How Can I Parse and Evaluate Mathematical Expressions as Strings in Java?

How to Parse and Evaluate Mathematical Expressions as Strings in Java

When working with mathematical expressions in Java, it's often necessary to parse and evaluate these expressions to obtain their values. This task can be achieved using various methods, including the BeanShell interpreter and ANTLR.

Using BeanShell Interpreter

The BeanShell interpreter is a powerful tool for evaluating strings as if they were Java code. It allows you to pass mathematical expressions as strings and retrieve the result:

Interpreter interpreter = new Interpreter();
interpreter.eval("result = 5+4*(7-15)");
System.out.println(interpreter.get("result")); // Prints -14

Using BeanShell is straightforward but requires caution since it evaluates the string as arbitrary code. Ensure that the input string is validated and trusted.

Using ANTLR

ANTLR (ANother Tool for Language Recognition) is a sophisticated language-parsing framework that can be used to parse mathematical expressions. By creating a grammar and defining actions to perform, you can build a parser that interprets mathematical statements and generates their values:

// Import ANTLR dependencies
import org.antlr.v4.runtime.*;
import org.antlr.v4.runtime.tree.*;

// Create a char stream from the expression string
CharStream charStream = CharStreams.fromString("5+4*(7-15)");

// Generate a lexer that tokens the input
Lexer lexer = new MathLexer(charStream);

// Generate a parser that parses the tokens
Parser parser = new MathParser(new CommonTokenStream(lexer));

// Parse the expression and generate a parse tree
ParseTree tree = parser.statement();

// Perform semantic actions to evaluate the expression
SemanticListener listener = new SemanticListener();
ParseTreeWalker.DEFAULT.walk(listener, tree);

// Retrieve the result from the listener
double result = listener.getResult(); // -14

ANTLR provides a more secure and structured approach to parsing mathematical expressions but requires more effort to set up.

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