JAVA TreeSet体会
一直以来，对TreeSet的理解受到了HashMap的影响。认为HashSet也是通过equals方法区分对象。最近在一个程序开发中使用了TreeSet，发现TreeSet区分对象是根据对象实现的Comparable或者Comparator接口中的Compare()方法或者CompareTo()方法。如果两个对象通过比较返回值为0，那么将一个对象插入到TreeSet后，另外一个对象将不能插入TreeSet。 HashSet通过HashMap实现，集合中的每一个对象都是一个key,根据根据对象的HashCode和equals方法区分。
可以通过修改以下代码查看结果。
import java.util.*;public class TestTreeSet {/** * @param args */public static void main(String[] args) {TreeSet<TestClass> treeSetWithComparator = new TreeSet<TestClass>(new TestClassComparator());HashSet<TestClass> testHashSet = new HashSet<TestClass>();TestClass testObj1 = new TestClass(5);TestClass testObj2 = new TestClass(5);System.out.printf("\ntestObj1.getI == %d && testObj2.getI == %d\n",testObj1.getI(),testObj2.getI());System.out.println("testObj1 equals testObj2? " + testObj1.equals(testObj2));System.out.println("testObj1 compares with testObj2? " + treeSetWithComparator.comparator().compare(testObj1, testObj2));System.out.println("testObj1 == testObj2? " + (testObj1 == testObj2));System.out.println("Hashcode of testObj1 " + testObj1.hashCode());System.out.println("Hashcode of testObj2 " + testObj2.hashCode());System.out.println("Add testObj1 into treeset: " + treeSetWithComparator.add(testObj1));System.out.println("Add testObj2 into treeset: " + treeSetWithComparator.add(testObj2));System.out.println("Add testObj1 into HashSet: " + testHashSet.add(testObj1));System.out.println("Add testObj2 into HashSet: " + testHashSet.add(testObj2));treeSetWithComparator.clear();testHashSet.clear();testObj1.setI(5);testObj2.setI(6);System.out.printf("\ntestObj1.getI == %d && testObj2.getI == %d\n",testObj1.getI(),testObj2.getI());System.out.println("testObj1 equals testObj2? " + testObj1.equals(testObj2));System.out.println("testObj1 compares with testObj2? " + treeSetWithComparator.comparator().compare(testObj1, testObj2));System.out.println("testObj1 == testObj2? " + (testObj1 == testObj2));System.out.println("Hashcode of testObj1 " + testObj1.hashCode());System.out.println("Hashcode of testObj2 " + testObj2.hashCode());System.out.println("Add testObj1 into treeset: " + treeSetWithComparator.add(testObj1));System.out.println("Add testObj2 into treeset: " + treeSetWithComparator.add(testObj2));System.out.println("Add testObj1 into HashSet: " + testHashSet.add(testObj1));System.out.println("Add testObj2 into HashSet: " + testHashSet.add(testObj2));}}class TestClass{private int i = 0;public TestClass(int i){this.i = i;}int getI(){return i;}void setI(int i){this.i = i;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + i;return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;TestClass other = (TestClass) obj;if (i != other.i)return false;return true;}}class TestClassComparator implements Comparator<TestClass>{@Overridepublic int compare(TestClass obj1, TestClass obj2) {return obj1.getI() - obj2.getI();}}