added interpreter sample

interpreter/pom.xml

@@ -0,0 +1,23 @@
+<?xml version="1.0"?>
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.0-SNAPSHOT</version>
+  </parent>
+  <groupId>com.iluwatar</groupId>
+  <artifactId>interpreter</artifactId>
+  <version>1.0-SNAPSHOT</version>
+  <name>interpreter</name>
+  <url>http://maven.apache.org</url>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <version>3.8.1</version>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

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

@@ -0,0 +1,61 @@
+package com.iluwatar;
+
+import java.util.Stack;
+
+public class App 
+{
+	
+	/**
+	 * 
+	 * Expressions can be evaluated using prefix, infix or postfix notations
+	 * This sample uses postfix, where operator comes after the operands
+	 * 
+	 */
+    public static void main( String[] args )
+    {
+    	String tokenString = "4 3 2 - 1 + *";
+		Stack<Expression> stack = new Stack<>();
+
+		String[] tokenList = tokenString.split(" ");
+		for (String s : tokenList) {
+			if (isOperator(s)) {
+				Expression rightExpression = stack.pop();
+				Expression leftExpression = stack.pop();
+				System.out.println(String.format("popped from stack left: %d right: %d", 
+						leftExpression.interpret(), rightExpression.interpret()));
+				Expression operator = getOperatorInstance(s, leftExpression,
+						rightExpression);
+				System.out.println(String.format("operator: %s", operator));
+				int result = operator.interpret();
+				NumberExpression resultExpression = new NumberExpression(result);
+				stack.push(resultExpression);
+				System.out.println(String.format("push result to stack: %d", resultExpression.interpret()));
+			} else {
+				Expression i = new NumberExpression(s);
+				stack.push(i);
+				System.out.println(String.format("push to stack: %d", i.interpret()));
+			}
+		}
+		System.out.println(String.format("result: %d", stack.pop().interpret()));    	
+    }
+    
+    public static boolean isOperator(String s) {
+		if (s.equals("+") || s.equals("-") || s.equals("*"))
+			return true;
+		else
+			return false;
+	}
+
+	public static Expression getOperatorInstance(String s, Expression left,
+			Expression right) {
+		switch (s) {
+		case "+":
+			return new PlusExpression(left, right);
+		case "-":
+			return new MinusExpression(left, right);
+		case "*":
+			return new MultiplyExpression(left, right);
+		}
+		return null;
+	}	
+}

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

@@ -0,0 +1,9 @@
+package com.iluwatar;
+
+public abstract class Expression {
+
+	public abstract int interpret();
+	
+	@Override
+	public abstract String toString();
+}

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

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

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

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

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";
+	}
+
+}

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 "+";
+	}
+
+}

pom.xml

@@ -32,6 +32,7 @@
     <module>proxy</module>
     <module>chain</module>
     <module>command</module>
+    <module>interpreter</module>
   </modules>
   
 <build>