Java annotations

Question

I've created simple annotation in Java

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
public @interface Column {
    String columnName();
}

and class

public class Table {

    @Column(columnName = "id")
    private int colId;

    @Column(columnName = "name")
    private String colName;

    private int noAnnotationHere;

    public Table(int colId, String colName, int noAnnotationHere) {
       this.colId = colId;
       this.colName = colName;
       this.noAnnotationHere = noAnnotationHere;
    }  
}

I need to iterate over all fields, that are annotated with Column and get name and value of field and annotation. But I've got problem with getting value of each field, since all of them are of different data type.

Is there anything that would return collection of fields that have certain annotation? I managed to do it with this code, but I don't think that reflection is good way to solve it.

Table table = new Table(1, "test", 2);

for (Field field : table.getClass().getDeclaredFields()) {
    Column col;
    // check if field has annotation
    if ((col = field.getAnnotation(Column.class)) != null) {
        String log = "colname: " + col.columnName() + "\n";
        log += "field name: " + field.getName() + "\n\n";

        // here i don't know how to get value of field, since all get methods
        // are type specific

        System.out.println(log);
    }
}

Do I have to wrap every field in object, which would implement method like getValue(), or is there some better way around this? Basicly all I need is string representation of each field that is annotated.

edit: yep field.get(table) works, but only for public fields, is there any way how to do this even for private fields? Or do I have to make getter and somehow invoke it?

Answer 1

Every object should has toString() defined. (And you can override this for each class to get a more meaningful representation).

So you where your "// here I don't know" comment is, you could have:

Object value = field.get(table);
// gets the value of this field for the instance 'table'

log += "value: " + value + "\n";
// implicitly uses toString for you
// or will put 'null' if the object is null

Answer 2

Reflection is exactly the way to solve it. Finding out things about types and their members at execution time is pretty much the definition of reflection! The way you've done it looks fine to me.

To find the value of the field, use field.get(table)