Just putting the comment by @pvpkiran in an answer.

Use ObjectMapper class from com.fasterxml.jackson.databind

ObjectMapper objectMapper = new ObjectMapper();

Converting from Object to String:

String jsonString = objectMapper.writeValueAsString(link);

Converting from String to Object:

Link link = objectMapper.readValue(jsonString, type)

Try this,

You can't create a new string like your doing.

    String string = "{\"id\":100,\"firstName\":\"Adam\"}";
    Student student = mapper.readValue(string, Student.class);

You can convert them by registering custom deserializer as below:

public class PersonDeserializer extends JsonDeserializer<Person> {

    @Override
    public Person deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException {
        ObjectCodec oc = jp.getCodec();
        JsonNode node = oc.readTree(jp);

        final Integer age = node.get("age").asInt();
        final String name = node.get("name").asText();
        final String hobbyListAsString = node.get("hobby_list").asText();

        ObjectMapper mapper = new ObjectMapper();
        // convert JSON array to List of objects
        List<Hobby> hobbyList = Arrays.asList(mapper.readValue(hobbyListAsString, Hobby[].class));

        Person person = new Person();
        person.setName(name);
        person.setAge(age);
        person.setHobbyList(hobbyList);

        return person;
    }
}

and in pojo at root use annotation @JsonDeserialize(using = PersonDeserializer.class) so that above deserializer can be registered. Attaching reference below:

@JsonDeserialize(using = PersonDeserializer.class)
@Data
public class Person {
    @JsonProperty("name")
    private String name;

    @JsonProperty("age")
    private Integer age;

    @JsonProperty("hobby_list")
    private List<Hobby> hobbyList;
}

Then I was able to deserialize above hobby_list string to object

ObjectMapper mapper = new ObjectMapper();
Person person = mapper.readValue(content, Person.class);
System.out.println(person.getHobbyList());

The code should pass an instance of Type A to the writeValueAsString method not the actual class.

The ObjectMapper will be able to determine the instance's type via reflection, however if you don't pass an instance it cannot determine the field value's to place in the generated JSON.

Also be sure to catch or throw the appropriate exceptions.

public void save() {

try {
        A a = new A();
        ObjectMapper mapper = new ObjectMapper();
        String temp = mapper.writeValueAsString(a);

    } catch (JsonGenerationException e) {
       e.printStackTrace();
    } catch (JsonMappingException e) {
       e.printStackTrace();
    } catch (IOException e) {
       e.printStackTrace();
    }

}

You should also have acessors for the fields on class A and class B. NOTICE I included B in the previous sentence, since Jackson must be able to map all fields on the instance we provide to it.

public A   {
    private List<B> b;
    private int c;

    private B getB(){ return b;}
    private void setB(B b){this.b = b;}
    private int getC(){return c;}
    private void setC(int c){this.c = c;}
}

Here is a complete working example that I have tested.

import java.io.IOException;
import java.util.List;

import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;

public class A {

    private List<B> b;
    private int c;

    public List<B> getB() {
        return b;
    }

    public void setB(List<B> b) {
        this.b = b;
    }

    public int getC() {
        return c;
    }

    public void setC(int c) {
        this.c = c;
    }

    public static void main(String[] args) throws JsonGenerationException, JsonMappingException, IOException {
        A a = new A();
        ObjectMapper mapper = new ObjectMapper();
        String temp = mapper.writeValueAsString(a);
        System.out.println(temp);   
    }
}

class B{


}

If this simple example does not work the jackson-mapper-asl.jar file is most likely not on the build path. Download the file from here and place it somewhere on your local machine.

Next via Eclipse, Right Click on your Project and select properties.

Then select Build Path > Libraries > Add External Jars. Locate the jar and then hit OK.

[Update Sept 2020] Although my original answer here, from many years ago, seems to be helpful and is still getting upvotes, I now use the GSON library from Google, which I find to be more intuitive.

I've got the following code:

public void testJackson() throws IOException {  
    ObjectMapper mapper = new ObjectMapper(); 
    File from = new File("albumnList.txt"); 
    TypeReference<HashMap<String,Object>> typeRef 
            = new TypeReference<HashMap<String,Object>>() {};

    HashMap<String,Object> o = mapper.readValue(from, typeRef); 
    System.out.println("Got " + o); 
}   

It's reading from a file, but mapper.readValue() will also accept an InputStream and you can obtain an InputStream from a string by using the following:

new ByteArrayInputStream(astring.getBytes("UTF-8")); 

There's a bit more explanation about the mapper on my blog.