Common misunderstandings and precautions: equals(Object) method in Java

The equals method in Java is a method used to compare whether objects are equal. It is a method of the Object class and is very commonly used in actual development. However, due to insufficient understanding of the equals method and its implementation, many developers will make some common misunderstandings when using the equals method. This article will introduce the precautions and common misunderstandings about the equals method in Java to help readers better understand and use the equals method.

First, let us understand the basic usage of the equals method. In Java, all classes inherit from the Object class, and the equals method in the Object class is defined as follows:

public boolean equals(Object obj) {
    return (this == obj);
}

As you can see, the default implementation of the equals method in the Object class is to compare whether the references of the objects are the same. That is, determine whether two objects are the same object. But in actual development, we usually need to judge whether they are equal based on the contents of the object, so we need to override the equals method in the custom class. The following is an example of overriding the equals method:

public class Person {
    private String name;
    private int age;

    // 省略构造方法和其他代码

    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null || getClass() != obj.getClass()) {
            return false;
        }
        Person person = (Person) obj;
        return age == person.age && Objects.equals(name, person.name);
    }
}

In this example, we override the equals method of the Person class to determine whether two Person objects are equal based on name and age.

Next, we will introduce some precautions and common misunderstandings about the equals method that need to be paid attention to in actual development.

1. When rewriting the equals method, you need to follow the specifications
   When you rewrite the equals method, you need to follow certain specifications. According to the official Java documentation, the equals method needs to meet the following conditions:

• Reflexivity: x.equals(x) must return true.
• Symmetry: If x.equals(y) returns true, then y.equals(x) must also return true.
• Transitiveness: If x.equals(y) returns true, and y.equals(z) also returns true, then x.equals(z) must also return true.
• Consistency: If x and y do not change, then calling x.equals(y) multiple times should return the same result.
• For any non-null reference x, x.equals(null) must return false.

When rewriting the equals method, you need to ensure that the above conditions are met to ensure the correctness of the equals method.

2. Don’t forget to rewrite the hashCode method
   While rewriting the equals method, you also need to rewrite the hashCode method. According to the official Java documentation, if two objects are judged to be equal according to the equals method, then their hashCode values ​​must be equal. Therefore, when rewriting the equals method, be sure to rewrite the hashCode method at the same time to ensure that the object can be operated correctly when placed in a data structure such as a hash table.
3. Pay attention to handling null pointer exceptions
   When overriding the equals method, you need to pay attention to handling null pointer exceptions. When using the equals method to compare objects, you need to first determine whether it is null to avoid the occurrence of a null pointer exception.
4. Use the equals method of the Objects class
   When overriding the equals method, you can use the equals method of the Objects class to compare whether the properties of the objects are equal to avoid the occurrence of null pointer exceptions and type conversion exceptions. The equals method of the Objects class can correctly handle null values ​​and type conversions, so it is recommended to use the equals method of the Objects class when overriding the equals method.
5. Don’t confuse the equals method with ==
   When using the equals method, be sure to distinguish the difference between the equals method and the == operator. The equals method is used to compare whether the contents of objects are equal, and the == operator is used to compare whether the references of objects are equal. Therefore, when using the equals method, be sure to clearly understand the difference between the two.

To summarize, the equals method in Java is a method used to compare whether objects are equal, and is very commonly used in actual development. However, when using the equals method, you need to pay attention to the specifications and precautions for rewriting the equals method to avoid common misunderstandings. Correctly rewriting the equals method can improve the maintainability and robustness of the program, so I hope this article can help readers better understand and use the equals method.

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