How do I invoke a Java method when given the method name as a string

Question

If I have two variables   Object obj  String methodName    getName     Without knowing the class of obj  how can I call the method identified by methodName on it   The method being called has no parameters  and a String return value  It s a getter for a Java bean

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I do this like this   try       YourClass yourClass   new YourClass        Method method   YourClass class getMethod  yourMethodName   ParameterOfThisMethod class       method invoke yourClass  parameter     catch  Exception e        e printStackTrace

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For those who want a straight-forward code example in Java 7   Dog class   package com mypackage bean   public class Dog       private String name      private int age       public Dog                empty constructor            public Dog String name  int age            this name   name          this age   age             public String getName             return name             public void setName String name            this name   name             public int getAge             return age             public void setAge int age            this age   age             public void printDog String name  int age            System out println name     is     age     year s  old               ReflectionDemo class   package com mypackage demo   import java lang reflect     public class ReflectionDemo        public static void main String   args  throws Exception           String dogClassName    com mypackage bean Dog           Class lt   gt  dogClass   Class forName dogClassName      convert string classname to class         Object dog   dogClass newInstance       invoke empty constructor          String methodName                   with single parameter  return void         methodName    setName           Method setNameMethod   dog getClass   getMethod methodName  String class           setNameMethod invoke dog   Mishka       pass arg             without parameters  return string         methodName    getName           Method getNameMethod   dog getClass   getMethod methodName           String name    String  getNameMethod invoke dog      explicit cast             with multiple parameters         methodName    printDog           Class lt   gt    paramTypes    String class  int class           Method printDogMethod   dog getClass   getMethod methodName  paramTypes           printDogMethod invoke dog  name  3      pass args           Output  Mishka is 3 year s  old     You can invoke the constructor with parameters this way   Constructor lt   gt  dogConstructor   dogClass getConstructor String class  int class   Object dog   dogConstructor newInstance  Hachiko   10     Alternatively  you can remove  String dogClassName    com mypackage bean Dog   Class lt   gt  dogClass   Class forName dogClassName   Object dog   dogClass newInstance      and do  Dog dog   new Dog     Method method   Dog class getMethod methodName        method invoke dog          Suggested reading  Creating New Class Instances

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Step1 - Using string funClass to convert to class String funClass    package myclass   Class c   Class forName funClass      Step2 - instantiate an object of the class abov Object o   c newInstance      Prepare array of the arguments that your function accepts  lets say only one string here Class   paramTypes   new Class 1   paramTypes 0  String class  String methodName    mymethod     Instantiate an object of type method that returns you method name  Method m   c getDeclaredMethod methodName  paramTypes     invoke method with actual params m invoke o   testparam

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First  don t  Avoid this sort of code  It tends to be really bad code and insecure too  see section 6 of Secure Coding Guidelines for the Java Programming Language  version 2 0    If you must do it  prefer java beans to reflection  Beans wraps reflection allowing relatively safe and conventional access

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Student java  class Student      int rollno      String name       void m1 int x int y           System out println  add is    x y               private void m3 String name           this name name          System out println  danger yappa   name             void m4            System out println  This is m4              StudentTest java  import java lang reflect Method  public class StudentTest        public static void main String   args            try               Class cls Student class               Student s  Student cls newInstance                  String x  kichha               Method mm3 cls getDeclaredMethod  m3  String class               mm3 setAccessible true               mm3 invoke s x                Method mm1 cls getDeclaredMethod  m1  int class int class               mm1 invoke s 10 20                      catch Exception e               e printStackTrace

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Indexing  faster  You can use FunctionalInterface to save methods in a container to index them  You can use array container to invoke them by numbers or hashmap to invoke them by strings  By this trick  you can index your methods to invoke them dynamically faster   FunctionalInterface public interface Method       double execute int number      public class ShapeArea       private final static double PI   3 14       private Method   methods             this  square          this  circle             private double square int number            return number   number             private double circle int number            return PI   number   number             public double run int methodIndex  int number            return methods methodIndex  execute number            Lambda syntax You can also use lambda syntax  public class ShapeArea       private final static double PI   3 14       private Method   methods             number - gt                return number   number                     number - gt                return PI   number   number                         public double run int methodIndex  int number            return methods methodIndex  execute number

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You should use reflection - init a class object  then a method in this class  and then invoke this method on an object with optional parameters  Remember to wrap the following snippet in try-catch block  Hope it helps   Class lt   gt  aClass   Class forName FULLY QUALIFIED CLASS NAME   Method method   aClass getMethod methodName  YOUR PARAM 1 class  YOUR PARAM 2 class   method invoke OBJECT TO RUN METHOD ON  YOUR PARAM 1  YOUR PARAM 2

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Method method   someVariable class getMethod SomeClass   String status    String  method invoke method     SomeClass is the class and someVariable is a variable

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Use method invocation from reflection   Class lt   gt  c   Class forName  class name    Method method   c getDeclaredMethod  method name   parameterTypes   method invoke objectToInvokeOn  params     Where     class name  is the name of the class objectToInvokeOn is of type Object and is the object you want to invoke the method on  method name  is the name of the method you want to call parameterTypes is of type Class   and declares the parameters the method takes params is of type Object   and declares the parameters to be passed to the method

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To complete my colleague s answers  You might want to pay close attention to    static or instance calls  in one case  you do not need an instance of the class  in the other  you might need to rely on an existing default constructor that may or may not be there  public or non-public method call  for the latter you need to call setAccessible on the method within an doPrivileged block  other findbugs won t be happy  encapsulating into one more manageable applicative exception if you want to throw back the numerous java system exceptions  hence the CCException in the code below    Here is an old java1 4 code which takes into account those points          Allow for instance call  avoiding certain class circular dependencies   lt br   gt     Calls even private method if java Security allows it      param aninstance instance on which method is invoked  if null  static call      param classname name of the class containing the method      can be null - ignored  actually - if instance if provided  must be provided if static call      param amethodname name of the method to invoke     param parameterTypes array of Classes     param parameters array of Object     return resulting Object     throws CCException if any problem     public static Object reflectionCall final Object aninstance  final String classname  final String amethodname  final Class   parameterTypes  final Object   parameters  throws CCException       Object res      null      try           Class aclass      null          if aninstance    null                        aclass   Class forName classname                     else                       aclass   aninstance getClass                        Class   parameterTypes   new Class   String   class       final Method amethod   aclass getDeclaredMethod amethodname  parameterTypes           AccessController doPrivileged new PrivilegedAction         public Object run                     amethod setAccessible true                   return null     nothing to return                                   res   amethod invoke aninstance  parameters         catch  final ClassNotFoundException e            throw new CCException Error PROBLEM TO ACCESS classname CLASS  e         catch  final SecurityException e            throw new CCException Error PROBLEM TO ACCESS classname GenericConstants HASH DIESE  amethodname   METHOD SECURITY ISSUE  e         catch  final NoSuchMethodException e            throw new CCException Error PROBLEM TO ACCESS classname GenericConstants HASH DIESE  amethodname   METHOD NOT FOUND  e         catch  final IllegalArgumentException e            throw new CCException Error PROBLEM TO ACCESS classname GenericConstants HASH DIESE  amethodname   METHOD ILLEGAL ARGUMENTS String valueOf parameters  GenericConstants CLOSING ROUND BRACKET  e         catch  final IllegalAccessException e            throw new CCException Error PROBLEM TO ACCESS classname GenericConstants HASH DIESE  amethodname   METHOD ACCESS RESTRICTION  e         catch  final InvocationTargetException e        throw new CCException Error PROBLEM TO ACCESS classname GenericConstants HASH DIESE  amethodname   METHOD INVOCATION ISSUE  e              return res

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If you do the call several times you can use the new method handles introduced in Java 7  Here we go for your method returning a String   Object obj   new Point  100  200    String methodName    toString     Class lt String gt  resultType   String class   MethodType mt   MethodType methodType  resultType    MethodHandle methodHandle   MethodHandles lookup   findVirtual  obj getClass    methodName  mt    String result   resultType cast  methodHandle invoke  obj       System out println  result        java awt Point x 100 y 200

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This is working fine for me    public class MethodInvokerClass       public static void main String   args  throws NoSuchMethodException  SecurityException  IllegalAccessException  IllegalArgumentException  ClassNotFoundException  InvocationTargetException  InstantiationException           Class c   Class forName MethodInvokerClass class getName             Object o   c newInstance            Class   paramTypes   new Class 1           paramTypes 0  String class          String methodName    countWord            Method m   c getDeclaredMethod methodName  paramTypes            m invoke o   testparam      public void countWord String input       System out println  My input   input          Output    My input testparam  I am able to invoke the method by passing its name to another method  like main

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Please refer following code may help you   public static Method method    public static MethodClass obj  public static String testMethod  A    public static void main String args           obj new MethodClass        method obj getClass   getMethods        try               for int i 0 i lt method length i                          String name method i  getName                if name  testMethod                                   method i  invoke name  Test Parameters of A                                      catch Exception ex                System out println ex getMessage               Thanks

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Coding from the hip  it would be something like   java lang reflect Method method  try     method   obj getClass   getMethod methodName  param1 class  param2 class         catch  SecurityException e            catch  NoSuchMethodException e            The parameters identify the very specific method you need  if there are several overloaded available  if the method has no arguments  only give methodName    Then you invoke that method by calling  try     method invoke obj  arg1  arg2         catch  IllegalArgumentException e            catch  IllegalAccessException e            catch  InvocationTargetException e            Again  leave out the arguments in  invoke  if you don t have any  But yeah  Read about Java Reflection

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for me a pretty simple and fool proof way would be to simply make a method caller method like so   public static object methodCaller String methodName        if methodName equals  getName            return className getName        then when you need to call the method simply put something like this    calling a toString method is unnessary here  but i use it to have my programs to both rigid and self-explanitory  System out println methodCaller methodName  toString

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Here are the READY TO USE METHODS   To invoke a method  without Arguments   public static void callMethodByName Object object  String methodName  throws IllegalAccessException  InvocationTargetException  NoSuchMethodException       object getClass   getDeclaredMethod methodName  invoke object       To invoke a method  with Arguments       public static void callMethodByName Object object  String methodName  int i  String s  throws IllegalAccessException  InvocationTargetException  NoSuchMethodException           object getClass   getDeclaredMethod methodName  int class  String class  invoke object  i  s           Use the above methods as below   package practice   import java io IOException  import java lang reflect InvocationTargetException   public class MethodInvoke        public static void main String   args  throws ClassNotFoundException  NoSuchMethodException  SecurityException  IllegalAccessException  IllegalArgumentException  InvocationTargetException  IOException           String methodName1    methodA           String methodName2    methodB           MethodInvoke object   new MethodInvoke            callMethodByName object  methodName1           callMethodByName object  methodName2  1   Test               public static void callMethodByName Object object  String methodName  throws IllegalAccessException  InvocationTargetException  NoSuchMethodException           object getClass   getDeclaredMethod methodName  invoke object              public static void callMethodByName Object object  String methodName  int i  String s  throws IllegalAccessException  InvocationTargetException  NoSuchMethodException           object getClass   getDeclaredMethod methodName  int class  String class  invoke object  i  s              void methodA             System out println  Method A               void methodB int i  String s            System out println  Method B      n tParam1 -   i   n tParam 2 -   s             Output      Method A   Method B      9 Param1 - 1     9 Param 2 - Test

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With jooR it s merely   on obj  call methodName   params    get     Here is a more elaborate example   public class TestClass        public int add int a  int b    return a   b        private int mul int a  int b    return a   b        static int sub int a  int b    return a - b        import static org joor Reflect     public class JoorTest        public static void main String   args            int add   on new TestClass    call  add   1  2  get       public         int mul   on new TestClass    call  mul   3  4  get       private         int sub   on TestClass class  call  sub   6  5  get       static         System out println add          mul          sub             This prints      3  12  1

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using import java lang reflect     public static Object launchProcess String className  String methodName  Class lt   gt    argsTypes  Object   methodArgs          throws Exception        Class lt   gt  processClass   Class forName className      convert string classname to class     Object process   processClass newInstance       invoke empty constructor      Method aMethod   process getClass   getMethod methodName argsTypes       Object res   aMethod invoke process  methodArgs      pass arg     return res       and here is how you use it   String className    com example helloworld   String methodName    print   Class lt   gt    argsTypes    String class   String class   Object   methArgs      hello    world        launchProcess className  methodName  argsTypes  methArgs

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For those who are calling the method within the same class from a non-static method  see below codes  class Person       public void method1             try               Method m2   this getClass   getDeclaredMethod  quot method2 quot                m1 invoke this             catch  NoSuchMethodException e                e printStackTrace              catch  IllegalAccessException e                e printStackTrace              catch  InvocationTargetException e                e printStackTrace                         public void method2                Do something

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The method can be invoked like this  There are also more possibilities  check the reflection api   but this is the simplest one   import java lang reflect InvocationTargetException  import java lang reflect Method   import org junit Assert  import org junit Test   public class ReflectionTest        private String methodName    length       private String valueObject    Some object         Test     public void testGetMethod   throws SecurityException  NoSuchMethodException  IllegalArgumentException              IllegalAccessException  InvocationTargetException           Method m   valueObject getClass   getMethod methodName  new Class                Object ret   m invoke valueObject  new Object                Assert assertEquals 11  ret

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Object obj   Method method   obj getClass   getMethod  methodName   null    method invoke obj  null

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This sounds like something that is doable with the Java Reflection package   http   java sun com developer technicalArticles ALT Reflection index html  Particularly under Invoking Methods by Name   import java lang reflect     public class method2     public int add int a  int b           return a   b         public static void main String args             try          Class cls   Class forName  method2           Class partypes     new Class 2           partypes 0    Integer TYPE          partypes 1    Integer TYPE          Method meth   cls getMethod             add   partypes           method2 methobj   new method2            Object arglist     new Object 2           arglist 0    new Integer 37           arglist 1    new Integer 47           Object retobj              meth invoke methobj  arglist           Integer retval    Integer retobj          System out println retval intValue                 catch  Throwable e            System err println e

[java] How do I invoke a Java method when given the method name as a string?

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