如何计算出字符串 "3*4+5"的值啊

[小幸运(●ˇ∀ˇ●)]

如何计算出字符串 像"3*4+5"的值啊

[龙四]

jdk1.6可以这样做：

import javax.script.*;
public class TestScript {
	public static void main(String[] args) throws Exception {
		String exp = "3*4+5";
		ScriptEngineManager manager = new ScriptEngineManager();
		ScriptEngine engine = manager.getEngineByName("jexl");
		Object result = engine.eval(exp);
		
		System.out.println("结果类型:" + result.getClass().getName() + ",计算结果:" + result);

	}
}

[amos1989]

楼上的已经解决了。。

[龙四]

上面是jdk1.5的做法

需要下载

commons-jexl-2.0.1.jar
commons-logging-1.1.1.jar
bsf-api-3.1.jar

加到classpath中就可以运行了

[amos1989]

如果是JDK6以下的版本，解决的办法也有很多。

可以对字符串进行解析。

[龙四]

jdk1.6可以这样做：

import javax.script.*;
public class TestScript {
	public static void main(String[] args) throws Exception {
		String exp = "3*4+5";
		ScriptEngineManager manager = new ScriptEngineManager();
		ScriptEngine engine = manager.getEngineByName("js");
		Object result = engine.eval(exp);
		
		System.out.println("结果类型:" + result.getClass().getName() + ",计算结果:" + result);

	}
}

[joaryyu]

可以动态编译

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

import javax.tools.JavaCompiler;
import javax.tools.ToolProvider;


public class DyComp {
	String expr = null;	

	public static double execute(String expr) {
		DyComp dc = new DyComp(expr);
		double result = -99999;
		try {
			result = dc.result();
		} catch (IOException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		return result;
	}
	
	//私有构造
	private DyComp(String expr) {
		this.expr = expr;
	}
	
	public double result() throws IOException {
		String javafileContent = "import java.io.*;\r\n" +
				"public class ExpreDyComp {" + "\r\n" +
			"public static double cal() {\r\n" +
					"\tdouble var = " + this.expr + ";\r\n" +
					"return var;\r\n" +
					"}\r\n}";
		System.out.println("待生成的文件内容" + "\n" + javafileContent);
		File tmpFile = new File("ExpreDyComp.java");
		tmpFile.createNewFile();
		PrintWriter printWriter = new PrintWriter(new FileWriter(tmpFile));
		printWriter.write(javafileContent);
		printWriter.flush();
		printWriter.close();
		//对文件进行编译
		JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
		int compRes = compiler.run(null, null, null,"-d","bin","ExpreDyComp.java");
		if(compRes != 0) {
			System.out.println("编译出错");
		}
		
		Class exprClass = null;
		try {
			exprClass = Class.forName("ExpreDyComp");
		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
			System.err.println("找不到相应类");
		}
		Method m = null;
		try {
			if(exprClass != null) {
				m = exprClass.getMethod("cal");
			}
		} catch (SecurityException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (NoSuchMethodException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		if(m != null) {
			try {
				Double d = (Double)m.invoke(null);
				//System.out.println("值为" + d.doubleValue());
				return d.doubleValue();
			} catch (IllegalArgumentException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (IllegalAccessException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			} catch (InvocationTargetException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
		}
		return -9999;
	}
	
	private double readResultToFile(File inFile) throws IOException {
		BufferedReader bf = new BufferedReader(new FileReader(inFile));
		String content = bf.readLine();
		double res = 0;
		if(content != null) {
			res = Double.parseDouble(content);
		} else {
			System.out.println("文件" + inFile.getAbsolutePath() + "无数据");
		}
		bf.close();
		return res;
	}
	
	public static void main(String args[]) {
		System.out.println("动态编译");
		System.out.println("输入你要计算的表达式");
		InputStreamReader inputReader = new InputStreamReader(System.in);
		BufferedReader bfReader = new BufferedReader(inputReader);
		String expre = null;
		try {
			expre = bfReader.readLine();
		} catch (IOException e1) {
			// TODO Auto-generated catch block
			System.out.println("从键盘输入出现问题");
			e1.printStackTrace();
		}
		//定义结果数据
		double result = -99999;
		result = execute(expre);
		//打印到console
		System.out.println(expre + "计算结果如下" + "\n" + result);
	}
}

[小幸运(●ˇ∀ˇ●)]

果然可以，大神就是大神啊！！

[joaryyu]

[小幸运(●ˇ∀ˇ●)]

[Quote=引用 1 楼 ticmy 的回复:]
jdk1.6可以这样做：

Java code
import javax.script.*;
public class TestScript {
public static void main(String[] args) throws Exception {
String exp = "3*4+5";
ScriptEngineManager ……
[/Quote]
不过计算"3*4+5%5”会出错啊！！！！！！！11