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Learn how to effectively manage Bigram objects in Java utilizing HashSet while adhering to best practices for the `equals` and `hashCode` methods. This guide details how to avoid common pitfalls when overriding these methods to ensure that duplicates are correctly detected, complying with the contract of the Set interface. Understand the compilation errors associated with incorrect method signatures and see examples of output to clarify the behavior of the Bigram class in a Set context.
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Bigram: group/word of 2 letters • java.util.Set: A collection that contains no duplicate elements. • sets contain no pair of elements e1 and e2 such that e1.equals(e2), • and at most one null element. • HashSet uses a HashMap instance for imnplementation Good Java Programming
Code for Bigram class: override equals public class Bigram { private final char first; private final char second; public Bigram(char firstIn, char secondIn) { this.first = firstIn; this.second = secondIn; } // override the default equals method public boolean equals (Bigram b) { return ( (b.first == first) && (b.second == second) ); } public int hashcode(){ return 31* first + second; } public String toString() { String printString = "First: " + first + "\t Second: " + second; return printString; } } Good Java Programming
Driver Code // print 26 pairs (a a, b b, cc, …) . Note that Set does not take duplicate elements Set<Bigram> s = new HashSet<Bigram>(); for (int i=0; i<100; i++) { for (char ch = 'a'; ch <='z'; ch++) { s.add(new Bigram(ch, ch)); } } for (Bigram e : s) { System.out.println(e); } System.out.println("The size of the Hashset is: " + s.size()); What will be the output of this code? Good Java Programming
Driver Code: output? First :a Second: a First :b Second :b …. The size of the Hashset is 2600 Perhaps the override did not work? Good Java Programming
Code for Bigram class: override annotation public class Bigram { private final char first; private final char second; public Bigram(char firstIn, char secondIn) { this.first = firstIn; this.second = secondIn; } // override the default equals method @Override public boolean equals (Bigram b) { return ( (b.first == first) && (b.second == second) ); } public int hashcode(){ return 31* first + second; } public String toString() { String printString = "First: " + first + "\t Second: " + second; return printString; } } Good Java Programming
Compilation error! Bigram.java:16: method does not override or implement a method from a supertype [javac] @Override public boolean equals (Bigram b) { [javac] ^ [javac] 1 error Instead of overriding, we overloaded the equals method!! Caught by the compiler (Java 1.5 or newer) Good Java Programming
Fix the overridden method // override the default equals method @Override public boolean equals (Object obj) { boolean returnValue = false; if (obj instanceof Bigram) { Bigram b = (Bigram)obj; returnValue = (b.first == first) && (b.second == second);; } return returnValue; } Good Java Programming
Driver Code: output. Didn’t fix the bug? First :a Second: a First :b Second :b …. The size of the Hashset is 2600 Perhaps the override did not work? Good Java Programming
Code for Bigram class: override equals public class Bigram { private final char first; private final char second; public Bigram(char firstIn, char secondIn) { this.first = firstIn; this.second = secondIn; } // override the default equals method public boolean equals (Bigram b) { … } @Override public int hashcode(){ return 31* first + second; } public String toString() { String printString = "First: " + first + "\t Second: " + second; return printString; } } Good Java Programming
Compilation error! Bigram.java:34: method does not override or implement a method from a supertype [javac] @Override public int hashcode () [javac] ^ [javac] 1 error Instead of overloaded the hashCode method, we defined another method named hashcode Caught by the compiler (Java 1.5 or newer) Good Java Programming
Code for Bigram class: override equals public class Bigram { private final char first; private final char second; public Bigram(char firstIn, char secondIn) { this.first = firstIn; this.second = secondIn; } // override the default equals method public boolean equals (Bigram b) { … } @Override public int hashCode(){ return 31* first + second; } public String toString() { String printString = "First: " + first + "\t Second: " + second; return printString; } } Good Java Programming
Driver Code // print 26 pairs (a a, b b, cc, …) . Note that Set does not take duplicate elements Set<Bigram> s = new HashSet<Bigram>(); for (int i=0; i<100; i++) { for (char ch = 'a'; ch <='z'; ch++) { s.add(new Bigram(ch, ch)); } } for (Bigram e : s) { System.out.println(e); } System.out.println("The size of the Hashset is: " + s.size()); What will be the output of this code? Good Java Programming
Correct output First: l Second: l First: n Second: n First: h Second: h First: y Second: y First: j Second: j First: d Second: d First: w Second: w First: f Second: f First: u Second: u First: s Second: s First: b Second: b First: q Second: q First: m Second: m First: o Second: o First: i Second: i First: z Second: z First: k Second: k First: x Second: x First: v Second: v First: e Second: e First: t Second: t First: g Second: g First: r Second: r First: a Second: a First: p Second: p First: c Second: c The size of the Hashset is: 26 Good Java Programming
Design Guideline Consistently use the Override annotation Good Java Programming
