@linux1s1s 2017-07-24T09:33:38.000000Z 字数 4204 阅读 1676

Java equals hashCode 浅析

Java 2017-07

为了搞懂Java中equals()和hashCode()方法，我们先从一个具体的问题入手。

问题

直接看下面的实例Code

class Student {
    private int number;
    private String name;
    public Student(int number, String name) {
        this.number = number;
        this.name = name;
    }
    public int getNumber() {
        return number;
    }
    public void setNumber(int number) {
        this.number = number;
    }
    @Override
    public String toString() {
        return "Student{" +
                "number=" + number +
                ", name='" + name + '\'' +
                '}';
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

上面是典型的POJO类，接着我们实例化这个类，并且将实例化后的实例添加到Set容器中。

private void profile() {
    HashSet<Student> hs = new HashSet<>();
    Student s1, s2, s3;
    hs.add(s1 = new Student(1, "zhangsan"));
    hs.add(s2 = new Student(2, "lisi"));
    hs.add(s3 = new Student(1, "zhangsan"));
    Iterator<Student> it = hs.iterator();
    while (it.hasNext()) {
        Student s = it.next();
        Log.i("Student", s.toString());
    }
}

上面故意将实例化的Student有两处内容相同，接下来观察一下打印出来的结果

07-24 16:47:22.420 29850-29850/ I/Student: Student{number=2, name='lisi'}
07-24 16:47:22.420 29850-29850/ I/Student: Student{number=1, name='zhangsan'}
07-24 16:47:22.420 29850-29850/ I/Student: Student{number=1, name='zhangsan'}

可以看到Set容器中出现了我们认为重复的实例（内容重复），而这在实际工作中，需要排除这种内容完全重复的实例，想一想在哪些地方要注意处理（其实在C端开发中，比如Tab页面，Server给出数据，如果不小心给出完全相同的数据，那么C端就要避免出现几个完全相同的页面同时出现）。那么是什么原因导致出现上面的情况呢？

探究原因

我们先来看Set的add源码

java.util.HashSet.java

public boolean add(E e) {
    return map.put(e, PRESENT)==null;
}

java.util.HashMap.java

/**
 * Adds a new entry with the specified key, value and hash code to
 * the specified bucket.  It is the responsibility of this
 * method to resize the table if appropriate.
 *
 * Subclass overrides this to alter the behavior of put method.
 */
public V put(K key, V value) {
    if (table == EMPTY_TABLE) {
        inflateTable(threshold);
    }
    if (key == null)
        return putForNullKey(value);
    int hash = sun.misc.Hashing.singleWordWangJenkinsHash(key);
    int i = indexFor(hash, table.length);
    for (HashMapEntry<K,V> e = table[i]; e != null; e = e.next) {
        Object k;
        //1.Hash code 相同
        //2. key地址相同或者key满足equals条件
        //仅当以上条件都满足的情况下，不会将key,value添加到Entry容器
        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;
}

定位到L22，我们看到在判断语句中分别调用了hash和equals()方法，如果hash code不同则直接加入Entry容器中，或者加入容器的实例不满足equals条件也会直接加入Entry容器。所以根据L19开始的注释得知，Student内容相同的两个实例必然满足如下条件：

Hash code不相同
Student地址不相同且Student不满足equals条件

我们来验证一下刚才的推测：

private void profile() {
    HashSet<Student> hs = new HashSet<>();
    Student s1, s2, s3;
    hs.add(s1 = new Student(1, "zhangsan"));
    hs.add(s2 = new Student(2, "lisi"));
    hs.add(s3 = new Student(1, "zhangsan"));
    Log.i("Student", "s1 hashcode: " + s1.hashCode());
    Log.i("Student", "s2 hashcode: " + s2.hashCode());
    Log.i("Student", "s3 hashcode: " + s3.hashCode());
    Log.i("Student", "s1.equals(s2): " + s1.equals(s2));
    Log.i("Student", "s2.equals(s3): " + s2.equals(s3));
    Log.i("Student", "s3.equals(s1): " + s3.equals(s1));
    Iterator<Student> it = hs.iterator();
    while (it.hasNext()) {
        Student s = it.next();
        Log.i("Student", s.toString());
    }
}

验证结果：

07-24 17:23:37.372 1454-1454/ I/Student: s1 hashcode: 36165334
07-24 17:23:37.372 1454-1454/ I/Student: s2 hashcode: 233823575
07-24 17:23:37.372 1454-1454/ I/Student: s3 hashcode: 215740484
07-24 17:23:37.372 1454-1454/ I/Student: s1.equals(s2): false
07-24 17:23:37.372 1454-1454/ I/Student: s2.equals(s3): false
07-24 17:23:37.372 1454-1454/ I/Student: s3.equals(s1): false
07-24 17:23:37.372 1454-1454/ I/Student: Student{number=2, name='lisi'}
07-24 17:23:37.372 1454-1454/ I/Student: Student{number=1, name='zhangsan'}
07-24 17:23:37.372 1454-1454/ I/Student: Student{number=1, name='zhangsan'}

经过上面的验证，猜测正确，那么如果想完成实际开发中的要求（相同内容的实例不能同时加入到Set容器中），必须重写Student这个类的equals()和hashCode()方法。

解决

重写Student类中的equals()和hashCode()方法

@Override
public boolean equals(Object obj) {
    Student s = (Student) obj;
    return this.number == s.number && this.name.equals(s.name);
}
@Override
public int hashCode() {
    return this.number * this.name.hashCode();
}

验证结果

07-24 17:29:26.653 7938-7938/ I/Student: s1 hashcode: -1432604556
07-24 17:29:26.653 7938-7938/ I/Student: s2 hashcode: 6644006
07-24 17:29:26.653 7938-7938/ I/Student: s3 hashcode: -1432604556
07-24 17:29:26.653 7938-7938/ I/Student: s1.equals(s2): false
07-24 17:29:26.653 7938-7938/ I/Student: s2.equals(s3): false
07-24 17:29:26.653 7938-7938/ I/Student: s3.equals(s1): true
07-24 17:29:26.653 7938-7938/ I/Student: Student{number=2, name='lisi'}
07-24 17:29:26.653 7938-7938/ I/Student: Student{number=1, name='zhangsan'}

看到这里的s1和s3 hashcode相等，而且s1和s3满足equals条件，这个时候Set容器就自动屏蔽了内容相同的实例。

TIPS：参看文章：Java提高篇——equals()与hashCode()方法详解

Java equals hashCode 浅析

问题

探究原因

解决

内容目录