Chapter 4: classes and interfaces. Item16: Compound takes precedence over inheritance.

1 package COM. twoslow. cha4; 2 3 Import Java. util. collection; 4 Import Java. util. hashset; 5 6/** 7 * Here we need to extend the hashset class and provide new functions to count the number of elements in the current set, 8 * the implementation method is to add a private domain variable to save the number of elements, 9 * and update the value in each method that adds the new element, 10 * then provide a public method to return the value 11 */12 public class instrumentedhashset <E> extends hashset <E> {13 14 private int addcount = 0; 15 16 public instrumentedhashset () {} 17 18 public instruments entedhashset (INT initcap, float loadfactor ) {19 super (initcap, loadfactor); 20} 21 22 @ override23 public Boolean add (E) {24 addcount ++; 25 return Super. add (E); 26} 27 28/* 29 * The addall method in hashset calls the add method. The addcount method is calculated twice. 30 */31 @ override32 public Boolean addall (collection <? Extends E> C) {33 addcount + = C. size (); 34 return Super. addall (c); 35} 36 37 public int getaddcount () {38 return addcount; 39} 40 41 42}

Revised Design:

 1 package com.twoslow.cha4; 2  3 import java.util.Collection; 4 import java.util.Iterator; 5 import java.util.Set; 6  7 public class ForwardingSet<E> implements Set<E>{ 8  9     private final Set<E> s ; 10     11     public ForwardingSet(Set<E> s) {12         this.s = s ;13     }14     15     @Override16     public int size() {17         return s.size();18     }19 20     @Override21     public boolean isEmpty() {22         return s.isEmpty();23     }24 25     @Override26     public boolean contains(Object o) {27         return s.contains(o);28     }29 30     @Override31     public Iterator<E> iterator() {32         return s.iterator();33     }34 35     @Override36     public Object[] toArray() {37         return s.toArray();38     }39 40     @Override41     public <T> T[] toArray(T[] a) {42         return s.toArray(a);43     }44 45     @Override46     public boolean add(E e) {47         return s.add(e);48     }49 50     @Override51     public boolean remove(Object o) {52         return s.remove(o);53     }54 55     @Override56     public boolean containsAll(Collection<?> c) {57         return s.containsAll(c);58     }59 60     @Override61     public boolean addAll(Collection<? extends E> c) {62         return s.addAll(c);63     }64 65     @Override66     public boolean retainAll(Collection<?> c) {67         return s.retainAll(c);68     }69 70     @Override71     public boolean removeAll(Collection<?> c) {72         return s.removeAll(c);73     }74 75     @Override76     public void clear() {77         s.clear();78     }79     80     @Override81     public boolean equals(Object obj) {82         return s.equals(obj);83     }84     85     @Override86     public int hashCode() {87         return s.hashCode();88     }89     90     @Override91     public String toString() {92         return s.toString();93     }94 95 }

 1 package com.twoslow.cha4; 2  3 import java.util.Collection; 4 import java.util.Set; 5  6 public class InstrumentedSet<E> extends ForwardingSet<E>{ 7  8     private int addCount = 0 ; 9     10     public InstrumentedSet(Set<E> s) {11         super(s);12     }13     14     @Override15     public boolean add(E e) {16         addCount++ ;17         return super.add(e);18     }19     20     @Override21     public boolean addAll(Collection<? extends E> c) {22         addCount += c.size() ;23         return super.addAll(c);24     }25 26     public int getAddCount() {27         return addCount;28     }29 30     31 }

 

Chapter 4: classes and interfaces. Item16: Compound takes precedence over inheritance.