前言：承接上一篇hashCode和identityHashCode 的关系，下面的两段简单的程序主要是检验一下系统生成的identityHashCode是否存在重复的情况。
1：可以*控制生成对象的个数，并且不受测试的类是否重写hashCode()方法的影响
import java.util.HashSet;
import java.util.Set; public class CheckSystemIdentity {
public static void main(String args[]) {
//声明set对象
Set<Integer> hashSet = new HashSet<Integer>(1024);
//通过循环遍历，检查生成的hashCode是否存在重复的现象
int colls = 0;
int cycleSize=1000000;
for (int n = 0; n < cycleSize; n++) {
Integer obj = new Integer(666);
int identityHashCode = System.identityHashCode(obj);
//System.out.println("identityHashCode is : "+identityHashCode);
Integer hashValue = Integer.valueOf(identityHashCode);
//System.out.println("hashValue is : "+hashValue+"\n");
if (hashSet.contains(hashValue)) {
System.err.println("System.identityHashCode() collision!");
colls++;
}
else {
hashSet.add(hashValue);
}
}
//System.out.println("Integer.MAX_VALUE is : "+Integer.MAX_VALUE+"\n");
System.out.println("created "+cycleSize+" different objects - " + colls + " times with the same value for System.identityHashCode()");
} }
2：利用死循环来检测系统生成的identityHashCode是否存在重复的情况
2-1：测试类是自定义的，没有重写hashCode()方法
import java.util.Hashtable;
import java.util.Map; public class HashCodeTest{
//试验对象，没有重写hashCode()方法
static class DummyObject{ } public static void report*(DummyObject obj1, DummyObject obj2) {
System.out.println("obj1.hashCode() = " + obj1.hashCode());
System.out.println("obj2.hashCode() = " + obj2.hashCode());
System.out.println("(obj1 == obj2) = " + (obj1 == obj2) + " (!)");
} public static void main(String[] args) {
Map<Integer,DummyObject> map = new Hashtable<Integer ,DummyObject>();
//通过死循环，检查生成的hashCode是否存在重复的情况
for (int count = 1; true; count++) {
DummyObject obj = new DummyObject();
if (map.containsKey(obj.hashCode())) {
System.out.println("* found after instantiating " + count + " objects.");
report*(map.get(obj.hashCode()), obj);
System.exit(0);
}
System.out.println("The method execute " + count + " and the object hashCode is "+obj.hashCode());
map.put(obj.hashCode(), obj);
}
}
}
2-2：测试类是String，重写了hashCode()方法和2-1正好再次的做一下对比
import java.util.Hashtable;
import java.util.Map; public class HashCodeTest { public static void report*(String obj1, String obj2) {
System.out.println("obj1.hashCode() = " + obj1.hashCode());
System.out.println("obj2.hashCode() = " + obj2.hashCode());
System.out.println("(obj1 == obj2) = " + (obj1 == obj2) + " (!)");
} public static void main(String[] args) {
Map<Integer,String> map = new Hashtable<Integer ,String>();
//通过死循环，检查生成的hashCode是否存在重复的情况
for (int count = 1; true; count++) {
String obj = new String();
if (map.containsKey(obj.hashCode())) {
System.out.println("* found after instantiating " + count + " objects.");
report*(map.get(obj.hashCode()), obj);
System.exit(0);
}
System.out.println("The method execute " + count + " and the object hashCode is "+obj.hashCode());
map.put(obj.hashCode(), obj);
}
}
}
3：程序相对简单，有兴趣的可以自己运行一下看看结果。
结论如下：
3-1：在我实验的环境中（win7+jdk7）identityHashCode没有出现重复的现象
3-2：没有重写的hashCode和identityHashCode是一致的，可以间接的反映一个对象的内存地址是什么
3-3：identityHashCode能更为准确的代表一个对象和其内存地址的hash关系，