在查看覆盖hashCode()的不同选项时,我被引导到Google的guava-libraries(
javadoc)中的Objects.hashCode(Object []). javadoc声明它委托给Arrays.hashCode(Object []).在许多不同的对象类型中使用此方法是否安全？这不容易发生哈希冲突,或者这不仅仅是因为容器通常只包含一种类型的对象？
举个简单的例子,考虑以下类,
public class Student {
private final String name;
public Student(String name) {
this.name = name;
}
@Override
public int hashCode() {
return Objects.hashCode(name);
}
}
public class Teacher {
private final String name;
public Teacher(String name) {
this.name = name;
}
@Override
public int hashCode() {
return Objects.hashCode(name);
}
}
public class HashCodeDriver {
public static void main(String[] args) {
final String name = "moe";
Student s = new Student(name);
Teacher t = new Teacher(name);
long studentHash = s.hashCode();
long teacherHash = t.hashCode();
System.out.println("studentHash=" + studentHash + " teacherHash=" + teacherHash);
if(studentHash == teacherHash) {
System.out.println("hash codes match");
}
else {
System.out.println("hash codes don't match");
}
}
}
输出：
studentHash=108322 teacherHash=108322 hash codes match
对象是两种不同的类型,但生成相同的哈希码.这不是问题吗？我应该将类作为第一个参数传入以防止此冲突吗？例如,
public class Student {
private final String name;
public Student(String name) {
this.name = name;
}
@Override
public int hashCode() {
return Objects.hashCode(Student.class,name);
}
}
public class Teacher {
private final String name;
public Teacher(String name) {
this.name = name;
}
@Override
public int hashCode() {
return Objects.hashCode(Teacher.class,name);
}
}
这就是javadoc警告只为这个方法提供单个对象的原因吗？来自javadoc,
Warning: When a single object is supplied,the returned hash code does not equal the hash code of that object.
解决方法
当2种不同类型的2个不同对象具有相同的哈希码时,这不是问题.
希望,当您打算构建HashMap时,您不会将学生和教师作为该地图的关键.即使在你想要做HashMap< Object,Object>的情况下也是如此.你会好的,因为
assertFalse( new Teacher( "John Smith" ).equals( new Student( "John Smith" ) );
这就是重写hashCode和equals的重要原因.
委托给Arrays.hashCode(Object [])的唯一缺点可能是,从性能的角度来看,有时它可能太昂贵了.
例如,在您的情况下,对于教师或学生来说,这将是一个更好的哈希方法.
@Override
public int hashCode() {
return name.hashCode();
}
