My use case is that I am trying to use the distinct method of Stream to remove students with the same roll number from a list of objects of class StudentCourseMapping.
POJO details are below:
public class StudentCourseMapping implements Serializable{
private String name;
private String dept;
private Integer roll;
private String course;
Below is the equals method:
@Override
public boolean equals(Object obj) {
StudentCourseMapping other = (StudentCourseMapping) obj;
if (roll == null) {
if (other.roll != null)
return false;
} else if (!roll.equals(other.roll))
return false;
return true;
}
Below is the implementation:
public class RemoveDuplicateUsingStream {
public static void main(String[] args) {
List<StudentCourseMapping> studentCourceList = JacksonJSONReaderObjectMapper.jsonReader();
studentCourceList.stream().distinct().forEach(System.out::println);
StudentCourseMapping s0 = studentCourceList.get(0);
StudentCourseMapping s1 = studentCourceList.get(1);
System.out.println(s0.equals(s1));
Set<Integer> st = new HashSet();
List<StudentCourseMapping>studentCourceList2 = studentCourceList.stream().filter(s -> st.add(s.getRoll()))
.collect(Collectors.toCollection(ArrayList::new));
System.out.println(studentCourceList2.size());
}
}
And the output is
StudentCourseMapping [name=Alu, dept=Physics, roll=12, course=Quantum Theory]
StudentCourseMapping [name=Alu, dept=Physics, roll=12, course=English]
StudentCourseMapping [name=Sam, dept=Commerce, roll=16, course=English]
StudentCourseMapping [name=Sam, dept=Commerce, roll=16, course=Accounts]
StudentCourseMapping [name=Joe, dept=Arts, roll=19, course=English]
StudentCourseMapping [name=Joe, dept=Arts, roll=19, course=Hindi]
true
3
JacksonJSONReaderObjectMapper.jsonReader() is a custom method which reads the below JSON. I am able to achieve the same by using filter and adding to HashSet but I really want to know what is wrong with my distinct implementation.
{
"studentCourseMapping": [
{
"name": "Alu",
"dept": "Physics",
"roll": 12,
"course": "Quantum Theory"
},
{
"name": "Alu",
"dept": "Physics",
"roll": 12,
"course": "English"
},
{
"name": "Sam",
"dept": "Commerce",
"roll": 16,
"course": "English"
},
{
"name": "Sam",
"dept": "Commerce",
"roll": 16,
"course": "Accounts"
},
{
"name": "Joe",
"dept": "Arts",
"roll": 19,
"course": "English"
},
{
"name": "Joe",
"dept": "Arts",
"roll": 19,
"course": "Hindi"
}
]
}
When I try to test the equals method directly it was working properly and returning true since both the s0 and s1 has roll as 12.
StudentCourseMapping s0 = studentCourceList.get(0);
StudentCourseMapping s1 = studentCourceList.get(1);
System.out.println(s0.equals(s1));
But when I am using distinct all the objects are getting printed and also while trying to debug in Eclipse the distinct method I wrote is not getting called, but the documentation says it should be called. I am using JDK 11:
Stream<T> distinct()Returns a stream consisting of the distinct elements (according to
Object.equals(Object))of this stream.
As @RealSkeptic and @Jesper have already said in the comments, in your StudentCourseMapping class you also need to override the hashCode() method according to your equals() implementation, in order to properly compare the elements in the stream and keep only the distinct ones.
In fact, although no hashCode() redefinition is mentioned in the distinct() documentation:
Returns a stream consisting of the distinct elements (according to Object.equals(Object)) of this stream.
It is mentioned though the use of equals(), whose documentation introduces the general contract for the hashcode method, which is amply covered in the hashCode() documentation. Quoting the equals() documentation:
Note that it is generally necessary to override the hashCode method whenever this method is overridden, so as to maintain the general contract for the hashCode method, which states that equal objects must have equal hash codes.
In fact, once the hashCode() method is properly overridden, your snippet produces the desired output. You can also test it at OneCompiler.
public class StudentCourseMapping implements Serializable {
private String name;
private String dept;
private Integer roll;
private String course;
//... your class implementation ...
@Override
public boolean equals(Object obj) {
if (obj == null) return false;
if (obj == this) return true;
if (obj.getClass() != getClass()) return false;
StudentCourseMapping student = (StudentCourseMapping) obj;
return Objects.equals(roll, student.roll);
}
@Override
public int hashCode() {
return Objects.hashCode(roll);
}
@Override
public String toString() {
return String.format("[name = %s, dept = %s, roll = %d, course = %s]", name, dept, roll, course);
}
}