any tool for java object to object mapping?

You could try Dozer.

Dozer is a Java Bean to Java Bean mapper that recursively copies data from one object to another. Typically, these Java Beans will be of different complex types.

Dozer supports simple property mapping, complex type mapping, bi-directional mapping, implicit-explicit mapping, as well as recursive mapping. This includes mapping collection attributes that also need mapping at the element level.

There are some libraries around there:

• Commons-BeanUtils: ConvertUtils -> Utility methods for converting String scalar values to objects of the specified Class, String arrays to arrays of the specified Class.

• Commons-Lang: ArrayUtils -> Operations on arrays, primitive arrays (like int[]) and primitive wrapper arrays (like Integer[]).

• Spring framework: Spring has an excellent support for PropertyEditors, that can also be used to transform Objects to/from Strings.

• Dozer: Dozer is a powerful, yet simple Java Bean to Java Bean mapper that recursively copies data from one object to another. Typically, these Java Beans will be of different complex types.

• ModelMapper: ModelMapper is an intelligent object mapping framework that automatically maps objects to each other. It uses a convention based approach to map objects while providing a simple refactoring safe API for handling specific use cases.

• MapStruct: MapStruct is a compile-time code generator for bean mappings, resulting in fast (no usage of reflection or similar), dependency-less and type-safe mapping code at runtime.

• Orika: Orika uses byte code generation to create fast mappers with minimal overhead.

• Selma: Compile-time code-generator for mappings

• JMapper: Bean mapper generation using Annotation, XML or API (seems dead, last updated 2 years ago)

• Smooks: The Smooks JavaBean Cartridge allows you to create and populate Java objects from your message data (i.e. bind data to) (suggested by superfilin in comments). (No longer under active development)

• Commons-Convert: Commons-Convert aims to provide a single library dedicated to the task of converting an object of one type to another. The first stage will focus on Object to String and String to Object conversions. (seems dead, last update 2010)

• Transmorph: Transmorph is a free java library used to convert a Java object of one type into an object of another type (with another signature, possibly parameterized). (seems dead, last update 2013)

• EZMorph: EZMorph is simple java library for transforming an Object to another Object. It supports transformations for primitives and Objects, for multidimensional arrays and transformations with DynaBeans (seems dead, last updated 2008)

• Morph: Morph is a Java framework that eases the internal interoperability of an application. As information flows through an application, it undergoes multiple transformations. Morph provides a standard way to implement these transformations. (seems dead, last update 2008)

• Lorentz: Lorentz is a generic object-to-object conversion framework. It provides a simple API to convert a Java objects of one type into an object of another type. (seems dead)

• OTOM: With OTOM, you can copy any data from any object to any other object. The possibilities are endless. Welcome to "Autumn". (seems dead)

Another one is Orika - https://github.com/orika-mapper/orika

Orika is a Java Bean mapping framework that recursively copies (among other capabilities) data from one object to another. It can be very useful when developing multi-layered applications.

Orika focuses on automating as much as possible, while providing customization through configuration and extension where needed.

Orika enables the developer to :

• Map complex and deeply structured objects
• "Flatten" or "Expand" objects by mapping nested properties to top-level properties, and vice versa
• Create mappers on-the-fly, and apply customizations to control some or all of the mapping
• Create converters for complete control over the mapping of a specific set of objects anywhere in the object graph--by type, or even by specific property name
• Handle proxies or enhanced objects (like those of Hibernate, or the various mock frameworks)
• Apply bi-directional mapping with one configuration
• Map to instances of an appropriate concrete class for a target abstract class or interface
• Handle reverse mappings
• Handle complex conventions beyond JavaBean specs.

Orika uses byte code generation to create fast mappers with minimal overhead.

ModelMapper is another library worth checking out. ModelMapper's design is different from other libraries in that it:

• Automatically maps object models by intelligently matching source and destination properties
• Provides a refactoring safe mapping API that uses actual code to map fields and methods rather than using strings
• Utilizes convention based configuration for simple handling of custom scenarios

Check out the ModelMapper site for more info:

http://modelmapper.org