比较两个对象的 Java 程序
对象是具有其状态和行为的类的实例。在 java 中，由于面向对象，当对象的范围结束时，它总是被垃圾收集器动态创建和自动销毁。
说明:说明一个类的对象的例子:
家具椅=新家具()；
家具沙发=新家具()；
//在这里，椅子和沙发是家具类的两个对象
接近:
有两种标准方法:
- 使用 等于()
- 没有覆盖
- 压倒一切
- 使用 hashCode() 和 equals()方法
示例 1: 虽然可以使用 equals()方法来比较两个字符串的值，但是默认情况下，在不覆盖的情况下比较两个对象并不真正有用。
Java 语言(一种计算机语言，尤用于创建网站)
// Java Program to compare two objects
// Importing java input output library
import java.io.*;
// Class 1
class Pet {
// attributes of class1
String name;
int age;
String breed;
// constructor of class 1
Pet(String name, int age, String breed)
{
// Assignment of current attributes
/// using this keyword with same
this.name = name;
this.age = age;
this.breed = breed;
}
}
/* Class 2 : where execution is shown
for class 1 */
public class GFG {
// Main driver method
public static void main(String args[])
{
// Objects of class1 (auxiliary class)
// are assigned value */
Pet dog1 = new Pet("Snow", 3, "German Shepherd");
Pet cat = new Pet("Jack", 2, "Tabby");
Pet dog2 = new Pet("Snow", 3, "German Shepherd");
// Checking objects are equal and
// printing output- true/false
System.out.println(dog1.equals(dog2));
}
}
Output
false
示例 2: 覆盖 equals()方法
虽然 dog1 和 dog2 的值是相同的，但是 equals() 方法总是检查两个对象的引用，即传递的两个对象是否引用相同的对象，而不是它们的值。因此，建议不要在比较对象时使用此方法而不重写它。为前面的示例实现 equals 方法:
Java 语言(一种计算机语言，尤用于创建网站)
// Java Program to Compare Two Objects
import java.io.*;
class Pet {
String name;
int age;
String breed;
Pet(String name, int age, String breed)
{
this.name = name;
this.age = age;
this.breed = breed;
}
@Override public boolean equals(Object obj)
{
// checking if the two objects
// pointing to same object
if (this == obj)
return true;
// checking for two condition:
// 1) object is pointing to null
// 2) if the objects belong to
// same class or not
if (obj == null
|| this.getClass() != obj.getClass())
return false;
Pet p1 = (Pet)obj; // type casting object to the
// intended class type
// checking if the two
// objects share all the same values
return this.name.equals(p1.name)
&& this.age == p1.age
&& this.breed.equals(p1.breed);
}
}
public class GFG {
public static void main(String args[])
{
Pet dog1 = new Pet("Snow", 3, "German Shepherd");
Pet cat = new Pet("Jack", 2, "Tabby");
Pet dog2 = new Pet("Snow", 3, "German Shepherd");
System.out.println(dog1.equals(dog2));
}
}
Output
true
示例 3: 在上面的示例中，equals()方法正在检查所有值是否匹配。然而，它可以以任何可能的方式被覆盖，即如果一个或两个值匹配，等等。例如，如果我们想要检查任意两个值以使 equals()方法认为这两个对象是相同的:
Java 语言(一种计算机语言，尤用于创建网站)
// Java Program to Compare Two Objects
// Importing java input/output libraries
import java.io.*;
// Class 1
class Pet {
// Attributes of objects
String name;
int age;
String breed;
// Constructor
Pet(String name, int age, String breed)
{
// Assigning current there it self
// using this keyword
this.name = name;
this.age = age;
this.breed = breed;
}
@Override public boolean equals(Object obj)
{
if (this == obj)
return true;
if (obj == null
|| this.getClass() != obj.getClass())
return false;
Pet p1 = (Pet)obj;
// Checking only if attribute- name
// and age is same and ignoring breed
return this.name.equals(p1.name)
&& this.age == p1.age;
}
}
public class GFG {
// Main driver method
public static void main(String args[])
{
// Assigning values to attributes of object
// of class 1
Pet dog1 = new Pet("Snow", 3, "German Shepherd");
Pet cat1 = new Pet("Jack", 2, "Tabby");
Pet dog2 = new Pet("Snow", 3, "German Shepherd");
Pet cat2 = new Pet("Jack", 2, "Persian");
// Checking if object are equal and
// printing boolean output
System.out.println(cat1.equals(cat2));
}
}
Output
true
使用 hashCode()和 equals()
这种方法更像是前一种方法的补充。在输入 equals()之前，使用 hashCode()检查哈希值大大减少了生成解决方案所需的时间。这样，两个对象之间的许多比较就不需要对其中的每个值进行比较。
示例 1:以上实现以及 hashCode() 的用法:
Java 语言(一种计算机语言，尤用于创建网站)
// Java Program to Compare Two Objects
// Importing java input/output libraries
import java.io.*;
// Class 1
class Pet {
// Attributes of objects of class
String name;
int age;
String breed;
// Constructor
Pet(String name, int age, String breed)
{
this.name = name;
this.age = age;
this.breed = breed;
}
// Overriding using hashCode() method
@Override public int hashCode()
{
/* overriding hashCode() method
to check the length of the names */
return this.name.length() % 10;
}
// Boolean function to check
@Override public boolean equals(Object obj)
{
if (this == obj)
return true;
if (obj == null
|| this.getClass() != obj.getClass())
return false;
Pet p1 = (Pet)obj;
return this.name.equals(p1.name)
&& this.age == p1.age && this.breed == p1.breed;
}
}
// main class (class2)
public class GFG {
// Main driver method
public static void main(String args[])
{
// Assigning values to object of class 1(Pet class)
Pet dog1 = new Pet("Snow", 3, "German Shepherd");
Pet cat1 = new Pet("Jack", 2, "Tabby");
Pet dog2 = new Pet("Snow", 3, "German Shepherd");
Pet cat2 = new Pet("Jack", 2, "Persian");
/* hashCode() generates true as the lengths of
the name value of the two objects are same*/
// Condition check using hashCode() method
if (dog1.hashCode() == cat1.hashCode())
/* On entering equals() method, it checks for
other values and hence, returns false */
System.out.println(dog1.equals(cat1));
else
System.out.println("Not equal");
}
}
Output
false
例 2:
Java 语言(一种计算机语言，尤用于创建网站)
// Java Program to Compare Two Objects
import java.io.*;
class Pet {
String name;
int age;
String breed;
Pet(String name, int age, String breed)
{
this.name = name;
this.age = age;
this.breed = breed;
}
@Override public int hashCode()
{
// overriding hashCode() method to first
// check the length of the names*/
return this.name.length() % 10;
}
@Override public boolean equals(Object obj)
{
if (this == obj)
return true;
if (obj == null
|| this.getClass() != obj.getClass())
return false;
Pet p1 = (Pet)obj;
return this.name.equals(p1.name)
&& this.age == p1.age && this.breed == p1.breed;
}
}
public class GFG {
public static void main(String args[])
{
Pet dog1 = new Pet("Snow", 3, "German Shepherd");
Pet cat1 = new Pet("Jack", 2, "Tabby");
Pet dog2 = new Pet("Snow", 3, "German Shepherd");
Pet cat2 = new Pet("Jack", 2, "Persian");
Pet dog3 = new Pet("Ray", 1, "Siberian Husky");
// here, hashCode() generates false and condition
// reverts to the else statement as soon as it finds out
// the lengths of the name value of the objects are
// differenT
if (dog1.hashCode() == dog3.hashCode())
System.out.println(dog1.equals(dog3));
else
System.out.println("Not equal");
}
}
Output
Not equal
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