werterzz

package com.example.fixmapstructobjectnullproblem.entity;

import Lombok.Data;

@Data
public class User {

   String username = "werterzz";
   String password = "password";
}

werterzz

package com.example.fixmapstructobjectnullproblem.dto;

import lombok.Data;

@Data
public class UserDto {
   String username;
}

werterzz

package com.example.fixmapstructobjectnullproblem.mapper;

import com.example.fixmapstructobjectnullproblem.dto.UserDto;
import com.example.fixmapstructobjectnullproblem.entity.User;
import org.mapstruct.Mapper;

@Mapper(componentModel = "spring")
public interface UserMapper {

   UserDto toUserDto(User user);

werterzz

package com.example.fixmapstructobjectnullproblem;

import com.example.fixmapstructobjectnullproblem.dto.UserDto;
import com.example.fixmapstructobjectnullproblem.entity.User;
import com.example.fixmapstructobjectnullproblem.mapper.UserMapper;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

werterzz

<dependency>
  <groupId>org.mapstruct</groupId>
  <artifactId>mapstruct</artifactId>
  <version>${org.mapstruct.version}</version>
</dependency>
<dependency>
  <groupId>org.mapstruct</groupId>
  <artifactId>mapstruct-processor</artifactId>
  <version>${org.mapstruct.version}</version>
</dependency>

werterzz

<dependency>
  <groupId>org.projectlombok</groupId>
  <artifactId>lombok</artifactId>
  <optional>true</optional>
</dependency>
<dependency>
  <groupId>org.mapstruct</groupId>
  <artifactId>mapstruct</artifactId>
  <version>${org.mapstruct.version}</version>
</dependency>

werterzz

List<String> fruits = Arrays.asList("orange", "apple", "banana", "cherry", "pear", "mango");
System.out.println("before map: " + fruits);
fruits = fruits.stream().map(fruit -> fruit.toUpperCase()).collect(Collectors.toList());
System.out.println("after map: " + fruits);

// output
// before map: [orange, apple, banana, cherry, pear, mango]
// after map: [ORANGE, APPLE, BANANA, CHERRY, PEAR, MANGO]

werterzz

List<String> fruits = Arrays.asList("orange", "apple", "banana", "cherry", "pear", "mango");
System.out.println("before filter: " + fruits);
fruits = fruits.stream().filter(fruit -> fruit.contains("e")).collect(Collectors.toList());
System.out.println("after filter: " + fruits);
        
// output
// before filter: [orange, apple, banana, cherry, pear, mango]
// after filter: [orange, apple, cherry, pear]

werterzz

List<String> fruits = Arrays.asList("orange", "apple", "banana", "cherry", "pear", "mango");
System.out.println("before sorted: " + fruits);
fruits = fruits.stream().sorted().collect(Collectors.toList());
System.out.println("after sorted: " + fruits);

// output
// before sorted: [orange, apple, banana, cherry, pear, mango]
// after sorted: [apple, banana, cherry, mango, orange, pear]

werterzz

List<String> fruits = Arrays.asList("orange", "apple", "banana", "cherry", "pear", "mango");
fruits.stream().map(fruit -> fruit.toUpperCase()).forEach(fruit -> System.out.println(fruit));

// output
// ORANGE
// APPLE
// BANANA
// CHERRY
// PEAR
// MANGO