比较两个对象的Java程序

对象是具有状态和行为的类的实例。在Java中，是面向对象的，当对象的作用域结束时，它总是由垃圾收集器动态创建和自动销毁。

插图：一个例子来说明一个类的对象：

Furniture chair=new Furniture();

Furniture sofa=new Furniture();

// Here, chair and sofa are two objects of the class Furniture

方法：

有两种标准方法：

1. 使用equals()
   • 没有覆盖
   • 与覆盖
2. 使用hashCode()和 equals() 方法

示例 1：虽然 equals() 方法可用于比较两个字符串的值，但默认情况下，在不覆盖它的情况下比较两个对象并没有什么用。

Java

//  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));
    }
}

输出

false

示例 2：覆盖 equals() 方法

尽管dog1和dog2的值相同，equals() 方法始终检查两个对象的引用，即传递的两个对象是否引用同一个对象，而不是它们的值。因此，建议不要在不覆盖它的情况下比较对象时使用此方法。为之前的例子实现 equals 方法：

Java

// 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));
    }
}

输出

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。但是，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。例如，如果我们想检查任何两个值以使 equals() 方法认为两个对象相同：

Java

// 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));
    }
}

输出

true

使用 hashCode() 和 equals()

这种方法更像是前一种方法的附加。在输入 equals() 之前使用 hashCode() 检查哈希值可以大大减少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经过它们内部每个值的比较。

示例1：上述实现以及 hashCode() 的使用：

Java

// 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");
    }
}

输出

false

示例 2：

Java

// 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");
    }
}

输出

Not equal