Java HashMap的返回值并不能证实我对equals和hashcode的理解 [英] Java HashMap return value not confirming with my understanding of equals and hashcode
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问题描述
以下代码示例的输出为:
<1> {1 - e = e2,2 - e1 = e1}
package com.sid.practice;
import java.util.HashMap;
import java.util.Map;
public class InputOutputPractice
{
public InputOutputPractice()
{
}
public static void main(String [] args)
{
Employee e = new InputOutputPractice()。new Employee(1,e);
Employee e1 = new InputOutputPractice()。new Employee(2,e1);
Employee e2 = new InputOutputPractice()。new Employee(1,e2);
Map m = new HashMap();
m.put(e,e);
m.put(e1,e1);
m.put(e2,e2);
System.out.println(m);
$ b $ class class Employee
{
public Employee(int id,String name)
{
this.id = ID;
this.name = name;
}
private int id;
私人字符串名称;
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
$ b @Override
public boolean equals(Object obj)
{
return((Employee)obj).getId()==(this .getId());
$ b $ @Override
public int hashCode()
{
return Integer.valueOf(getId())。hashCode();
}
@Override
public String toString()
{
return this.id + - + this.name;
}
}
}
我不明白对象 e2
能够覆盖Object e
中的键,但不能覆盖该值。在我的理解中,输出应该是:
lockquote
{1 - e2 = e2,2 - e1 = e1}
解决方案
其实,你已经把它倒过来了。该值被覆盖。关键是没有被取代,因为就 HashMap
而言,e和e2是相同的。
您的输出是 {1 - e = e2,2 - e1 = e1}
:
key = e,value =e2(它覆盖旧值e)
key = e1,value =e1
The output of the following code sample is:
{1--e=e2, 2--e1=e1}
package com.sid.practice;
import java.util.HashMap;
import java.util.Map;
public class InputOutputPractice
{
public InputOutputPractice()
{
}
public static void main(String[] args)
{
Employee e = new InputOutputPractice().new Employee(1, "e");
Employee e1 = new InputOutputPractice().new Employee(2, "e1");
Employee e2 = new InputOutputPractice().new Employee(1, "e2");
Map m = new HashMap();
m.put(e, "e");
m.put(e1, "e1");
m.put(e2, "e2");
System.out.println(m);
}
class Employee
{
public Employee(int id, String name)
{
this.id=id;
this.name = name;
}
private int id;
private String name;
public String getName()
{
return name;
}
public void setName(String name)
{
this.name = name;
}
public int getId()
{
return id;
}
public void setId(int id)
{
this.id = id;
}
@Override
public boolean equals(Object obj)
{
return ((Employee)obj).getId()==(this.getId());
}
@Override
public int hashCode()
{
return Integer.valueOf(getId()).hashCode();
}
@Override
public String toString()
{
return this.id + "--" + this.name;
}
}
}
I do not understand how the Object e2
was able to overwrite the key in Object e
, but not the value. In my understanding the output should have been:
{1--e2=e2, 2--e1=e1}
解决方案
Actually, you got it backwards. The value was overridden. The key wasn't replaced since as far as HashMap
is concerned, e and e2 are identical.
Your output is {1--e=e2, 2--e1=e1}
:
key = e, value = "e2" (which overrode the old value "e")
key = e1, value = "e1"
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