hashcode是在Object就已经定义了一个方法，名叫散列码，来看看Object是怎么描述它的
/**
* Returns a hash code value for the object. This method is
* supported for the benefit of hash tables such as those provided by
* {@link java.util.HashMap}.
* <p>
* The general contract of {@code hashCode} is:
* <ul>
* <li>Whenever it is invoked on the same object more than once during
* an execution of a Java application, the {@code hashCode} method
* must consistently return the same integer, provided no information
* used in {@code equals} comparisons on the object is modified.
* This integer need not remain consistent from one execution of an
* application to another execution of the same application.
* <li>If two objects are equal according to the {@code equals(Object)}
* method, then calling the {@code hashCode} method on each of
* the two objects must produce the same integer result.
* <li>It is <em>not</em> required that if two objects are unequal
* according to the {@link java.lang.Object#equals(java.lang.Object)}
* method, then calling the {@code hashCode} method on each of the
* two objects must produce distinct integer results. However, the
* programmer should be aware that producing distinct integer results
* for unequal objects may improve the performance of hash tables.
* </ul>
* <p>
* As much as is reasonably practical, the hashCode method defined by
* class {@code Object} does return distinct integers for distinct
* objects. (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.)
*
* @return a hash code value for this object.
* @see java.lang.Object#equals(java.lang.Object)
* @see java.lang.System#identityHashCode
*/
public native int hashCode();
hashcode如果没有覆盖重写，那么默认是由Object导出的对象存储地址。主要应用场景是HashMap和HashSet等等的Hash集合类里面
实例1⃣️：String的hashCode()方法
public static void main(String[] args) {
String a1=new String("张");
String a2=new String("张");
String a3=new String("吴");
HashSet<String> h1=new HashSet<>();
h1.add(a1);
h1.add(a2);
h1.add(a3);
System.out.println(h1);
}
输出结果：
我明明add了三个对象进去，为什么HashSet里面只有两个元素，这是因为String覆盖了Object的hashcode()方法，我们来看看String的hashcode()
/**
* Returns a hash code for this string. The hash code for a
* {@code String} object is computed as
* <blockquote><pre>
* s[0]*31^(n-1) + s[1]*31^(n-2) + ... + s[n-1]
* </pre></blockquote>
* using {@code int} arithmetic, where {@code s[i]} is the
* <i>i</i>th character of the string, {@code n} is the length of
* the string, and {@code ^} indicates exponentiation.
* (The hash value of the empty string is zero.)
*
* @return a hash code value for this object.
*/
public int hashCode() {
int h = hash;
if (h == 0 && value.length > 0) {
char val[] = value;
for (int i = 0; i < value.length; i++) {
h = 31 * h + val[i];
}
hash = h;
}
return h;
}
String的hashcode值为每个字符chat值相加。HashSet虽然添加了三个对象，但是其中两个对象的值是一样的，所以HashSet只保存两个元素。
实例2⃣️：Object的hashCode()方法
我们将实例1⃣️的中的String类型换成Object类型，代码如下：
public static void main(String[] args) {
Object a1=new Object();
Object a2=new Object();
Object a3=new Object();
HashSet<Object> h1=new HashSet<>();
h1.add(a1);
h1.add(a2);
h1.add(a3);
System.out.println(h1);
}
输出结果：
OK，HashSet保存了三个对象，没问题，因为Object的hashcode默认就是对象的存储地址
实例3⃣️自定义类覆盖hashCode()方法
我在这里定义了一个类，代码如下：
public class HashCodeObject {
private int a;
public int getA() {
return a;
}
public void setA(int a) {
this.a = a;
}
@Override
public boolean equals(Object obj) {
if(obj instanceof HashCodeObject){
return this.a==((HashCodeObject)obj).getA();
}
return false;
}
@Override
public int hashCode() {
return this.a;
}
}
然后，再将实例2⃣️的Object换成我们自己定义的类，如下
public static void main(String[] args) {
HashCodeObject a1=new HashCodeObject();
HashCodeObject a2=new HashCodeObject();
HashCodeObject a3=new HashCodeObject();
HashSet<HashCodeObject> h1=new HashSet<>();
h1.add(a1);
h1.add(a2);
h1.add(a3);
System.out.println(h1);
}
输出结果：
注意，覆盖hashcode()方法的同时也要覆盖equal()方法，不然达不到效果，并且equal()和hashcode()定义必须要一致，如果equal()返回true，那么hashcode应该
具有相同的值。