How to loop over a Class attributes in Java?

Question

How can I loop over a class attributes in java dynamically.

For eg :

public class MyClass{
    private type1 att1;
    private type2 att2;
    ...

    public void function(){
        for(var in MyClass.Attributes){
            System.out.println(var.class);
        }
    }
}

is this possible in Java?

Answer 1

There is no linguistic support to do what you're asking for.

You can reflectively access the members of a type at run-time using reflection (e.g. with Class.getDeclaredFields() to get an array of Field), but depending on what you're trying to do, this may not be the best solution.

See also

• Java Tutorials: Reflection API / Advanced Language Topics: Reflection

Related questions

• What is reflection, and why is it useful?
• Java Reflection: Why is it so bad?
• How could Reflection not lead to code smells?
• Dumping a java object’s properties

---

Example

Here's a simple example to show only some of what reflection is capable of doing.

import java.lang.reflect.*;

public class DumpFields {
    public static void main(String[] args) {
        inspect(String.class);
    }
    static <T> void inspect(Class<T> klazz) {
        Field[] fields = klazz.getDeclaredFields();
        System.out.printf("%d fields:%n", fields.length);
        for (Field field : fields) {
            System.out.printf("%s %s %s%n",
                Modifier.toString(field.getModifiers()),
                field.getType().getSimpleName(),
                field.getName()
            );
        }
    }
}

The above snippet uses reflection to inspect all the declared fields of class String; it produces the following output:

7 fields:
private final char[] value
private final int offset
private final int count
private int hash
private static final long serialVersionUID
private static final ObjectStreamField[] serialPersistentFields
public static final Comparator CASE_INSENSITIVE_ORDER

---

Effective Java 2nd Edition, Item 53: Prefer interfaces to reflection

These are excerpts from the book:

Given a Class object, you can obtain Constructor, Method, and Field instances representing the constructors, methods and fields of the class. [They] let you manipulate their underlying counterparts reflectively. This power, however, comes at a price:

• You lose all the benefits of compile-time checking.
• The code required to perform reflective access is clumsy and verbose.
• Performance suffers.

As a rule, objects should not be accessed reflectively in normal applications at runtime.

There are a few sophisticated applications that require reflection. Examples include [...omitted on purpose...] If you have any doubts as to whether your application falls into one of these categories, it probably doesn't.

Answer 2

Accessing the fields directly is not really good style in java. I would suggest creating getter and setter methods for the fields of your bean and then using then Introspector and BeanInfo classes from the java.beans package.

MyBean bean = new MyBean();
BeanInfo beanInfo = Introspector.getBeanInfo(MyBean.class);
for (PropertyDescriptor propertyDesc : beanInfo.getPropertyDescriptors()) {
    String propertyName = propertyDesc.getName();
    Object value = propertyDesc.getReadMethod().invoke(bean);
}

Answer 3

While I agree with Jörn's answer if your class conforms to the JavaBeabs spec, here is a good alternative if it doesn't and you use Spring.

Spring has a class named ReflectionUtils that offers some very powerful functionality, including doWithFields(class, callback), a visitor-style method that lets you iterate over a classes fields using a callback object like this:

public void analyze(Object obj){
    ReflectionUtils.doWithFields(obj.getClass(), field -> {

        System.out.println("Field name: " + field.getName());
        field.setAccessible(true);
        System.out.println("Field value: "+ field.get(obj));

    });
}

But here's a warning: the class is labeled as "for internal use only", which is a pity if you ask me

Answer 4

Simple way to iterate over class fields and obtain values from object:

 Class<?> c = obj.getClass();
 Field[] fields = c.getDeclaredFields();
 Map<String, Object> temp = new HashMap<String, Object>();

 for( Field field : fields ){
      try {
           temp.put(field.getName().toString(), field.get(obj));
      } catch (IllegalArgumentException e1) {
      } catch (IllegalAccessException e1) {
      }
 }

Answer 5

Java has Reflection (java.reflection.*), but I would suggest looking into a library like Apache Beanutils, it will make the process much less hairy than using reflection directly.

Answer 6

Here is a solution which sorts the properties alphabetically and prints them all together with their values:

public void logProperties() throws IllegalArgumentException, IllegalAccessException {
  Class<?> aClass = this.getClass();
  Field[] declaredFields = aClass.getDeclaredFields();
  Map<String, String> logEntries = new HashMap<>();

  for (Field field : declaredFields) {
    field.setAccessible(true);

    Object[] arguments = new Object[]{
      field.getName(),
      field.getType().getSimpleName(),
      String.valueOf(field.get(this))
    };

    String template = "- Property: {0} (Type: {1}, Value: {2})";
    String logMessage = System.getProperty("line.separator")
            + MessageFormat.format(template, arguments);

    logEntries.put(field.getName(), logMessage);
  }

  SortedSet<String> sortedLog = new TreeSet<>(logEntries.keySet());

  StringBuilder sb = new StringBuilder("Class properties:");

  Iterator<String> it = sortedLog.iterator();
  while (it.hasNext()) {
    String key = it.next();
    sb.append(logEntries.get(key));
  }

  System.out.println(sb.toString());
}