Java basic Object class

The Object class is located in the java.lang package and is the ancestor of all Java classes. Every class in Java extends from it.

If you do not explicitly specify the parent class when defining a Java class, it will inherit the Object class by default. For example:

public class Demo{
  // ...
}

is actually the abbreviation of the following code:

public class Demo extends Object{
  // ...
}

In Java, only basic types are not objects, such as numerical values, Character and Boolean values ​​are not objects. All array types, whether object arrays or basic type arrays, inherit from the Object class.

The Object class defines some useful methods. Since it is a root class, these methods exist in other classes. They are usually overloaded or overridden to implement their respective specific functions.

equals() method

The equals() method in the Object class is used to detect whether an object is equivalent to another object. The syntax is:
public boolean equals(Object obj)
For example:

obj1.equals(obj2);

In Java, the basic meaning of data equality is that the values ​​of two data are equal. When comparing through equals() and "==", the reference type data compares the reference, that is, the memory address, and the basic data type compares the value.

Note:

•The equals() method can only compare reference types, and "==" can compare reference types and basic types.
•When comparing using the equals() method, for classes File, String, Date and wrapper classes, the type and content are compared without considering whether the references are to the same instance.
•When comparing with "==", the data types on both sides of the symbol must be consistent (except for automatically converted data types), otherwise a compilation error will occur. However, the two data compared with the equals method only need to be reference types. .

hashCode() method

Hash code (hashCode) is a value obtained from an object according to a certain algorithm. Hash codes have no rules. If x and y are different objects, x.hashCode() and y.hashCode() will basically not be the same.

The hashCode() method is mainly used to implement operations such as quick search in a collection, and can also be used for object comparison.

In Java, the regulations for hashCode are as follows:

•During the execution of the same application, calling hashCode() on the same object must return the same integer result— —The premise is that none of the information compared by equals() has been modified. As for the call results obtained by the same application in different execution periods, they do not need to be consistent.
•If two objects are considered equal by the equals() method, then calling hashCode() on both objects must yield the same integer result.
•If two objects are considered unequal by the equals() method, then calling hashCode() on the two objects need not produce different integer results. However, programmers should be aware that producing different integer results for different objects may improve the efficiency of hashTable (you will learn about it later, a class in the collections framework).

Simply put: if two objects are the same, then their hashCode values ​​must be the same; if the hashCode values ​​of two objects are the same, they are not necessarily the same. According to the Java specification, generally overriding the equals() method should also override the hashCode() method.

toString() method

toString() method is another important method defined in the Object class. It is the string representation of the object. The syntax is:
public String toString()
The return value is String type, used to describe information about the current object. The toString() method implemented in the Object class returns the type and memory address information of the current object, but it has been rewritten in some subclasses (such as String, Date, etc.), and can also be rewritten in user-defined types as needed. toString() method to return more applicable information.

In addition to explicitly calling the toString() method of the object, the toString() method will be automatically called when connecting String to other types of data.

The above methods are often used in Java. Here is only a brief introduction to let everyone know about the Object class and other classes. For detailed description, please refer to the Java API documentation.

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