sorting xml using java comparator

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Java

<students>
	<student>
		<data>
			<key>stu-id</key>
			<value>56170</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description>Computer Science Department </dept-data></value>
		</data>
	</student>
	<student>
		<data>
			<key>stu-id</key>
			<value>56171</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description>Business Department </dept-data></value>
		</data>
	</student>
	<student>
		<data>
			<key>stu-id</key>
			<value>50278</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description>Economics Department </dept-data></value>
		</data>
	</student>
	<student>
		<data>
			<key>stu-id</key>
			<value>60241</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description>English Department </dept-data></value>
		</data>
	</student>
	<student>
		<data>
			<key>stu-id</key>
			<value>58123</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description> Physics Department </dept-data></value>
		</data>
	</student>
	<student>
		<data>
			<key>stu-id</key>
			<value>56126</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description>Chemistry Department </dept-data></value>
		</data>
	</student>
</students>

need to sort the above file with java so I wrote a comparator class

public class Comp implements Comparator<StudentType> {




        public class Comp implements Comparator<StudentType> {




        public int compare(StudentType o1,StudentType o2)
            return (o1.getStudentList().get(1).getValue().compareTo(o2.getStudentList().get(1).getValue()));


        }

}

Calling as

List<StudentType> students = response.getStudents().getStudentList();


          Collections.sort(students, comparator);

New sorted file looks like this

<students>
<student>
		<data>
			<key>stu-id</key>
			<value>56171</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description>Business Department </dept-data></value>
		</data>
	</student>
	<student>
		<data>
			<key>stu-id</key>
			<value>56170</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description>Computer Science Department </dept-data></value>
		</data>
	</student>
	<student>
		<data>
			<key>stu-id</key>
			<value>56170</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description>Computer Science Department </dept-data></value>
		</data>
	</student>
	<student>
		<data>
			<key>stu-id</key>
			<value>50278</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description>Economics Department </dept-data></value>
		</data>
	</student>
	<student>
		<data>
			<key>stu-id</key>
			<value>50278</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description>Economics Department </dept-data></value>
		</data>
	</student>
	<student>
		<data>
			<key>stu-id</key>
			<value>58123</value>
		</data>
		<data>
			<key>dept</key>
			<value><?xml version="1.0" encoding="UTF-8"?>Description> Physics Department </dept-data></value>
		</data>
	</student>
</students>

I don't know why the camparator class is sorting and deleting some data and duplicating it

Thanks