CODEWITHSUNDEEP
javastringreflection
user1768830
user1768830

Reputation:

How to get string value from a Java field via reflection?

I have a method:

public void extractStringFromField(Class<?> classToInspect) {
    Field[] allFields = classToInspect.getDeclaredFields();

    for(Field field : allFields) {
        if(field.getType().isAssignableFrom(String.class)) {
            System.out.println("Field name: " + field.getName());

            // How to get the actual value of the string?!?!
            // String strValue = ???
        }
    }
}

When this runs I get output like:

Field name: java.lang.String

Now how do I extract the actual string value into strValue, using reflection?

Upvotes: 24

Views: 53371

Answers (5)

Slaysn
Slaysn

Reputation: 11

Just had the same issue. This Thread somewhat helped. Just for reference if somebody else stumbles upon this thread. I used the StringBuilder class to convert so basically:

StringBuilder builder = new StringBuilder();
builder.append(field.get(object))

Which has multiple advantages. One that you do not explicitly cast (which btw. causes problems with primitive types int vs. Integer) but also of being more efficient if you have multiple string operations sequentialy. In time critical code parts.

Upvotes: 1

Simon S
Simon S

Reputation: 303

String strValue = field.getName().toString();

Full code looks like this: 

public static void extractStringFromField(Class<?> Login) {
    Field[] allFields = Login.getDeclaredFields();

    for(Field field : allFields) {
        String strValue = field.getName().toString();
//          if(field.getType().isAssignableFrom(String.class)) {
            System.out.println("Field name: " + strValue);
        }
    }

Upvotes: 0

Dmitry Knyazev
Dmitry Knyazev

Reputation: 21

Just usefull example code for reflection fields:

Field[] fields = InsanceName.getDeclaredFields();
for (Field field : fields) {      //array for fields names

System.out.println("Fields: " + Modifier.toString(field.getModifiers())); // modyfiers
System.out.println("Fields: " + field.getType().getName());  //type var name
System.out.println("Fields: " + field.getName());        //real var name
field.setAccessible(true);                                //var readable
System.out.println("Fields: " + field.get(InsanceName));  //get var values  
System.out.println("Fields: " + field.toString());     //get "String" values
System.out.println("");  //some space for readable code
}

Upvotes: 2

Sazzadur Rahaman
Sazzadur Rahaman

Reputation: 7126

In ideal situations,Class does not hold data. It merely holds the information about the structure and behavior of its instances and Instances of the Classes hold your data to use. So your extractStringFromField method can not extract values unless you pass any instances (from where it will actually extract values).

If the name of the parameter of the reference, you are passing to extract value is instance, then you can easily get what you want like bellow:

String strValue = (String)field.get(instance);

Upvotes: 8

Owen
Owen

Reputation: 1736

It looks like you need a reference to an instance of the class. You would want to call get and pass in the reference, casting the return to a String.

You can use get as follows:

String strValue = (String) field.get (objectReference);

Upvotes: 38

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