Java HashMap碰撞示例不起作用

Question

我的理解是Java的HashMap实现使用“桶”指向处理冲突的值列表，并且只有键的hashCode（）和对象的equals（）才会覆盖对象。对于要添加的对象及其碰撞的对象，它们是相同的。

我尝试使用HashMap来尝试查看碰撞行为，但无论我做什么，它总是会覆盖。

我在这里做错了什么（注意我故意将“count”从hashCode中取出并等于方法）？

public class HashMapTest {

public static void main(String[] args) {
    HashMapTest test = new HashMapTest();
    test.execute();
}

public void execute() {
    HashMap<String, Node> nodeMap = new HashMap<String, Node>();
    Node node1 = new Node("data1", 1);
    Node node2 = new Node("data2", 2);

    String key = "1";

    System.out.println("node1 hash: " + node1.hashCode());
    System.out.println("node2 hash: " + node2.hashCode());
    System.out.println("node1 hash == node2 hash? " + (node1.hashCode() == node2.hashCode() ? "true" : "false"));
    System.out.println("node1.equals(node2)? " + (node1.equals(node2) ? "true" : "false"));

    nodeMap.put(key, node1);
    System.out.println("added node1 to hash map");
    System.out.println("hash map size: " + nodeMap.size());
    System.out.println("hash map entry set size: " + nodeMap.entrySet().size());
    System.out.println("hash map contains node1? " + (nodeMap.containsValue(node1) ? "true" : "false"));
    System.out.println("hash map contains node2? " + (nodeMap.containsValue(node2) ? "true" : "false"));

    nodeMap.put(key, node2);
    System.out.println("added node2 to hash map");
    System.out.println("hash map size: " + nodeMap.size());
    System.out.println("hash map entry set size: " + nodeMap.entrySet().size());
    System.out.println("hash map contains node1? " + (nodeMap.containsValue(node1) ? "true" : "false"));
    System.out.println("hash map contains node2? " + (nodeMap.containsValue(node2) ? "true" : "false"));
}

protected class Node {

    private String data;

    private Integer count;

    public Node(String data, Integer count) {
        this.data = data;
        this.count = count;
    }

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

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Node other = (Node) obj;
        if (data == null) {
            if (other.data != null)
                return false;
        }
        else
            if (!data.equals(other.data))
                return false;
        return true;
    }


    public String getData() {
        return data;
    }


    public void setData(String data) {
        this.data = data;
    }


    public Integer getCount() {
        return count;
    }


    public void setCount(Integer count) {
        this.count = count;
    }

}

}

输出：

node1 hash: 95356390
node2 hash: 95356391
node1 hash == node2 hash? false
node1.equals(node2)? false
added node1 to hash map
hash map size: 1
hash map entry set size: 1
hash map contains node1? true
hash map contains node2? false
added node2 to hash map
hash map size: 1
hash map entry set size: 1
hash map contains node1? false
hash map contains node2? true

Answer 1

散列映射中的键是散列的，而不是值。在您的情况下，您正在调用put(key, node)，并且键是常量，因此第二个put将覆盖第一个{{1}}。如果节点是密钥，那么您将有两个条目。

Answer 2

你应该看看关键而不是价值。

Answer 3

如果您阅读了javadoc for HashMap，则会注意到put()方法定义如下

  

put（对象键，对象值）

     

将指定值与此映射中的指定键相关联。

当您使用相同的密钥拨打put()两次时，它会覆盖与该密钥相关联的原始值。

除此之外，该表使用键的哈希值而不是值来查找正确的存储桶。

除此之外，你对班级的理解是正确的。

但是，我不相信您可以查看来自公共API的冲突，因为这很可能是在类中处理的。

Answer 4

这是使用HashMap＆lt; Node，Node＆gt;的更新代码。我还更改了equals（）方法以包含count字段，但将其从hashCode（）方法中删除。这成功地创建了一个碰撞，如果你用调试器查看HashMap，你可以看到它在碰撞桶中构建的列表：

public class HashMapTest {

public static void main(String[] args) {
    HashMapTest test = new HashMapTest();
    test.execute();
}

public void execute() {
    HashMap<Node, Node> nodeMap = new HashMap<Node, Node>();
    Node node1 = new Node("data1", 1);
    Node node2 = new Node("data2", 2);
    Node node3 = new Node("data1", 2);
    Node node4 = new Node("data1", 1);

    System.out.println("node1 hash: " + node1.hashCode());
    System.out.println("node2 hash: " + node2.hashCode());
    System.out.println("node3 hash: " + node3.hashCode());
    System.out.println("node1 hash == node2 hash? " + (node1.hashCode() == node2.hashCode() ? "true" : "false"));
    System.out.println("node2 hash == node3 hash? " + (node2.hashCode() == node3.hashCode() ? "true" : "false"));
    System.out.println("node1 hash == node3 hash? " + (node1.hashCode() == node3.hashCode() ? "true" : "false"));
    System.out.println("node1.equals(node2)? " + (node1.equals(node2) ? "true" : "false"));
    System.out.println("node2.equals(node3)? " + (node2.equals(node3) ? "true" : "false"));
    System.out.println("node1.equals(node3)? " + (node1.equals(node3) ? "true" : "false"));
    System.out.println("");

    nodeMap.put(node1, node1);
    System.out.println("added node1 to hash map");
    System.out.println("hash map size: " + nodeMap.size());
    System.out.println("hash map entry set size: " + nodeMap.entrySet().size());
    System.out.println("hash map contains node1? " + (nodeMap.containsValue(node1) ? "true" : "false"));
    System.out.println("hash map contains node2? " + (nodeMap.containsValue(node2) ? "true" : "false"));
    System.out.println("hash map contains node3? " + (nodeMap.containsValue(node3) ? "true" : "false"));
    System.out.println("node1's count from map: " + nodeMap.get(node1).getCount());
    System.out.println("");

    nodeMap.put(node2, node2);
    System.out.println("added node2 to hash map");
    System.out.println("hash map size: " + nodeMap.size());
    System.out.println("hash map entry set size: " + nodeMap.entrySet().size());
    System.out.println("hash map contains node1? " + (nodeMap.containsValue(node1) ? "true" : "false"));
    System.out.println("hash map contains node2? " + (nodeMap.containsValue(node2) ? "true" : "false"));
    System.out.println("hash map contains node3? " + (nodeMap.containsValue(node3) ? "true" : "false"));
    System.out.println("node1's count from map: " + nodeMap.get(node1).getCount());
    System.out.println("");

    // note that if node4 is used then it replaces the value that stored node1
    nodeMap.put(node3, node3);
    System.out.println("added node3 to hash map");
    System.out.println("hash map size: " + nodeMap.size());
    System.out.println("hash map entry set size: " + nodeMap.entrySet().size());
    System.out.println("hash map contains node1? " + (nodeMap.containsValue(node1) ? "true" : "false"));
    System.out.println("hash map contains node2? " + (nodeMap.containsValue(node2) ? "true" : "false"));
    System.out.println("hash map contains node3? " + (nodeMap.containsValue(node3) ? "true" : "false"));
    System.out.println("node1's count from map: " + nodeMap.get(node1).getCount());
}

protected class Node {

    private String data;

    private Integer count;

    public Node(String data, Integer count) {
        this.data = data;
        this.count = count;
    }

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

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Node other = (Node) obj;
        if (!getOuterType().equals(other.getOuterType()))
            return false;
        if (count == null) {
            if (other.count != null)
                return false;
        }
        else
            if (!count.equals(other.count))
                return false;
        if (data == null) {
            if (other.data != null)
                return false;
        }
        else
            if (!data.equals(other.data))
                return false;
        return true;
    }


    public String getData() {
        return data;
    }


    public void setData(String data) {
        this.data = data;
    }


    public Integer getCount() {
        return count;
    }


    public void setCount(Integer count) {
        this.count = count;
    }

    private HashMapTest getOuterType() {
        return HashMapTest.this;
    }

}

}

输出：

node1 hash: 1077170390
node2 hash: 1077170391
node3 hash: 1077170390
node1 hash == node2 hash? false
node2 hash == node3 hash? false
node1 hash == node3 hash? true
node1.equals(node2)? false
node2.equals(node3)? false
node1.equals(node3)? false

added node1 to hash map
hash map size: 1
hash map entry set size: 1
hash map contains node1? true
hash map contains node2? false
hash map contains node3? false
node1's count from map: 1

added node2 to hash map
hash map size: 2
hash map entry set size: 2
hash map contains node1? true
hash map contains node2? true
hash map contains node3? false
node1's count from map: 1

added node3 to hash map
hash map size: 3
hash map entry set size: 3
hash map contains node1? true
hash map contains node2? true
hash map contains node3? true
node1's count from map: 1