Summary of Object class knowledge points in Java

This article brings you relevant knowledge about java, which mainly introduces related issues about the Object class, including what the Object class is, the equals method and hashCode method in the Object class Let’s take a look at the content below. I hope it will be helpful to everyone.

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1. What is the Object class?

Object is a special class in the Java class library and the parent class of all classes.

In other words, Java allows any type of object to be assigned to a variable of type Object.

Except for the Object class, all classes in Java have inheritance relationships.

The Object class is located in the java.lang package and will be automatically imported during compilation. When a class is defined, if no inherited parent class is specified, the default parent class is the Object class.

class Person {}
class Student {}
public class Test03 {
    public static void func(Object o) {  }

    public static void main(String[] args) {
        func(new Person());
        func(new Student());
    }
    public static void main1(String[] args) {
        Object o = new Person();
        Object o2 = new Student();
    }
}

Let’s take a look at the methods in the Object class

This article will briefly introduce hashCode(), equals()

2. The equals method in the Object class

Look at this piece of code below

class Person {
    public String dz;
    public Person(String dz) {
        this.dz = dz;
    }
    @Override
    public String toString() {
        return "Person{" +
                "dz='" + dz + '\'' +
                '}';
    }
}
class Student {

}
public class Test03 {

    public static void main(String[] args) {
        Person person = new Person("beijing");
        Person person1 = new Person("beijing");
        System.out.println(person == person1);
}

If you compare directly like this and see if the addresses are the same, false will be output, and the output is wrong

So if you want to compare, is there any way

In java A method provided is equals, which can be used to compare whether two objects are the same.

   System.out.println(person.equals(person1));

If you write this directly in main, false will still be output.

This is because there is no equals, the default is inherited from Object

You can see that the comparison is still this and obj, which are the parameters currently passed in, so it is false.

So how to use equals for comparison

Then we need to rewrite an equals before we can compare

    @Override
    public boolean equals(Object obj) {
        //判断当前obj参数是否为null
        if (obj == null) {
            return false;
        }
        if (this == obj) {
            return true;
        }
        //不是Person类对象
        if (!(obj instanceof Person)) {
            return false;
        }
        Person ret = (Person) obj;
        return this.dz.equals(ret.dz);
    }

We can use String to help rewrite Write equals

At this time equals can be compared

The content in the comparison object When they are the same, you need to override equals

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3. The hashCode method in the Object class

Using hashCode can calculate the specific object location, which is the memory address , and then call the Integer.toHexString() method to output this address in hexadecimal

,

The two dz are the same, and then directly use The hashCode calculates the memory address, and then the code finds that the memory address is different.

It means that it is not possible to directly enter the hashCode like this. Then you need to re-implement a hashCode,

below Rewrite hashCode,

    @Override
    public int hashCode() {
        return Objects.hash(dz);
    }

and then run the code to try. Sure enough, after rewriting, the memory address is the same

## So, hashCode is To locate the position

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4. The compiler automatically generates equals and hashCode

Generally, when rewriting, these two are rewritten at the same time

This is the generated equals and hashCode

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

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