How to use java Set

Question

I m trying to make it working for quite some time but just can t seem to get it  I have object Tower built of Block s  I ve already made it working using arrays  but I wanted to learn Set s  I d like to get similar functionality to this   public class Tower     public Tower       public Tower add Block k1                      if block already in tower  return  Block already in tower     public Tower delete Block k1                      if block already dleted  show  No such block in tower         Someone gave me some code  but I constantly get errors when trying to use it    Set lt Block gt  tower   new HashSet lt Block gt      boolean added   tower add  k1    if  added     System out println  Added 1 block       else   System out println  Tower already contains this block         How to implement it

User · Answer

Did you override equals and hashCode in the Block class?

EDIT:

I assumed you mean it doesn't work at runtime... did you mean that or at compile time? If compile time what is the error message? If it crashes at runtime what is the stack trace? If it compiles and runs but doesn't work right then the equals and hashCode are the likely issue.

User · Answer

The first thing you need to study is the java util Set API   Here s a small example of how to use its methods       Set lt Integer gt  numbers   new TreeSet lt Integer gt          numbers add 2       numbers add 5        System out println numbers        2  5       System out println numbers contains 7        false       System out println numbers add 5        false      System out println numbers size         2       int sum   0      for  int n   numbers            sum    n            System out println  Sum       sum       Sum   7       numbers addAll Arrays asList 1 2 3 4 5        System out println numbers        1  2  3  4  5        numbers removeAll Arrays asList 4 5 6 7        System out println numbers        1  2  3        numbers retainAll Arrays asList 2 3 4 5        System out println numbers        2  3     Once you re familiar with the API  you can use it to contain more interesting objects  If you haven t familiarized yourself with the equals and hashCode contract  already  now is a good time to start   In a nutshell     Override both or none  never just one   very important  because it must satisfied property  a equals b     true -- gt  a hashCode      b hashCode     Be careful with writing boolean equals Thing other  instead  this is not a proper  Override   For non-null references x  y  z  equals must be    reflexive  x equals x   symmetric  x equals y  if and only if y equals x  transitive  if x equals y   amp  amp  y equals z   then x equals z  consistent  x equals y  must not change unless the objects have mutated x equals null     false  The general contract for hashCode is    consistent  return the same number unless mutation happened consistent with equals  if x equals y   then x hashCode      y hashCode     strictly speaking  object inequality does not require hash code inequality but hash code inequality necessarily requires object inequality   What counts as mutation should be consistent between equals and hashCode    Next  you may want to impose an ordering of your objects  You can do this by making your type implements Comparable  or by providing a separate Comparator   Having either makes it easy to sort your objects  Arrays sort  Collections sort List    It also allows you to use SortedSet  such as TreeSet     Further readings on stackoverflow    Overriding equals and hashCode in Java When to use Comparable vs Comparator

User · Answer

It s difficult to answer this question with the information given   Nothing looks particularly wrong with how you are using HashSet   Well  I ll hazard a guess that it s not a compilation issue and  when you say  getting errors   you mean  not getting the behavior  you  want    I ll also go out on a limb and suggest that maybe your Block s equals an hashCode methods are not properly overridden

User · Answer

Since it is a HashSet you will need to override hashCode and equals methods  http   preciselyconcise com java collections d set php has an example explaining how to implement hashCode and equals methods

[java] How to use java.Set

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