,25 @@
+package com.iluwatar;
+
+public class NumberExpression extends Expression {
+
+	private int number;
+
+	public NumberExpression(int number) {
+		this.number = number;
+	}
+	
+	public NumberExpression(String s) {
+		this.number = Integer.parseInt(s);
+	}
+	
+	@Override
+	public int interpret() {
+		return number;
+	}
+
+	@Override
+	public String toString() {
+		return "number";
+	}
+
+}