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;
	}

	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + i;
		return result;
	}
	@Override
	public 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>{
	@Override
	public int compare(TestClass obj1, TestClass obj2) {
		return obj1.getI() - obj2.getI();
	}
}