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POIn TreeSet, Sorting & Uniqueness of custom objects based on different properties
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copied!<p>Below is my Student class</p> <pre><code>class Student implements Comparable { String name; int rollNo; @Override public int compareTo(Object obj) { return ((Student)obj).name.compareTo(this.name); } } </code></pre> <p>latest modification: but still no getting the right result</p> <pre><code>@Override public int compareTo(Object obj) { Student s = (Student) obj; if (name.equals(s.name)) { // achieving uniqueness return 0; } else { if (rollNo < s.rollNo) { return -1; } else if (rollNo > s.rollNo) { return 1; } else { // this makes `name` the second ordering option. // names don't equal here return name.compareTo(s.name); } } } </code></pre> <p>If I create object of TreeSet<Student>, I am getting sorted list of Student objects based on unique name & ordered by name also.</p> <p>But I need unique student-name in my TreeSet<Student> with order by student-rollNo.</p> <p>Is it possible with Comparator? Can anybody help me, Every suggestion is appreciated. Thanks.</p> <p><b>UPDATE: here is the complete program:</b></p> <pre><code>public class Student implements Comparable { int rollNo; String name; Student(String n,int rno) { rollNo=rno; name=n; } /** * @param args */ public static void main(String[] args) { TreeSet<Student> ts = new TreeSet<Student>(); ts.add(new Student("bbb",2)); ts.add(new Student("aaa",4)); ts.add(new Student("bbb",2)); ts.add(new Student("ccc",3)); ts.add(new Student("aaa",1)); ts.add(new Student("bbb",2)); ts.add(new Student("bbb",5)); System.out.println(ts); } @Override public int compareTo(Object obj) { Student s = (Student) obj; if (name.equals(s.name)) { // achieving uniqueness return 0; } else { if (rollNo < s.rollNo) { return -1; } else if (rollNo > s.rollNo) { return 1; } else { // this makes `name` the second ordering option. // names don't equal here return name.compareTo(s.name); } } } @Override public String toString() { return name + rollNo; } } </code></pre> <p><b>Update:2: Thank you all for your suggestions, I still need some more :)</b></p> <pre><code> /* * Actual scenario is having different properties, * So here I am just relating my actual scenario with Student class */ class Student implements Comparable { // sorting required on rollNo int rollNo; // Unique name is required String name; Student(String n, int rno) { rollNo = rno; name = n; } /** * * @param args */ public static void main(String[] args) { TreeSet<Student> tsName = new TreeSet<Student>(); // here by default, order & uniqueness by name only tsName.add(new Student("ccc", 2)); tsName.add(new Student("aaa", 4)); tsName.add(new Student("ddd", 1)); tsName.add(new Student("bbb", 3)); tsName.add(new Student("ddd", 5)); // output: aaa:4, bbb:3, ccc:2, ddd:1 System.out.println(tsName); // creating new comparator for student RollNo TreeSet<Student> tsRollNo = new TreeSet<Student>(new Comparator<Student>() { public int compare(Student stud1, Student stud2) { return new Integer(stud1.rollNo).compareTo(stud2.rollNo); } }); tsRollNo.addAll(tsName); System.out.println(tsRollNo); // now got the desire output: ddd:1, ccc:2, bbb:3, aaa:4 } public boolean equals(Object obj) { // internally not used to check equality while adding objects // in TreeSet System.out.println("equals() for " + this + " & " + ((Student) obj)); return false;// return false/true doesn't make any sense here } @Override public int compareTo(Object obj) { Student s = (Student) obj; // internally inside TreeSet, compareTo is used to decide // whether two objects are equal or not, // i.e. compareTo will return 0 for same object(here student name) System.out.println("compareTo() for " + this + " & " + ((Student) obj)); // achieving uniqueness return name.compareTo(s.name); } @Override public String toString() { return name + ":" + rollNo; } } </code></pre> <p><strong>OUTPUT</strong>:</p> <pre><code>compareTo() for aaa:4 & ccc:2 compareTo() for ddd:1 & ccc:2 compareTo() for bbb:3 & ccc:2 compareTo() for bbb:3 & aaa:4 compareTo() for ddd:5 & ccc:2 compareTo() for ddd:5 & ddd:1 [aaa:4, bbb:3, ccc:2, ddd:1] [ddd:1, ccc:2, bbb:3, aaa:4] </code></pre> <p><B>Friends, whatever I got by using two Comparators, Is it possible to achieve the same while adding the objects ?? I cannot first Add elements & then use new comparator to achieve the desired order. <br>I am manipulating thousands of values so need to consider performance also. </B></p>

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