Setting private fields using Java Reflection

Question

While using java reflection, Can we set a private field without having to tell the parameter type.

For example, if this is my Child class,

package reflection;
public class Child {
 private String name;
 private Integer value;
 private boolean flag;
 public String getLName()
 {
 return this.name;
 }
 public void setName(String name)
 {
 this.name = name;
 }
 public Integer getValue()
 {
 return this.value;
 }
 public void setValue(Integer value)
 {
 this.value = value;
 }
 public boolean getFlag()
 {
 return this.flag;
 }
 public void setFlag(boolean flag)
 {
 this.flag = flag;
 }
 public String toString()
 {
 return "Name" + this.name;
 }
}

I want to set fields of this Child class in my Tester class.

package reflection;
public class Tester {
 public static void main(String args[]) throws Exception
 {
 Class<?> clazz = Class.forName("reflection.Child");
 Object cc = clazz.newInstance();
 cc.getClass().getMethod("setName", String.class).invoke(cc,"AAA");
 }
}

Here I am setting the value of Name field. In the line,

cc.getClass().getMethod("setName", String.class).invoke(cc,"AAA");

I have used String.class. Is there a way to do this, without having to tell the field type. Can Java automatically identify the type somehow? This is because I will get the name, value and flag data from a csv file, and I want to set all the three fields in a single line using a loop. I will declare an array of String with values - "setName", "setValue" and "setFlag" and then I want to use something of the following

cc.getClass().getMethod(array[index]).invoke(cc,data);

I know the above statement is wrong, but is there some alternative to this?

• JB Nizet: Loop through the methods of the class, and find the one called setName. Pray that it isn't overloaded. Note that this has nothing to do with private fields. You're calling public methods here, not setting private fields. That said, like almost all questions related to reflection, reflection is probably not the good way to solve your problem. Why not use an array of Consumer<String>, containing for example n -> c.setName(n), instead of an array of strings?

Answer 1

Get all the methods and find the matching one, which has type info about the parameters:

String name;
String value;
Method[] methods = Child.class.getMethods();
for (Method method : methods) {
 if (!method.getName().equals(name))
 continue;
 Class<?> paramType = method.getParameterTypes()[0];
 //You will have to figure how to convert the String value to the parameter.
 method.invoke(child, paramType.cast(value)); // for example
}

• Grzegorz Górkiewicz: Is there a way not to loop through all methods?
• Bohemian: @GrzegorzGórkiewicz not unless you know the parameter type, which you said you don't know. If you know the parameter type, you can call Child.class.getMethod(name, parameterClass) to get the exact method.
• Grzegorz Górkiewicz: But the parameter type is always... value.getClass()... i.e. data.getClass() as in my answer or?
• Bohemian: But you said your values are from a CSV, which are all Strings
• Grzegorz Górkiewicz: Yep. The OP (not me) would have to use some redundant ifs or switches to infer type, when following my answer.

Answer 2

You could use Apache Commons FieldUtils.writeDeclaredField

Child childObject = new Child();
FieldUtils.writeDeclaredField(childObject, "name", "John", true);