added interpreter sample

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

@@ -0,0 +1,23 @@
+package com.iluwatar;
+
+public class PlusExpression extends Expression {
+
+	private Expression leftExpression;
+	private Expression rightExpression;
+
+	public PlusExpression(Expression leftExpression, Expression rightExpression) {
+		this.leftExpression = leftExpression;
+		this.rightExpression = rightExpression;
+	}
+	
+	@Override
+	public int interpret() {
+		return leftExpression.interpret() + rightExpression.interpret();
+	}
+
+	@Override
+	public String toString() {
+		return "+";
+	}
+
+}