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