Is there an eval function in Java

Question

I have a string like the following   String str    4 5     Now I have to get the result of 20 by using the string   I know in some other languages the eval   function will do this  How can I do this in Java

User · Answer

Writing your own library is not that hard as u might thing. Here is link for Shunting-yard algorithm with step by step algorithm explenation. Although, you will have to parse the input for tokens first.

There are 2 other questions wich can give you some information too: Turn a String into a Math Expression? What's a good library for parsing mathematical expressions in java?

User · Answer

The following resolved the issue   ScriptEngineManager mgr   new ScriptEngineManager    ScriptEngine engine   mgr getEngineByName  JavaScript    String str    4 5   System out println engine eval str

User · Answer

There is no standard Java class or method that will do what you want   Your options include    Select and use some third-party expression evaluation library   For example JEL or any of the half dozen libraries listed here  Wrap the expression in the Java source code for a class with an eval method  send that to the Java compiler  and then load the resulting compiled class  Use some scripting language that can be called from Java as an expression evaluator   Possibilities include Javascript  BeanShell  and so on  Write your own expression evaluator from scratch    The first approach is probably simplest   The second and third approaches are a potential security risk if you get the expression to be evaluated from an untrusted user    Think code injection

User · Answer

There are very few real use cases in which being able to evaluate a String as a fragment of Java code is necessary or desirable  That is  asking how to do this is really an XY problem  you actually have a different problem  which can be solved a different way   First ask yourself  where did this String that you wish to evaluate come from  Did another part of your program generate it  or was it input provided by the user    Another part of my program generated it  so  you want one part of your program to decide the kind of operation to perform   but not perform the operation  and a second part that performs the chosen operation  Instead of generating and then evaluating  a String  use the Strategy  Command or Builder design pattern  as appropriate for your particular case  It is user input  the user could input anything  including commands that  when executed  could cause your program to misbehave  crash  expose information that should be secret  damage persistent information  such as the content of a database   and other such nastiness   The only way to prevent that would be to parse the String yourself   check it was not malicious  and then evaluate it  But parsing it yourself is much of the work that the requested evalfunction would do  so you have saved yourself nothing  Worse still  checking that arbitrary Java was not malicious is impossible  because checking that is the halting problem  It is user input  but the syntax and semantics of permitted text to evaluate is greatly restricted  No general purpose facility can easily implement a general purpose parser and evaluator for whatever restricted syntax and semantics you have chosen  What you need to do is implement a parser and evaluator for your chosen syntax and semantics  If the task is simple  you could write a simple recursive-descent or finite-state-machine parser by hand  If the task is difficult  you could use a compiler-compiler  such as ANTLR  to do some of the work for you  I just want to implement a desktop calculator   A homework assignment  eh  If you could implement the evaluation of the input expression using a provided eval function  it would not be much of a homework assignment  would it  Your program would be three lines long  Your instructor probably expects you to write the code for a simple arithmetic parser evaluator  There is well known algorithm  shunting-yard  which you might find useful

User · Answer

There are some perfectly capable answers here  However for non-trivial script it may be desirable to retain the code in a cache  or for debugging purposes  or even to have dynamically self-updating code   To that end  sometimes it s simpler or more robust to interact with Java via command line  Create a temporary directory  output your script and any assets  create the jar  Finally import your new code   It s a bit beyond the scope of normal eval   use in most languages  though you could certainly implement eval by returning the result from some function in your jar    Still  thought I d mention this method as it does fully encapsulate everything Java can do without 3rd party tools  in case of desperation  This method allows me to turn HTML templates into objects and save them  avoiding the need to parse a template at runtime

User · Answer

You can use the ScriptEngine class and evaluate it as a Javascript string   ScriptEngineManager manager   new ScriptEngineManager    ScriptEngine engine   manager getEngineByName  js    Object result   engine eval  4 5      There may be a better way  but this one works

User · Answer

No  you can not have a generic  eval  in Java  or any compiled language   Unless you re willing to write a Java compiler AND a JVM to be executed inside of your Java program   Yes  you can have some library to evaluate numeric algebraic expressions like the one above - see this thread for discussion

User · Answer

With Java 9  we get access to jshell  so one can write something like this   import jdk jshell JShell  import java lang StringBuilder  import java io BufferedReader  import java io InputStreamReader  import java io IOException   public class Eval       public static void main String   args  throws IOException           try JShell js   JShell create    BufferedReader br   new BufferedReader new InputStreamReader System in                   js onSnippetEvent snip - gt                    if  snip status      jdk jshell Snippet Status VALID                        System out println        snip value                                                    System out print   gt                  for  String line   br readLine    line    null  line   br readLine                      js eval js sourceCodeAnalysis   analyzeCompletion line  source                     System out print   gt                                        Sample run    gt  1   2   4   3   1  gt  32   121   3872  gt  4   5   20  gt  121   51   6171  gt    Slightly op  but that s what Java currently has to offer

User · Answer

As there are many answers  I m adding my implementation on top of eval   method with some additional features like support for factorial  evaluating complex expressions etc    package evaluation   import java math BigInteger  import java util EmptyStackException  import java util Scanner  import java util Stack  import javax script ScriptEngine  import javax script ScriptEngineManager  import javax script ScriptException   public class EvalPlus       private static Scanner scanner   new Scanner System in        public static void main String   args            System out println  This Evaluation is based on BODMAS rule n            evaluate               private static void evaluate             StringBuilder finalStr   new StringBuilder            System out println  Enter an expression to evaluate             String expr   scanner nextLine             if isProperExpression expr                 expr   replaceBefore expr               char   temp   expr toCharArray                String operators      -                   for int i   0  i  lt  temp length  i                      if  i    0  amp  amp  temp i              i    temp length-1  amp  amp  temp i          amp  amp  temp i                                 finalStr append temp i                      else if  i  gt  0  amp  amp  i  lt  temp length -1      i  temp length-1  amp  amp  temp i                                 if temp i                                    StringBuilder str   new StringBuilder                            for int k   i-1  k  gt   0  k--                                if Character isDigit temp k                                      str insert 0  temp k                                   else                                   break                                                                                  Long prev   Long valueOf str toString                             BigInteger val   new BigInteger  1                            for Long j   prev  j  gt  1  j--                                val   val multiply BigInteger valueOf j                                                      finalStr setLength finalStr length   - str length                             finalStr append       val                                 if temp length  gt  i 1                                char next   temp i 1                               if operators indexOf next     -1                                     finalStr append                                                                                     else                           finalStr append temp i                                                                      expr   finalStr toString                if expr    null  amp  amp   expr isEmpty                      ScriptEngineManager mgr   new ScriptEngineManager                    ScriptEngine engine   mgr getEngineByName  JavaScript                    try                       System out println  Result      engine eval expr                        evaluate                      catch  ScriptException e                        System out println e getMessage                                     else                   System out println  Please give an expression                    evaluate                            else               System out println  Not a valid expression                evaluate                         private static String replaceBefore String expr            expr   expr replace                     expr   expr replace            replace  -     -   replace            replace            replace                     return expr             private static boolean isProperExpression String expr            expr   expr replaceAll                       char   arr   expr toCharArray            Stack lt Character gt  stack   new Stack lt Character gt             int i  0          while i  lt  arr length                try                   if arr i                                stack push arr i                      else                       stack pop                                    catch  EmptyStackException e                    stack push arr i                              i                      return stack isEmpty              Please find the updated gist anytime here  Also comment if any issues are there  Thanks

User · Answer

I could advise you to use Exp4j  It is easy to understand as you can see from the following example code   Expression e   new ExpressionBuilder  3   sin y  - 2    x - 2         variables  x    y        build        setVariable  x   2 3       setVariable  y   3 14   double result   e evaluate

User · Answer

As previous answers  there is no standard API in Java for this    You can add groovy jar files to your path and groovy util Eval me  4 5   gets your job done

User · Answer

A fun way to solve your problem could be coding an eval   function on your own  I ve done it for you   You can use FunctionSolver library simply by typing FunctionSolver solveByX function value  inside your code  The function attribute is a String which represents the function you want to solve  the value attribute is the value of the independent variable  of your function  which MUST be x    If you want to solve a function which contains more than one independent variable  you can use FunctionSolver solve function values  where the values attribute is an HashMap String Double  which contains all your independent attributes  as Strings  and their respective values  as Doubles    Another piece of information  I ve coded a simple version of FunctionSolver  so its supports only Math methods which return a double value and which accepts one or two double values as fields  just use FunctionSolver usableMathMethods   if you re curious   These methods are  bs  sin  cos  tan  atan2  sqrt  log  log10  pow  exp  min  max  copySign  signum  IEEEremainder  acos  asin  atan  cbrt  ceil  cosh  expm1  floor  hypot  log1p  nextAfter  nextDown  nextUp  random  rint  sinh  tanh  toDegrees  toRadians  ulp   Also  that library supports the following operators        -    even if java normally does not support the   operator    One last thing  while creating this library I had to use reflections to call Math methods  I think it s really cool  just have a look at this if you are interested in   That s all  here it is the code  and the library    package core    import java lang reflect InvocationTargetException   import java lang reflect Method   import java util ArrayList   import java util Arrays   import java util HashMap    public abstract class FunctionSolver    public static double solveNumericExpression  String expression  throws NoSuchMethodException  IllegalAccessException  IllegalArgumentException  InvocationTargetException       return solve expression  new HashMap lt  gt         public static double solveByX  String function  double value  throws NoSuchMethodException  IllegalAccessException  IllegalArgumentException  InvocationTargetException        HashMap lt String  Double gt  values   new HashMap lt  gt         values put  x   value       return solveComplexFunction function  function  values      public static double solve  String function  HashMap lt String Double gt  values  throws NoSuchMethodException  IllegalAccessException  IllegalArgumentException  InvocationTargetException        return solveComplexFunction function  function  values      private static double solveComplexFunction  String function  String motherFunction  HashMap lt String  Double gt  values  throws NoSuchMethodException  IllegalAccessException  IllegalArgumentException  InvocationTargetException        int position   0      while position  lt  function length              if  alphabetic contains    function charAt position                  if  position    0     alphabetic contains    function charAt position-1                      int endIndex   -1                  for  int j   position   j  lt  function length  -1   j                          if  alphabetic contains    function charAt j                                 amp  amp   alphabetic contains    function charAt j 1                              endIndex   j                          break                                                          if  endIndex    -1  amp  alphabetic contains    function charAt function length  -1                          endIndex   function length  -1                                    if  endIndex    -1                        String alphabeticElement   function substring position  endIndex 1                       if  Arrays asList usableMathMethods    contains alphabeticElement                               Start analyzing a Math function                         int closeParenthesisIndex   -1                          int openedParenthesisquantity   0                          int commaIndex   -1                          for  int j   endIndex 1   j  lt  function length     j                                  if  function substring j j 1  equals                                         openedParenthesisquantity                                 else if  function substring j j 1  equals                                         openedParenthesisquantity--                                  if  openedParenthesisquantity    0                                        closeParenthesisIndex   j                                      break                                                                 else if  function substring j j 1  equals       amp  openedParenthesisquantity    0                                    if  commaIndex    -1                                        commaIndex   j                                   else                                      throw new IllegalArgumentException  The argument of math function  which is   alphabeticElement    has too many commas                                                                                                                      if  closeParenthesisIndex    -1                                throw new IllegalArgumentException  The argument of a Math function  which is   alphabeticElement    hasn t got the closing bracket                                                           String functionArgument   function substring endIndex 2 closeParenthesisIndex                           if  commaIndex    -1                                double firstParameter   solveComplexFunction functionArgument substring 0 commaIndex  motherFunction values                               double secondParameter   solveComplexFunction functionArgument substring commaIndex 1  motherFunction values                               Method mathMethod   Math class getDeclaredMethod alphabeticElement  new Class lt   gt     double class  double class                                mathMethod setAccessible true                               String newKey   getNewKey values                               values put newKey   Double  mathMethod invoke null  firstParameter  secondParameter                                function   function substring 0  position  newKey                                           closeParenthesisIndex    function length  -1        function substring closeParenthesisIndex 1                              else                               double firstParameter   solveComplexFunction functionArgument  motherFunction  values                               Method mathMethod   Math class getDeclaredMethod alphabeticElement  new Class lt   gt     double class                                mathMethod setAccessible true                               String newKey   getNewKey values                               values put newKey   Double  mathMethod invoke null  firstParameter                                function   function substring 0  position  newKey                                           closeParenthesisIndex    function length  -1        function substring closeParenthesisIndex 1                                                       else if   values containsKey alphabeticElement                             throw new IllegalArgumentException  Found a group of letters    alphabeticElement    which is neither a variable nor a Math function                                                                              position              return solveBracketsFunction function motherFunction values      private static double solveBracketsFunction  String function String motherFunction HashMap lt String  Double gt  values  throws IllegalArgumentException       function   function replace               String openingBrackets              String closingBrackets              int parenthesisIndex   0      do           int position   0          int openParenthesisBlockIndex   -1          String currentOpeningBracket   openingBrackets charAt parenthesisIndex              String currentClosingBracket   closingBrackets charAt parenthesisIndex              if  contOccouranceIn currentOpeningBracket function     contOccouranceIn currentClosingBracket function                 throw new IllegalArgumentException  Error  brackets are misused in the function   function                     while  position  lt  function length                  if  function substring position position 1  equals currentOpeningBracket                     if  position    0  amp  amp   operators contains function substring position-1 position                          throw new IllegalArgumentException  Error in function  there must be an operator following a   currentClosingBracket   breacket                                      openParenthesisBlockIndex   position               else if  function substring position position 1  equals currentClosingBracket                     if  position    function length  -1  amp  amp   operators contains function substring position 1 position 2                          throw new IllegalArgumentException  Error in function  there must be an operator before a   currentClosingBracket   breacket                                      String newKey   getNewKey values                   values put newKey  solveBracketsFunction function substring openParenthesisBlockIndex 1 position  motherFunction  values                    function   function substring 0 openParenthesisBlockIndex  newKey                               position    function length  -1        function substring position 1                     position   -1                            position                      parenthesisIndex         while  parenthesisIndex  lt  openingBrackets length         return solveBasicFunction function motherFunction  values      private static double solveBasicFunction  String function  String motherFunction  HashMap lt String  Double gt  values  throws IllegalArgumentException       if   firstContainsOnlySecond function  alphanumeric operators             throw new IllegalArgumentException  The function   function   is not a basic function              if  function contains                 function contains                 function contains  --             function contains                 function contains                 function contains  -              function contains  -                               -     -    -         -        gt  Those values are admitted                     throw new IllegalArgumentException  Operators are misused in the function              function   function replace               int position      int operatorIndex   0      String currentOperator      do           currentOperator   operators substring operatorIndex operatorIndex 1           if  currentOperator equals                     currentOperator                   operatorIndex             else if  currentOperator equals                     currentOperator   -               operatorIndex                      operatorIndex            position   0          while  position  lt  function length                  if   position    0  amp  amp       function charAt position   equals  -    amp  amp       function charAt position   equals       amp  amp  operators contains    function charAt position                        position    function length  -1  amp  amp  operators contains    function charAt position                      throw new IllegalArgumentException  Operators are misused in the function                              if  currentOperator contains function substring position  position 1    amp  position    0                    int firstTermBeginIndex   position                  while  firstTermBeginIndex  gt  0                        if   alphanumeric contains    function charAt firstTermBeginIndex     amp   operators contains    function charAt firstTermBeginIndex-1                              break                                            firstTermBeginIndex--                                    if  firstTermBeginIndex    0  amp  amp   function charAt firstTermBeginIndex-1      -    function charAt firstTermBeginIndex-1                                 if  firstTermBeginIndex    1                            firstTermBeginIndex--                       else if  operators contains     function charAt firstTermBeginIndex-2                              firstTermBeginIndex--                                                          String firstTerm   function substring firstTermBeginIndex position                   int secondTermLastIndex   position                  while  secondTermLastIndex  lt  function length  -1                        if   alphanumeric contains    function charAt secondTermLastIndex     amp   operators contains    function charAt secondTermLastIndex 1                               break                                            secondTermLastIndex                                      String secondTerm   function substring position 1 secondTermLastIndex 1                   double result                  switch  function substring position position 1                         case      result   solveSingleValue firstTerm values  solveSingleValue secondTerm values   break                      case      result   solveSingleValue firstTerm values  solveSingleValue secondTerm values   break                      case      result   solveSingleValue firstTerm values  solveSingleValue secondTerm values   break                      case  -   result   solveSingleValue firstTerm values -solveSingleValue secondTerm values   break                      case      result   Math pow solveSingleValue firstTerm values  solveSingleValue secondTerm values    break                      default  throw new IllegalArgumentException  Unknown operator    currentOperator                                     String newAttribute   getNewKey values                   values put newAttribute  result                   function   function substring 0 firstTermBeginIndex  newAttribute function substring secondTermLastIndex 1 function length                     deleteValueIfPossible firstTerm  values  motherFunction                   deleteValueIfPossible secondTerm  values  motherFunction                   position   -1                            position                   while  operatorIndex  lt  operators length         return solveSingleValue function  values      private static double solveSingleValue  String singleValue  HashMap lt String  Double gt  values  throws IllegalArgumentException       if  isDouble singleValue             return Double parseDouble singleValue        else if  firstContainsOnlySecond singleValue  alphabetic            return getValueFromVariable singleValue  values        else if  firstContainsOnlySecond singleValue  alphanumeric  -               String   composition   splitByLettersAndNumbers singleValue           if  composition length    2                throw new IllegalArgumentException  Wrong expression    singleValue            else               if  composition 0  equals  -                      composition 0     -1                else if  composition 1  equals  -                      composition 1     -1                else if  composition 0  equals                         composition 0      1                else if  composition 1  equals                         composition 1      1                             if  isDouble composition 0                      return Double parseDouble composition 0   getValueFromVariable composition 1   values                else if  isDouble composition 1                     return Double parseDouble composition 1   getValueFromVariable composition 0   values                else                   throw new IllegalArgumentException  Wrong expression    singleValue                                else           throw new IllegalArgumentException  Wrong expression    singleValue            private static double getValueFromVariable  String variable  HashMap lt String  Double gt  values  throws IllegalArgumentException       Double val   values get variable       if  val    null            throw new IllegalArgumentException  Unknown variable    variable        else           return val                 FunctionSolver help tools           private static final String alphabetic    abcdefghilmnopqrstuvzwykxy   private static final String numeric    0123456789    private static final String alphanumeric   alphabetic numeric  private static final String operators        -     -- gt  Operators order in important   private static boolean firstContainsOnlySecond String firstString  String secondString         for  int j   0   j  lt  firstString length     j              if   secondString contains firstString substring j  j 1                  return false                      return true     private static String getNewKey  HashMap lt String  Double gt  hashMap         String alpha    abcdefghilmnopqrstuvzyjkx       for  int j   0   j  lt  alpha length     j              String k   alpha substring j j 1           if   hashMap containsKey k   amp   Arrays asList usableMathMethods    contains k                 return k                      for  int j   0   j  lt  alpha length     j              for  int i   0   i  lt  alpha length     i                  String k   alpha substring j j 1  alpha substring i i 1               if   hashMap containsKey k   amp   Arrays asList usableMathMethods    contains k                     return k                                    throw new NullPointerException       public static String   usableMathMethods                      Only methods that          return a double type         present one or two parameters  which are double type               Method   mathMethods   Math class getDeclaredMethods        ArrayList lt String gt  usableMethodsNames   new ArrayList lt  gt         for  Method method   mathMethods            boolean usable   true          int argumentsCounter   0          Class lt   gt    methodParametersTypes   method getParameterTypes            for  Class lt   gt  parameter   methodParametersTypes                if   parameter getSimpleName   equalsIgnoreCase  double                      usable   false                  break               else                   argumentsCounter                                    if   method getReturnType   getSimpleName   toLowerCase   equals  double                  usable   false                    if  usable  amp  argumentsCounter lt  2                usableMethodsNames add method getName                         return usableMethodsNames toArray new String usableMethodsNames size         private static boolean isDouble  String number        try           Double parseDouble number           return true       catch  Exception ex            return false           private static String   splitByLettersAndNumbers  String val        if   firstContainsOnlySecond val  alphanumeric   -              throw new IllegalArgumentException  Wrong passed value   lt  lt   val   gt  gt               ArrayList lt String gt  response   new ArrayList lt  gt         String searchingFor      int lastIndex   0      if  firstContainsOnlySecond    val charAt 0   numeric   -              searchingFor   alphabetic       else           searchingFor   numeric   -             for  int j   0   j  lt  val length     j              if  searchingFor contains val charAt j                     response add val substring lastIndex  j                lastIndex   j              if  searchingFor equals numeric   -                      searchingFor   alphabetic               else                   searchingFor   numeric   -                                     response add val substring lastIndex val length          return response toArray new String response size         private static void deleteValueIfPossible  String val  HashMap lt String  Double gt  values  String function        if  values get val     null  amp  function    null            if   function contains val                 values remove val                      private static int contOccouranceIn  String howManyOfThatString  String inThatString        return inThatString length   - inThatString replace howManyOfThatString      length

[java] Is there an eval() function in Java?

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