Object is a class provided by Java by default. Except for the Object class, all classes in Java have inheritance relationships. By default, it will inherit the Object parent class. That is, objects of all classes can be received using the reference of Object.
Example: Use Object to receive objects of all classes
class Person{}
class Student{}
public class Test {
public static void main(String[] args) {
function(new Person());
function(new Student());
}
public static void function(Object obj) {
System.out.println(obj);
}
}
//执行结果：
Person@1b6d3586
Student@4554617cSo during development, the Object class is the highest unified type of parameters. But the Object class also has some well-defined methods. As follows:
Here are three of the methods: toString() method, equals() method, hashcode() method
When we want to print the contents of the object, we can do it by overriding the toString method in the Object class!
The following explains why the toString() method should be overridden
The following code wants to print a Person object:
public class Person {
String name;
String gender;
int age;
public Person(String name, String gender, int age) {
this.name = name;
this.gender = gender;
this.age = age;
}
public static void main(String[] args) {
Person person = new Person("Jim","男", 18);
System.out.println(person);
}
}Look at the execution result and the printed content is not the specific content of the object.
Observe the println method source code here. In the source code, the valueOf method in the String class is called. When jumping to the source code at valueOf, you can find that the method body actually The toString method is called again,
Now let’s look at the specific implementation of toString,
getClass() .getName() returns the runtime class (Class) of this Object, and returns the name of the entity (class, interface, array class, basic type or void) represented by this Class object in the form of String
hashCode( ) method returns the "address"
Integer.toHexString(hashCode()) Gets the hash code value of this object (int type), and uses the wrapper class Integer class to get the hash code of this int type value, convert it to a hexadecimal unsigned integer, and represent the converted hexadecimal integer in the form of a string
So the output function println is implemented by calling toString at the bottom level. If you want To realize the specific content of the printing object, we only need to rewrite the toString method in the Object class according to our own ideas
public class Person {
String name;
String gender;
int age;
@Override
public String toString() {
return "Person{" +
"name='" + name + '\'' +
", gender='" + gender + '\'' +
", age=" + age +
'}';
}
public Person(String name, String gender, int age) {
this.name = name;
this.gender = gender;
this.age = age;
}
public static void main(String[] args) {
Person person = new Person("xin","男", 21);
System.out.println(person);
}
}Execution result:
In Java, when == is compared:
If the left and right sides of == are basic type variables, what is compared is whether the value in the variable is Same
If the left and right sides of == are reference type variables, the comparison is whether the reference variable addresses are the same
If you want to compare the contents of the objects , the equals method in Object must be rewritten, because the equals method also compares according to the address by default. The following is the source code of the equals method:
Object comparison code example:
class Person{
private String name ;
private int age ;
public Person(String name, int age) {
this.age = age ;
this.name = name ;
}
@Override
public boolean equals(Object obj) {
if (obj == null) {
return false ;
}
if(this == obj) {
return true ;
}
//不是Person类对象
if (!(obj instanceof Person)) {
return false ;
}
Person person = (Person) obj ; // 向下转型，比较属性值
return this.name.equals(person.name) && this.age==person.age ;
}
}
public class Test {
public static void main(String[] args) {
Person p1 = new Person("xin", 20);
Person p2 = new Person("xin", 20);
Person p3 = new Person("rong", 21);
System.out.println(p1.equals(p2));
System.out.println(p1.equals(p3));
}
}Execution result:
The function of hashCode() in the hash table is to obtain the hash code of the object and then determine the position of the object in the hash table. hashCode method source code:
class Person {
public String name;
public int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
}
public class TestDemo4 {
public static void main(String[] args) {
Person per1 = new Person("xin", 21) ;
Person per2 = new Person("xin", 21) ;
System.out.println(per1.hashCode());
System.out.println(per2.hashCode());
}
}
import java.util.Objects;
class Person {
public String name;
public int age;
public Person(String name, int age) {
this.name = name;
this.age = age;
}
@Override
public int hashCode() {
return Objects.hash(name, age);
}
}
public class TestDemo4 {
public static void main(String[] args) {
Person per1 = new Person("xin", 21) ;
Person per2 = new Person("xin", 21) ;
System.out.println(per1.hashCode());
System.out.println(per2.hashCode());
}
}
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