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POcompareTo() implementation problems
 

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  1. POcompareTo() implementation problems
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    <p>Hi i'm having trouble implementing the compareTo method. I've looked for answers but nothing has been any help. I'm trying to fill a TreeSet with various sizes of circles. I need compareTo in my circle class to be able to store them this way. </p> <pre><code>import java.util.*; import java.lang.*; abstract class Shape { private String name; //e.g."circlel", "rectangle3" Shape(String name0) { name = name0; } abstract double area (); // area of shape abstract double perim(); // length of perimeter of shape void put() { // display shape details System.out.println(name + " with area " + area() + " and perimeter " + perim() ); } } class Circle extends Shape implements Comparable { private static String name; private int radius; Circle(String n, int r) { super(n); radius = r; } public double area() { return Math.PI * radius * radius; } public double perim() { return 2 * Math.PI * radius; } public int compareTo(Circle c) { if(c.name == name &amp;&amp; c.radius == radius) { return 0; } else { return 1; } } } </code></pre> <p>Edit: Thanks I was forgetting something: Circle is not abstract and does not overide abstract method compareTo(Object) in Comparable</p> <p>Thanks for the help on that, now that I have gotten down to testing the class, when try to add a circle to the treeset this exception pops up any ideas,</p> <pre><code>Exception in thread "main" java.lang.NullPointerException at Circle.compareTo(Shape.java:47) at Circle.compareTo(Shape.java:23) at java.util.TreeMap.compare(Unknown Source) at java.util.TreeMap.put(Unknown Source) at java.util.TreeSet.add(Unknown Source) at CircleTreeSet.main(CircleTreeSet.java:24) </code></pre>
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    1. USBungalo Soldier
    1. USBungalo Soldier
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    1. COWould be useful with more information on what errors you get.
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      1. USNiklas Lindblad
    2. COYou need to declare it as `class Circle extends Shape implements Comparable<Circle>`. Also read [How do I compare strings in Java?](http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java). Also, as currently implemented there will never be a negative return value, which is needed for _smaller_ circles.
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      1. USjlordo
 

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