关于hashmap中hashcode的问题

java api里的Object.hashcode():

(This is typically implemented by converting the internal address of the object into an integer, but this implementation technique is not required by the Java programming language.)

看代码可以知道, hashcode()是一个native函数, 但实际JVM会不会直接用对象地址来做hashcode有待探讨, 因为现代的JVM 堆都是分代管理的, 一个Object很可能在一次 GC后改变其对象地址.而对一个对象来说,　其生命周期内的hashcode是不会变的.

Effective Java里提到自定义hashcode()的recipe(实际上在Eclipse里, 右键-> Source -> Generate hashcode and Equals可以代劳).大概例子这样：

  private String s;
    private int a;
    private short b;
    private Date d;

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + a;
        result = prime * result + b;
        result = prime * result + ((d == null) ? 0 : d.hashCode());
        result = prime * result + ((s == null) ? 0 : s.hashCode());
        return result;
    }