import java.util.HashMap;public interface Expression {    public double evaluate(HashMap<String, Double> context) throws Exception;}

import java.util.HashMap;public class Multiplication implements Expression {    private final Expression operand1;    private final Expression operand2;    public Multiplication(Expression operand1, Expression operand2) {        this.operand1 = operand1;        this.operand2 = operand2;    }    public double evaluate(HashMap<String, Double> context) throws Exception {        return operand1.evaluate(context) * operand2.evaluate(context);    }}

import java.util.HashMap;public class Division implements Expression {    private final Expression operand1;    private final Expression operand2;    public Division(Expression operand1, Expression operand2) {        this.operand1 = operand1;        this.operand2 = operand2;    }    public double evaluate(HashMap<String, Double> context) throws Exception {        return operand1.evaluate(context) / operand2.evaluate(context);    }}

import java.util.HashMap;public class Constant implements Expression {    private final double value;    public Constant(double value) {        this.value = value;    }    public double evaluate(HashMap<String, Double> context) {        return value;    }}

import java.util.HashMap;public class Variable implements Expression {    private final String name;    public Variable(String name) {        this.name = name;    }    public double evaluate(HashMap<String, Double> context) throws Exception {        Double value = context.get(name);        if (value == null)            throw new Exception("Unbound variable: " + name);        return value;    }}

HashMap<String, Double> context = new HashMap<String, Double>();context.put("base", 10.0);context.put("height", 5.0);Expression two = new Constant(2);Expression base = new Variable("base");Expression height = new Variable("height");Expression multi = new Multiplication(base, height);Expression triangleArea = new Division(multi, two);double result = triangleArea.evaluate(context);