Is it possible in Java to check if objects fields are null and then add default value to all those attributes?

StackOverflow https://stackoverflow.com/questions/2146129
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Question

I need to make sure that no object attribute is null and add default value in case if it is null. Is there any easy way to do this, or do I have to do it manually by checking every attribute by its getters and setters?

Was it helpful?

Solution

You can use reflection to iterate over the object's field, and set them. You'd obviously need some sort of mapping between types or even field names and required default values but this can be done quite easily in a loop. For example:

for (Field f : obj.getClass().getFields()) {
  f.setAccessible(true);
  if (f.get(obj) == null) {
     f.set(obj, getDefaultValueForType(f.getType()));
  }
}

[Update]

With modern Java, you can use annotations to set the default values for fields on a per class basis. A complete implementation might look like this:

// DefaultString.java:
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@Retention(RetentionPolicy.RUNTIME)
public @interface DefaultString {
    String value();
}

// DefaultInteger.java:
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

@Retention(RetentionPolicy.RUNTIME)
public @interface DefaultInteger {
    int value();
}

// DefaultPojo.java:
import java.lang.annotation.Annotation;
import java.lang.reflect.Field;

public class DefaultPojo {

    public void setDefaults() {
        for (Field f : getClass().getFields()) {
            f.setAccessible(true);
            try {
                if (f.get(this) == null) {
                    f.set(this, getDefaultValueFromAnnotation(f.getAnnotations()));
                }
            } catch (IllegalAccessException e) { // shouldn't happen because I used setAccessible
            }
        }
    }

    private Object getDefaultValueFromAnnotation(Annotation[] annotations) {
        for (Annotation a : annotations) {
            if (a instanceof DefaultString)
                return ((DefaultString)a).value();
            if (a instanceof DefaultInteger)
                return ((DefaultInteger)a).value();
        }
        return null;
    }

}

// Test Pojo
public class TestPojo extends DefaultPojo {
    @DefaultString("Hello world!")
    public String stringValue;
    @DefaultInteger(42);
    public int integerValue;
}

Then default values for a TestPojo can be set just by running test.setDetaults()

OTHER TIPS

You need to manually filter input to constructors and setters. Well... you could use reflection but I wouldn't advise it. Part of the job of constructors and setters is to validate input. That can include things like:

public void setPrice(double price) {
  if (price < 0.0d) {
    throw new IllegalArgumentException("price cannot be negative " + price);
  }
  this.price = price;
}

and

public void setName(String name) {
  if (name == null) {
    throw new NullPointerException("name cannot be null");
  }
  this.name = name;
}

You could use wrapper functions for the actual check and throwing the exception.

Maybe check Hibernate Validator 4.0, the Reference Implementation of the JSR 303: Bean Validation.

This is an example of an annotated class:

public class Address {

    @NotNull 
    private String line1;
    private String line2;
    private String zip;
    private String state;

    @Length(max = 20)
    @NotNull
    private String country;

    @Range(min = -2, max = 50, message = "Floor out of range")
    public int floor;

        ...
}

For an introduction, see Getting started with JSR 303 (Bean Validation) – part 1 and part 2 or the "Getting started" section of the reference guide which is part of the Hibernate Validator distribution.

Non-reflective solution for Java 8, without using a series of if's, would be to stream all fields and check for nullness:

return Stream.of(id, name).allMatch(Objects::isNull);

This remains quite easy to maintain while avoiding the reflection hammer. This will return true for null attributes.

You can create a function that returns a boolean value and checks every attribute. You can call that function to do the job for you.

Alternatively, you can initialize the object with default values. That way there is no need for you to do any checking.

I don't have enough context to give you a correct answer, but I'll suggest you to make you code immutable as much as possible. Use public final fields. No more getters or setters : every field has to be defined by the constructor. Your code is shorter, more readable and prevents you from writing code with side effects.

It doesn't prevent you from passing null arguments to your constructor though... You can still check every argument as suggested by @cletus, but I'll suggest you to throw IllegalArgumentException instead of NullPointerException that doesn't give no new hint about what you've done.

Anyway, that's what I do as much as I can and it improved my code (readability, stability) to a great extend. Everyone in my team does so and we are very happy with that. We learned that when we try to write some erlang code where everything is immutable.

Hope this helps.

I tried this and it works without any issues to validate if the field is empty. I have answered your question partially as I haven't personally tried to add default values to attributes

if(field.getText()!= null && !field.getText().isEmpty())

Hope it helps

This is not to check for null, instead this will be helpful in converting an existing object to an empty object(fresh object). I dont know whether this is relevant or not, but I had such a requirement.

@SuppressWarnings({ "unchecked" })
static void emptyObject(Object obj) 
{
    Class c1 = obj.getClass();
    Field[] fields = c1.getDeclaredFields();

    for(Field field : fields)
    {
        try
        {
            if(field.getType().getCanonicalName() == "boolean")
            {
                field.set(obj, false);
            }
            else if(field.getType().getCanonicalName() == "char")
            {
                field.set(obj, '\u0000');
            }
            else if((field.getType().isPrimitive()))
            {
                field.set(obj, 0);
            }
            else
            {
                field.set(obj, null);
            }
        }
        catch(Exception ex)
        {

        }
    }
}
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