HashSet源码分析1

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class SetTest {
public static void main(String[] args) {
Set<String> set = new HashSet<>();
set.add("abc");
set.add("xyz");
set.add("abc");

for (Iterator<String> it = set.iterator(); it.hasNext();) {
System.out.println(it.next());
}
}
}

查看add方法

/**
* Adds the specified element to this set if it is not already present.
* More formally, adds the specified element <tt>e</tt> to this set if
* this set contains no element <tt>e2</tt> such that
* <tt>(e==null ? e2==null : e.equals(e2))</tt>.
* If this set already contains the element, the call leaves the set
* unchanged and returns <tt>false</tt>.
*
* @param e element to be added to this set
* @return <tt>true</tt> if this set did not already contain the specified
* element
*/
public boolean add(E e) {
return map.put(e, PRESENT)==null;
}

查看map.put方法：

/**
* Associates the specified value with the specified key in this map.
* If the map previously contained a mapping for the key, the old
* value is replaced.
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
*         <tt>null</tt> if there was no mapping for <tt>key</tt>.
*         (A <tt>null</tt> return can also indicate that the map
*         previously associated <tt>null</tt> with <tt>key</tt>.)
*/
public V put(K key, V value) {
if (table == EMPTY_TABLE) {
inflateTable(threshold);
}
if (key == null)
return putForNullKey(value);
int hash = hash(key);
int i = indexFor(hash, table.length);
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}

modCount++;
addEntry(hash, key, value, i);
return null;
}

关于set：当向Set集合中添加对象时，集合首先要计算出待添加对象的hashCode值，根据该值来得到一个位置，用于存放当前的对象。如果该位置没有对象存在，那么集合Set就会认为该对象在集合中不存在，直接添加进去。如果该位置已经有一个对象存在，接着将准备添加到集合中的对象与该位置上的对象进行equals方法比较，如果该equals方法返回false，那么集合认为该集合中不存在该对象的，再进行一次散列，将该对象放到散列后计算的地址里。如果equals方法返回true，那么集合认为该对象已经存在了，不会再将对象添加到集合中。

在上面的例子中我们的执行结果为：

true
true
false
abc
xyz

第三个abc插入失败了，原因就是String类型的字符串“abc”的hashcode是相等的而且equals也是相等的(equals方法使一个字符一个字符的进行比较)。

再看下面的例子：

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class SetTest {
public static void main(String[] args) {
/*    Set<String> set = new HashSet<>();
System.out.println(set.add("abc"));
System.out.println(set.add("xyz"));
System.out.println(set.add("abc"));

for (Iterator<String> it = set.iterator(); it.hasNext();) {
System.out.println(it.next());
}*/
Set<People> set2 = new HashSet<>();
set2.add(new People("zhangsan"));
set2.add(new People("lisi"));
set2.add(new People("zhangsan"));
for(Iterator<People> it = set2.iterator();it.hasNext();){
System.out.println(it.next().getName());
}
}
}
class People{
String name;
public People(String name){
this.name = name;
}
public String getName(){
return this.name;
}
}

运行结果为：

lisi
zhangsan
zhangsan

同样是zhangsan却能添加两次。

如果想根据name来判断，修改代码如下：

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class SetTest {
public static void main(String[] args) {
/*
* Set<String> set = new HashSet<>();
* System.out.println(set.add("abc"));
* System.out.println(set.add("xyz"));
* System.out.println(set.add("abc"));
*
* for (Iterator<String> it = set.iterator(); it.hasNext();) {
* System.out.println(it.next()); }
*/
/*
* String a = "abc"; String b = "abc"; System.out.println(a.hashCode());
* System.out.println(b.hashCode());
*/
Set<People> set2 = new HashSet<>();
set2.add(new People("zhangsan"));
set2.add(new People("lisi"));
set2.add(new People("zhangsan"));
for (Iterator<People> it = set2.iterator(); it.hasNext();) {
System.out.println(it.next().getName());
}
}
}

class People {
String name;

public People(String name) {
this.name = name;
}

public String getName() {
return this.name;
}

@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj instanceof People) {
People people = (People) obj;
if(this.name.equals(people.getName()));
return true;
}
return false;
}
@Override
public int hashCode() {
return this.name.hashCode();
}
}

执行结果为：

lisi
zhangsan

当重写equals方法时，必须要重写hashCode方法。如果一个类的两个对象，使用equals方法比较时返回true，那么这两个对象必须要具有相同的hashCode。